Обсуждение: track_planning causing performance regression

Hi,

During fully-cached SELECT-only test using pgbench, Postgres v13Beta1 shows
~45% performance drop [2] at high DB connection counts (when compared with v12.3)

Disabling pg_stat_statements.track_planning (which is 'On' by default)
brings the TPS numbers up to v12.3 levels.

The inflection point (in this test-case) is 128 Connections, beyond which the
TPS numbers are consistently low. Looking at the mailing list [1], this issue
didn't surface earlier possibly since the regression is trivial at low connection counts.

It would be great if this could be optimized further, or track_planning
disabled (by default) so as to not trip users upgrading from v12 with pg_stat_statement
enabled (but otherwise not particularly interested in track_planning).

These are some details around the above test:

pgbench: scale - 100 / threads - 16
test-duration - 30s each
server - 96 vCPUs / 768GB - r5.24xl (AWS EC2 instance)
client - 72 vCPUs / 144GB - c5.18xl (AWS EC2 instance) (co-located with the DB server - Same AZ)
v12 - REL_12_STABLE (v12.3)
v13Beta1 - REL_13_STABLE (v13Beta1)
max_connections = 10000
shared_preload_libraries = 'pg_stat_statements'
shared_buffers 128MB


Reference:
1) https://www.postgresql.org/message-id/1554150919882-0.post%40n3.nabble.com

2) Fully-cached-select-only TPS drops >= 128 connections.

Conn      v12.3          v13Beta1        v13Beta1 (track_planning=off)
1         6,764          6,734            6,905
2         14,978         14,961           15,316
4         31,641         32,012           36,961
8         71,989         68,848           69,204
16        129,056        131,157          132,773
32        231,910        226,718          253,316
64        381,778        371,782          385,402
128       534,661  ====> 353,944          539,231
256       636,794  ====> 248,825          643,631
512       574,447  ====> 213,033          555,099
768       493,912  ====> 214,801          502,014
1024      484,993  ====> 222,492          490,716
1280      480,571  ====> 223,296          483,843
1536      475,030  ====> 228,137          477,153
1792      472,145  ====> 229,027          474,423
2048      471,385  ====> 228,665          470,238


3) perf - v13Beta1

-   88.38%     0.17%  postgres      postgres               [.] PostgresMain
   - 88.21% PostgresMain
      - 80.09% exec_simple_query
         - 25.34% pg_plan_queries
            - 25.28% pg_plan_query
               - 25.21% pgss_planner
                  - 14.36% pgss_store
                     + 13.54% s_lock
                  + 10.71% standard_planner
         + 18.29% PortalRun
         - 15.12% PortalDrop
            - 14.73% PortalCleanup
               - 13.78% pgss_ExecutorEnd
                  - 13.72% pgss_store
                     + 12.83% s_lock
                 0.72% standard_ExecutorEnd
         + 6.18% PortalStart
         + 4.86% pg_analyze_and_rewrite
         + 3.52% GetTransactionSnapshot
         + 2.56% pg_parse_query
         + 1.83% finish_xact_command
           0.51% start_xact_command
      + 3.93% pq_getbyte
      + 3.40% ReadyForQuery



4) perf - v12.3

v12.3
-   84.32%     0.21%  postgres      postgres               [.] PostgresMain
   - 84.11% PostgresMain
      - 72.56% exec_simple_query
         + 26.71% PortalRun
         - 15.33% pg_plan_queries
            - 15.29% pg_plan_query
               + 15.21% standard_planner
         + 7.81% PortalStart
         + 6.76% pg_analyze_and_rewrite
         + 4.37% GetTransactionSnapshot
         + 3.69% pg_parse_query
         - 2.96% PortalDrop
            - 2.42% PortalCleanup
               - 1.35% pgss_ExecutorEnd
                  - 1.22% pgss_store
                       0.57% s_lock
                 0.77% standard_ExecutorEnd
         + 2.16% finish_xact_command
         + 0.78% start_xact_command
         + 0.59% pg_rewrite_query
      + 5.67% pq_getbyte
      + 4.73% ReadyForQuery

-
robins

Hi,

On Mon, Jun 29, 2020 at 7:49 AM Tharakan, Robins <tharar@amazon.com> wrote:
>
> During fully-cached SELECT-only test using pgbench, Postgres v13Beta1 shows
> ~45% performance drop [2] at high DB connection counts (when compared with v12.3)
>
> Disabling pg_stat_statements.track_planning (which is 'On' by default)
> brings the TPS numbers up to v12.3 levels.
>
> The inflection point (in this test-case) is 128 Connections, beyond which the
> TPS numbers are consistently low. Looking at the mailing list [1], this issue
> didn't surface earlier possibly since the regression is trivial at low connection counts.
>
> It would be great if this could be optimized further, or track_planning
> disabled (by default) so as to not trip users upgrading from v12 with pg_stat_statement
> enabled (but otherwise not particularly interested in track_planning).
>
> These are some details around the above test:
>
> pgbench: scale - 100 / threads - 16
> test-duration - 30s each
> server - 96 vCPUs / 768GB - r5.24xl (AWS EC2 instance)
> client - 72 vCPUs / 144GB - c5.18xl (AWS EC2 instance) (co-located with the DB server - Same AZ)
> v12 - REL_12_STABLE (v12.3)
> v13Beta1 - REL_13_STABLE (v13Beta1)
> max_connections = 10000
> shared_preload_libraries = 'pg_stat_statements'
> shared_buffers 128MB

I can't reproduce this on my laptop, but I can certainly believe that
running the same 3 queries using more connections than available cores
will lead to extra overhead.

I disagree with the conclusion though.  It seems to me that if you
really have this workload that consists in these few queries and want
to get better performance, you'll anyway use a connection pooler
and/or use prepared statements, which will make this overhead
disappear entirely, and will also yield an even bigger performance
improvement.  A quick test using pgbench -M prepared, with
track_planning enabled, with still way too many connections already
shows a 25% improvement over the -M simple without track_planning.

On 2020/06/29 16:05, Julien Rouhaud wrote:
> Hi,
> 
> On Mon, Jun 29, 2020 at 7:49 AM Tharakan, Robins <tharar@amazon.com> wrote:
>>
>> During fully-cached SELECT-only test using pgbench, Postgres v13Beta1 shows

Thanks for the benchmark!


>> ~45% performance drop [2] at high DB connection counts (when compared with v12.3)

That's bad :(


>>
>> Disabling pg_stat_statements.track_planning (which is 'On' by default)
>> brings the TPS numbers up to v12.3 levels.
>>
>> The inflection point (in this test-case) is 128 Connections, beyond which the
>> TPS numbers are consistently low. Looking at the mailing list [1], this issue
>> didn't surface earlier possibly since the regression is trivial at low connection counts.
>>
>> It would be great if this could be optimized further, or track_planning
>> disabled (by default) so as to not trip users upgrading from v12 with pg_stat_statement
>> enabled (but otherwise not particularly interested in track_planning).

Your benchmark result seems to suggest that the cause of the problem is
the contention of per-query spinlock in pgss_store(). Right?
This lock contention is likely to happen when multiple sessions run
the same queries.

One idea to reduce that lock contention is to separate per-query spinlock
into two; one is for planning, and the other is for execution. pgss_store()
determines which lock to use based on the given "kind" argument.
To make this idea work, also every pgss counters like shared_blks_hit
need to be separated into two, i.e., for planning and execution.


>> These are some details around the above test:
>>
>> pgbench: scale - 100 / threads - 16
>> test-duration - 30s each
>> server - 96 vCPUs / 768GB - r5.24xl (AWS EC2 instance)
>> client - 72 vCPUs / 144GB - c5.18xl (AWS EC2 instance) (co-located with the DB server - Same AZ)
>> v12 - REL_12_STABLE (v12.3)
>> v13Beta1 - REL_13_STABLE (v13Beta1)
>> max_connections = 10000
>> shared_preload_libraries = 'pg_stat_statements'
>> shared_buffers 128MB
> 
> I can't reproduce this on my laptop, but I can certainly believe that
> running the same 3 queries using more connections than available cores
> will lead to extra overhead.
> 
> I disagree with the conclusion though.  It seems to me that if you
> really have this workload that consists in these few queries and want
> to get better performance, you'll anyway use a connection pooler
> and/or use prepared statements, which will make this overhead
> disappear entirely, and will also yield an even bigger performance
> improvement.  A quick test using pgbench -M prepared, with
> track_planning enabled, with still way too many connections already
> shows a 25% improvement over the -M simple without track_planning.

I understand your point. But IMO the default setting basically should
be safer value, i.e., off at least until the problem disappears.

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

On Mon, Jun 29, 2020 at 10:55 AM Fujii Masao
<masao.fujii@oss.nttdata.com> wrote:
>
> On 2020/06/29 16:05, Julien Rouhaud wrote:
> > On Mon, Jun 29, 2020 at 7:49 AM Tharakan, Robins <tharar@amazon.com> wrote:
> >>
> >> During fully-cached SELECT-only test using pgbench, Postgres v13Beta1 shows
>
> Thanks for the benchmark!
>
>
> >> ~45% performance drop [2] at high DB connection counts (when compared with v12.3)
>
> That's bad :(
>
>
> >>
> >> Disabling pg_stat_statements.track_planning (which is 'On' by default)
> >> brings the TPS numbers up to v12.3 levels.
> >>
> >> The inflection point (in this test-case) is 128 Connections, beyond which the
> >> TPS numbers are consistently low. Looking at the mailing list [1], this issue
> >> didn't surface earlier possibly since the regression is trivial at low connection counts.
> >>
> >> It would be great if this could be optimized further, or track_planning
> >> disabled (by default) so as to not trip users upgrading from v12 with pg_stat_statement
> >> enabled (but otherwise not particularly interested in track_planning).
>
> Your benchmark result seems to suggest that the cause of the problem is
> the contention of per-query spinlock in pgss_store(). Right?
> This lock contention is likely to happen when multiple sessions run
> the same queries.
>
> One idea to reduce that lock contention is to separate per-query spinlock
> into two; one is for planning, and the other is for execution. pgss_store()
> determines which lock to use based on the given "kind" argument.
> To make this idea work, also every pgss counters like shared_blks_hit
> need to be separated into two, i.e., for planning and execution.

This can probably remove some overhead, but won't it eventually hit
the same issue when multiple connections try to plan the same query,
given the number of different queries and very low execution runtime?
It'll also quite increase the shared memory consumption.

I'm wondering if we could instead use atomics to store the counters.
The only downside is that we won't guarantee per-row consistency
anymore, which may be problematic.

On 2020/06/29 18:17, Julien Rouhaud wrote:
> On Mon, Jun 29, 2020 at 10:55 AM Fujii Masao
> <masao.fujii@oss.nttdata.com> wrote:
>>
>> On 2020/06/29 16:05, Julien Rouhaud wrote:
>>> On Mon, Jun 29, 2020 at 7:49 AM Tharakan, Robins <tharar@amazon.com> wrote:
>>>>
>>>> During fully-cached SELECT-only test using pgbench, Postgres v13Beta1 shows
>>
>> Thanks for the benchmark!
>>
>>
>>>> ~45% performance drop [2] at high DB connection counts (when compared with v12.3)
>>
>> That's bad :(
>>
>>
>>>>
>>>> Disabling pg_stat_statements.track_planning (which is 'On' by default)
>>>> brings the TPS numbers up to v12.3 levels.
>>>>
>>>> The inflection point (in this test-case) is 128 Connections, beyond which the
>>>> TPS numbers are consistently low. Looking at the mailing list [1], this issue
>>>> didn't surface earlier possibly since the regression is trivial at low connection counts.
>>>>
>>>> It would be great if this could be optimized further, or track_planning
>>>> disabled (by default) so as to not trip users upgrading from v12 with pg_stat_statement
>>>> enabled (but otherwise not particularly interested in track_planning).
>>
>> Your benchmark result seems to suggest that the cause of the problem is
>> the contention of per-query spinlock in pgss_store(). Right?
>> This lock contention is likely to happen when multiple sessions run
>> the same queries.
>>
>> One idea to reduce that lock contention is to separate per-query spinlock
>> into two; one is for planning, and the other is for execution. pgss_store()
>> determines which lock to use based on the given "kind" argument.
>> To make this idea work, also every pgss counters like shared_blks_hit
>> need to be separated into two, i.e., for planning and execution.
> 
> This can probably remove some overhead, but won't it eventually hit
> the same issue when multiple connections try to plan the same query,
> given the number of different queries and very low execution runtime?

Yes. But maybe we can expect that the idea would improve
the performance to the near same level as v12?


> It'll also quite increase the shared memory consumption.

Yes.


> I'm wondering if we could instead use atomics to store the counters.
> The only downside is that we won't guarantee per-row consistency
> anymore, which may be problematic.

Yeah, we can consider more improvements against this issue.
But I'm afraid these (maybe including my idea) basically should
be items for v14...

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

On Mon, Jun 29, 2020 at 11:38 AM Fujii Masao
<masao.fujii@oss.nttdata.com> wrote:
>
> >> Your benchmark result seems to suggest that the cause of the problem is
> >> the contention of per-query spinlock in pgss_store(). Right?
> >> This lock contention is likely to happen when multiple sessions run
> >> the same queries.
> >>
> >> One idea to reduce that lock contention is to separate per-query spinlock
> >> into two; one is for planning, and the other is for execution. pgss_store()
> >> determines which lock to use based on the given "kind" argument.
> >> To make this idea work, also every pgss counters like shared_blks_hit
> >> need to be separated into two, i.e., for planning and execution.
> >
> > This can probably remove some overhead, but won't it eventually hit
> > the same issue when multiple connections try to plan the same query,
> > given the number of different queries and very low execution runtime?
>
> Yes. But maybe we can expect that the idea would improve
> the performance to the near same level as v12?

A POC patch should be easy to do and see how much it solves this
problem.  However I'm not able to reproduce the issue, and IMHO unless
we specifically want to be able to distinguish planner-time counters
from execution-time counters, I'd prefer to disable track_planning by
default than going this way, so that users with a sane usage won't
have to suffer from a memory increase.

> > I'm wondering if we could instead use atomics to store the counters.
> > The only downside is that we won't guarantee per-row consistency
> > anymore, which may be problematic.
>
> Yeah, we can consider more improvements against this issue.
> But I'm afraid these (maybe including my idea) basically should
> be items for v14...

Yes, that's clearly not something I'd vote to push in v13 at this point.

On 2020/06/29 18:53, Julien Rouhaud wrote:
> On Mon, Jun 29, 2020 at 11:38 AM Fujii Masao
> <masao.fujii@oss.nttdata.com> wrote:
>>
>>>> Your benchmark result seems to suggest that the cause of the problem is
>>>> the contention of per-query spinlock in pgss_store(). Right?
>>>> This lock contention is likely to happen when multiple sessions run
>>>> the same queries.
>>>>
>>>> One idea to reduce that lock contention is to separate per-query spinlock
>>>> into two; one is for planning, and the other is for execution. pgss_store()
>>>> determines which lock to use based on the given "kind" argument.
>>>> To make this idea work, also every pgss counters like shared_blks_hit
>>>> need to be separated into two, i.e., for planning and execution.
>>>
>>> This can probably remove some overhead, but won't it eventually hit
>>> the same issue when multiple connections try to plan the same query,
>>> given the number of different queries and very low execution runtime?
>>
>> Yes. But maybe we can expect that the idea would improve
>> the performance to the near same level as v12?
> 
> A POC patch should be easy to do and see how much it solves this
> problem.  However I'm not able to reproduce the issue, and IMHO unless
> we specifically want to be able to distinguish planner-time counters
> from execution-time counters, I'd prefer to disable track_planning by
> default than going this way, so that users with a sane usage won't
> have to suffer from a memory increase.

Agreed. +1 to change that default to off.

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

On 2020/06/29 18:56, Fujii Masao wrote:
> 
> 
> On 2020/06/29 18:53, Julien Rouhaud wrote:
>> On Mon, Jun 29, 2020 at 11:38 AM Fujii Masao
>> <masao.fujii@oss.nttdata.com> wrote:
>>>
>>>>> Your benchmark result seems to suggest that the cause of the problem is
>>>>> the contention of per-query spinlock in pgss_store(). Right?
>>>>> This lock contention is likely to happen when multiple sessions run
>>>>> the same queries.
>>>>>
>>>>> One idea to reduce that lock contention is to separate per-query spinlock
>>>>> into two; one is for planning, and the other is for execution. pgss_store()
>>>>> determines which lock to use based on the given "kind" argument.
>>>>> To make this idea work, also every pgss counters like shared_blks_hit
>>>>> need to be separated into two, i.e., for planning and execution.
>>>>
>>>> This can probably remove some overhead, but won't it eventually hit
>>>> the same issue when multiple connections try to plan the same query,
>>>> given the number of different queries and very low execution runtime?
>>>
>>> Yes. But maybe we can expect that the idea would improve
>>> the performance to the near same level as v12?
>>
>> A POC patch should be easy to do and see how much it solves this
>> problem.  However I'm not able to reproduce the issue, and IMHO unless
>> we specifically want to be able to distinguish planner-time counters
>> from execution-time counters, I'd prefer to disable track_planning by
>> default than going this way, so that users with a sane usage won't
>> have to suffer from a memory increase.
> 
> Agreed. +1 to change that default to off.

Attached patch does this.

I also add the following into the description about each *_plan_time column
in the docs. IMO this is helpful for users when they see that those columns
report zero by default and try to understand why.

(if <varname>pg_stat_statements.track_planning</varname> is enabled, otherwise zero)

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

On Mon, Jun 29, 2020 at 1:14 PM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
>
> On 2020/06/29 18:56, Fujii Masao wrote:
> >
> >
> > On 2020/06/29 18:53, Julien Rouhaud wrote:
> >> On Mon, Jun 29, 2020 at 11:38 AM Fujii Masao
> >> <masao.fujii@oss.nttdata.com> wrote:
> >>>
> >>>>> Your benchmark result seems to suggest that the cause of the problem is
> >>>>> the contention of per-query spinlock in pgss_store(). Right?
> >>>>> This lock contention is likely to happen when multiple sessions run
> >>>>> the same queries.
> >>>>>
> >>>>> One idea to reduce that lock contention is to separate per-query spinlock
> >>>>> into two; one is for planning, and the other is for execution. pgss_store()
> >>>>> determines which lock to use based on the given "kind" argument.
> >>>>> To make this idea work, also every pgss counters like shared_blks_hit
> >>>>> need to be separated into two, i.e., for planning and execution.
> >>>>
> >>>> This can probably remove some overhead, but won't it eventually hit
> >>>> the same issue when multiple connections try to plan the same query,
> >>>> given the number of different queries and very low execution runtime?
> >>>
> >>> Yes. But maybe we can expect that the idea would improve
> >>> the performance to the near same level as v12?
> >>
> >> A POC patch should be easy to do and see how much it solves this
> >> problem.  However I'm not able to reproduce the issue, and IMHO unless
> >> we specifically want to be able to distinguish planner-time counters
> >> from execution-time counters, I'd prefer to disable track_planning by
> >> default than going this way, so that users with a sane usage won't
> >> have to suffer from a memory increase.
> >
> > Agreed. +1 to change that default to off.
>
> Attached patch does this.

Patch looks good to me.

> I also add the following into the description about each *_plan_time column
> in the docs. IMO this is helpful for users when they see that those columns
> report zero by default and try to understand why.
>
> (if <varname>pg_stat_statements.track_planning</varname> is enabled, otherwise zero)

+1

Do you intend to wait for other input before pushing?  FWIW I'm still
not convinced that the exposed problem is representative of any
realistic workload.  I of course entirely agree with the other
documentation changes.

On Mon, Jun 29, 2020 at 1:55 AM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
> > I disagree with the conclusion though.  It seems to me that if you
> > really have this workload that consists in these few queries and want
> > to get better performance, you'll anyway use a connection pooler
> > and/or use prepared statements, which will make this overhead
> > disappear entirely, and will also yield an even bigger performance
> > improvement.  A quick test using pgbench -M prepared, with
> > track_planning enabled, with still way too many connections already
> > shows a 25% improvement over the -M simple without track_planning.
>
> I understand your point. But IMO the default setting basically should
> be safer value, i.e., off at least until the problem disappears.

+1 -- this regression seems unacceptable to me.

-- 
Peter Geoghegan

On Mon, Jun 29, 2020 at 3:23 PM Peter Geoghegan <pg@bowt.ie> wrote:
> +1 -- this regression seems unacceptable to me.

I added an open item to track this.

Thanks
-- 
Peter Geoghegan

Hi,

On 2020-06-29 09:05:18 +0200, Julien Rouhaud wrote:
> I can't reproduce this on my laptop, but I can certainly believe that
> running the same 3 queries using more connections than available cores
> will lead to extra overhead.

> I disagree with the conclusion though.  It seems to me that if you
> really have this workload that consists in these few queries and want
> to get better performance, you'll anyway use a connection pooler
> and/or use prepared statements, which will make this overhead
> disappear entirely, and will also yield an even bigger performance
> improvement.

It's an extremely common to have have times where there's more active
queries than CPUs. And a pooler won't avoid that fully, at least not
without drastically reducing overall throughput.

Greetings,

Andres Freund

Hi,

On 2020-06-29 17:55:28 +0900, Fujii Masao wrote:
> One idea to reduce that lock contention is to separate per-query spinlock
> into two; one is for planning, and the other is for execution. pgss_store()
> determines which lock to use based on the given "kind" argument.
> To make this idea work, also every pgss counters like shared_blks_hit
> need to be separated into two, i.e., for planning and execution.

I suspect that the best thing would be to just turn the spinlock into an
lwlock. Spinlocks deal terribly with contention. I suspect it'd solve
the performance issue entirely. And it might even be possible, further
down the line, to just use a shared lock, and use atomics for the
counters.

Greetings,

Andres Freund

On 2020/06/29 22:23, Ants Aasma wrote:
> On Mon, 29 Jun 2020 at 12:17, Julien Rouhaud <rjuju123@gmail.com <mailto:rjuju123@gmail.com>> wrote:
> 
>     On Mon, Jun 29, 2020 at 10:55 AM Fujii Masao
>     <masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>> wrote:
>      >
>      > On 2020/06/29 16:05, Julien Rouhaud wrote:
>      > > On Mon, Jun 29, 2020 at 7:49 AM Tharakan, Robins <tharar@amazon.com <mailto:tharar@amazon.com>> wrote:
>      > >>
>      > >> During fully-cached SELECT-only test using pgbench, Postgres v13Beta1 shows
>      >
>      > Thanks for the benchmark!
>      >
>      >
>      > >> ~45% performance drop [2] at high DB connection counts (when compared with v12.3)
>      >
>      > That's bad :(
>      >
>      >
>      > >>
>      > >> Disabling pg_stat_statements.track_planning (which is 'On' by default)
>      > >> brings the TPS numbers up to v12.3 levels.
>      > >>
>      > >> The inflection point (in this test-case) is 128 Connections, beyond which the
>      > >> TPS numbers are consistently low. Looking at the mailing list [1], this issue
>      > >> didn't surface earlier possibly since the regression is trivial at low connection counts.
>      > >>
>      > >> It would be great if this could be optimized further, or track_planning
>      > >> disabled (by default) so as to not trip users upgrading from v12 with pg_stat_statement
>      > >> enabled (but otherwise not particularly interested in track_planning).
>      >
>      > Your benchmark result seems to suggest that the cause of the problem is
>      > the contention of per-query spinlock in pgss_store(). Right?
>      > This lock contention is likely to happen when multiple sessions run
>      > the same queries.
>      >
>      > One idea to reduce that lock contention is to separate per-query spinlock
>      > into two; one is for planning, and the other is for execution. pgss_store()
>      > determines which lock to use based on the given "kind" argument.
>      > To make this idea work, also every pgss counters like shared_blks_hit
>      > need to be separated into two, i.e., for planning and execution.
> 
>     This can probably remove some overhead, but won't it eventually hit
>     the same issue when multiple connections try to plan the same query,
>     given the number of different queries and very low execution runtime?
>     It'll also quite increase the shared memory consumption.
> 
>     I'm wondering if we could instead use atomics to store the counters.
>     The only downside is that we won't guarantee per-row consistency
>     anymore, which may be problematic.
> 
> 
> 
> The problem looks to be that spinlocks are terrible with overloaded CPU and a contended spinlock. A process holding
thespinlock might easily get scheduled out leading to excessive spinning by everybody. I think a simple thing to try
wouldbe to replace the spinlock with LWLock.
 

Yes. Attached is the POC patch that replaces per-counter spinlock with LWLock.

> 
> I did a prototype patch that replaces spinlocks with futexes, but was not able to find a workload where it mattered.

I'm not familiar with futex, but could you tell me why you used futex instead
of LWLock that we already have? Is futex portable?

> We have done a great job at eliminating spinlocks from contended code paths. Robins, perhaps you could try it to see
ifit reduces the regression you are observing.
 

Yes. Also we need to check that this change doesn't increase performance
overhead in other workloads.

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

On 2020/06/30 20:30, Ants Aasma wrote:
> On Tue, 30 Jun 2020 at 08:43, Fujii Masao <masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>> wrote:
> 
>      > The problem looks to be that spinlocks are terrible with overloaded CPU and a contended spinlock. A process
holdingthe spinlock might easily get scheduled out leading to excessive spinning by everybody. I think a simple thing
totry would be to replace the spinlock with LWLock.
 
> 
>     Yes. Attached is the POC patch that replaces per-counter spinlock with LWLock.
> 
> 
> Great. I think this is the one that should get considered for testing.
> 
>      > I did a prototype patch that replaces spinlocks with futexes, but was not able to find a workload where it
mattered.
> 
>     I'm not familiar with futex, but could you tell me why you used futex instead
>     of LWLock that we already have? Is futex portable?
> 
> 
> Futex is a Linux kernel call that allows to build a lock that has uncontended cases work fully in user space almost
exactlylike a spinlock, while falling back to syscalls that wait for wakeup in case of contention. It's not
portable, butprobably something similar could be implemented for other operating systems. I did not pursue this further
becauseit became apparent that every performance critical spinlock had already been removed.
 
> 
> To be clear, I am not advocating for this patch to get included. I just had the patch immediately available and it
couldhave confirmed that using a better lock fixes things.
 

Understood. Thanks for the explanation!

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

On 2020/06/30 7:29, Peter Geoghegan wrote:
> On Mon, Jun 29, 2020 at 3:23 PM Peter Geoghegan <pg@bowt.ie> wrote:
>> +1 -- this regression seems unacceptable to me.
> 
> I added an open item to track this.

Thanks!
I'm thinking to change the default value of track_planning to off for v13.

Ants and Andres suggested to replace the spinlock used in pgss_store() with
LWLock. I agreed with them and posted the POC patch doing that. But I think
the patch is an item for v14. The patch may address the reported performance
issue, but may cause other performance issues in other workloads. We would
need to measure how the patch affects the performance in various workloads.
It seems too late to do that at this stage of v13. Thought?

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

Hi,

On 2020-06-30 14:43:39 +0900, Fujii Masao wrote:
> > I did a prototype patch that replaces spinlocks with futexes, but was not able to find a workload where it
mattered.
> 
> I'm not familiar with futex, but could you tell me why you used futex instead
> of LWLock that we already have? Is futex portable?

We can't rely on futexes, they're linux only. I also don't see much of a
reason to use spinlocks (rather than lwlocks) here in the first place.


> diff --git a/contrib/pg_stat_statements/pg_stat_statements.c b/contrib/pg_stat_statements/pg_stat_statements.c
> index cef8bb5a49..aa506f6c11 100644
> --- a/contrib/pg_stat_statements/pg_stat_statements.c
> +++ b/contrib/pg_stat_statements/pg_stat_statements.c
> @@ -39,7 +39,7 @@
>   * in an entry except the counters requires the same.  To look up an entry,
>   * one must hold the lock shared.  To read or update the counters within
>   * an entry, one must hold the lock shared or exclusive (so the entry doesn't
> - * disappear!) and also take the entry's mutex spinlock.
> + * disappear!) and also take the entry's partition lock.
>   * The shared state variable pgss->extent (the next free spot in the external
>   * query-text file) should be accessed only while holding either the
>   * pgss->mutex spinlock, or exclusive lock on pgss->lock.  We use the mutex to
> @@ -115,6 +115,11 @@ static const uint32 PGSS_PG_MAJOR_VERSION = PG_VERSION_NUM / 100;
>  
>  #define JUMBLE_SIZE                1024    /* query serialization buffer size */
>  
> +#define    PGSS_NUM_LOCK_PARTITIONS()        (pgss_max)
> +#define    PGSS_HASH_PARTITION_LOCK(key)    \
> +    (&(pgss->base +    \
> +       (get_hash_value(pgss_hash, key) % PGSS_NUM_LOCK_PARTITIONS()))->lock)
> +
>  /*
>   * Extension version number, for supporting older extension versions' objects
>   */
> @@ -207,7 +212,7 @@ typedef struct pgssEntry
>      Size        query_offset;    /* query text offset in external file */
>      int            query_len;        /* # of valid bytes in query string, or -1 */
>      int            encoding;        /* query text encoding */
> -    slock_t        mutex;            /* protects the counters only */
> +    LWLock           *lock;            /* protects the counters only */
>  } pgssEntry;

Why did you add the hashing here? It seems a lot better to just add an
lwlock in-place instead of the spinlock? The added size is neglegible
compared to the size of pgssEntry.

Greetings,

Andres Freund

Hi,

On 2020-06-30 14:30:03 +0300, Ants Aasma wrote:
> Futex is a Linux kernel call that allows to build a lock that has
> uncontended cases work fully in user space almost exactly like a spinlock,
> while falling back to syscalls that wait for wakeup in case of contention.
> It's not portable, but probably something similar could be implemented for
> other operating systems. I did not pursue this further because it became
> apparent that every performance critical spinlock had already been removed.

Our lwlock implementation does have that property already, though. While
the kernel wait is implemented using semaphores, those are implemented
using futexes internally (posix ones, not sysv ones, so only after
whatever version we switched the default to posix semas on linux).

I'd rather move towards removing spinlocks from postgres than making
their implementation more complicated...

Greetings,

Andres Freund

On Tue, Jun 30, 2020 at 6:40 AM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
> Ants and Andres suggested to replace the spinlock used in pgss_store() with
> LWLock. I agreed with them and posted the POC patch doing that. But I think
> the patch is an item for v14. The patch may address the reported performance
> issue, but may cause other performance issues in other workloads. We would
> need to measure how the patch affects the performance in various workloads.
> It seems too late to do that at this stage of v13. Thought?

I agree that it's too late for v13.

Thanks
--
Peter Geoghegan

On 2020/07/01 4:03, Andres Freund wrote:
> Hi,
> 
> On 2020-06-30 14:43:39 +0900, Fujii Masao wrote:
>>> I did a prototype patch that replaces spinlocks with futexes, but was not able to find a workload where it
mattered.
>>
>> I'm not familiar with futex, but could you tell me why you used futex instead
>> of LWLock that we already have? Is futex portable?
> 
> We can't rely on futexes, they're linux only.

Understood. Thanks!



> I also don't see much of a
> reason to use spinlocks (rather than lwlocks) here in the first place.
> 
> 
>> diff --git a/contrib/pg_stat_statements/pg_stat_statements.c b/contrib/pg_stat_statements/pg_stat_statements.c
>> index cef8bb5a49..aa506f6c11 100644
>> --- a/contrib/pg_stat_statements/pg_stat_statements.c
>> +++ b/contrib/pg_stat_statements/pg_stat_statements.c
>> @@ -39,7 +39,7 @@
>>    * in an entry except the counters requires the same.  To look up an entry,
>>    * one must hold the lock shared.  To read or update the counters within
>>    * an entry, one must hold the lock shared or exclusive (so the entry doesn't
>> - * disappear!) and also take the entry's mutex spinlock.
>> + * disappear!) and also take the entry's partition lock.
>>    * The shared state variable pgss->extent (the next free spot in the external
>>    * query-text file) should be accessed only while holding either the
>>    * pgss->mutex spinlock, or exclusive lock on pgss->lock.  We use the mutex to
>> @@ -115,6 +115,11 @@ static const uint32 PGSS_PG_MAJOR_VERSION = PG_VERSION_NUM / 100;
>>   
>>   #define JUMBLE_SIZE                1024    /* query serialization buffer size */
>>   
>> +#define    PGSS_NUM_LOCK_PARTITIONS()        (pgss_max)
>> +#define    PGSS_HASH_PARTITION_LOCK(key)    \
>> +    (&(pgss->base +    \
>> +       (get_hash_value(pgss_hash, key) % PGSS_NUM_LOCK_PARTITIONS()))->lock)
>> +
>>   /*
>>    * Extension version number, for supporting older extension versions' objects
>>    */
>> @@ -207,7 +212,7 @@ typedef struct pgssEntry
>>       Size        query_offset;    /* query text offset in external file */
>>       int            query_len;        /* # of valid bytes in query string, or -1 */
>>       int            encoding;        /* query text encoding */
>> -    slock_t        mutex;            /* protects the counters only */
>> +    LWLock           *lock;            /* protects the counters only */
>>   } pgssEntry;
> 
> Why did you add the hashing here? It seems a lot better to just add an
> lwlock in-place instead of the spinlock? The added size is neglegible
> compared to the size of pgssEntry.

Because pgssEntry is not array entry but hashtable entry. First I was
thinking to assign per-process lwlock to each entry in the array at the
startup. But each entry is created every time new entry is required.
So lwlock needs to be assigned to each entry at that creation time.
We cannnot easily assign lwlock to all the entries at the startup.

Also each entry can be dropped from the hashtable. In this case,
maybe already-assigned lwlock needs to be moved back to "freelist"
so that it will be able to be assigned again to new entry later. We can
implement this probably, but which looks a bit complicated.

Since the hasing addresses these issues, I just used it in POC patch.
But I'd like to hear better idea!

> +#define      PGSS_NUM_LOCK_PARTITIONS()              (pgss_max)

Currently pgss_max is used as the number of lwlock for entries.
But if too large number of lwlock is useless (or a bit harmful?), we can
set the upper limit here, e.g., max(pgss_max, 10000).

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

Hi,

On 2020-07-01 22:20:50 +0900, Fujii Masao wrote:
> On 2020/07/01 4:03, Andres Freund wrote:
> > Why did you add the hashing here? It seems a lot better to just add an
> > lwlock in-place instead of the spinlock? The added size is neglegible
> > compared to the size of pgssEntry.
> 
> Because pgssEntry is not array entry but hashtable entry. First I was
> thinking to assign per-process lwlock to each entry in the array at the
> startup. But each entry is created every time new entry is required.
> So lwlock needs to be assigned to each entry at that creation time.
> We cannnot easily assign lwlock to all the entries at the startup.

But why not just do it exactly at the place the SpinLockInit() is done
currently?

Greetings,

Andres Freund

On 2020/07/02 1:54, Andres Freund wrote:
> Hi,
> 
> On 2020-07-01 22:20:50 +0900, Fujii Masao wrote:
>> On 2020/07/01 4:03, Andres Freund wrote:
>>> Why did you add the hashing here? It seems a lot better to just add an
>>> lwlock in-place instead of the spinlock? The added size is neglegible
>>> compared to the size of pgssEntry.
>>
>> Because pgssEntry is not array entry but hashtable entry. First I was
>> thinking to assign per-process lwlock to each entry in the array at the
>> startup. But each entry is created every time new entry is required.
>> So lwlock needs to be assigned to each entry at that creation time.
>> We cannnot easily assign lwlock to all the entries at the startup.
> 
> But why not just do it exactly at the place the SpinLockInit() is done
> currently?

Sorry I failed to understand your point... You mean that new lwlock should
be initialized at the place the SpinLockInit() is done currently instead of
requesting postmaster to initialize all the lwlocks required for pgss
at _PG_init()?

Regards,


-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

On 2020/07/01 7:37, Peter Geoghegan wrote:
> On Tue, Jun 30, 2020 at 6:40 AM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
>> Ants and Andres suggested to replace the spinlock used in pgss_store() with
>> LWLock. I agreed with them and posted the POC patch doing that. But I think
>> the patch is an item for v14. The patch may address the reported performance
>> issue, but may cause other performance issues in other workloads. We would
>> need to measure how the patch affects the performance in various workloads.
>> It seems too late to do that at this stage of v13. Thought?
> 
> I agree that it's too late for v13.

Thanks for the comment!

So I pushed the patch and changed default of track_planning to off.

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

On Thu, Jul 2, 2020 at 7:39 PM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
> So I pushed the patch and changed default of track_planning to off.

I have closed out the open item I created for this.

Thanks!
-- 
Peter Geoghegan

On 2020/07/03 11:43, Peter Geoghegan wrote:
> On Thu, Jul 2, 2020 at 7:39 PM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
>> So I pushed the patch and changed default of track_planning to off.
> 
> I have closed out the open item I created for this.

Thanks!!

I added the patch that replaces spinlock with lwlock in pgss, into CF-2020-09.

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

On 2020/07/03 13:05, Pavel Stehule wrote:
> Hi
> 
> pá 3. 7. 2020 v 4:39 odesílatel Fujii Masao <masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>>
napsal:
> 
> 
> 
>     On 2020/07/01 7:37, Peter Geoghegan wrote:
>      > On Tue, Jun 30, 2020 at 6:40 AM Fujii Masao <masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>>
wrote:
>      >> Ants and Andres suggested to replace the spinlock used in pgss_store() with
>      >> LWLock. I agreed with them and posted the POC patch doing that. But I think
>      >> the patch is an item for v14. The patch may address the reported performance
>      >> issue, but may cause other performance issues in other workloads. We would
>      >> need to measure how the patch affects the performance in various workloads.
>      >> It seems too late to do that at this stage of v13. Thought?
>      >
>      > I agree that it's too late for v13.
> 
>     Thanks for the comment!
> 
>     So I pushed the patch and changed default of track_planning to off.
> 
> 
> Maybe there can be documented so enabling this option can have a negative impact on performance.

Yes. What about adding either of the followings into the doc?

     Enabling this parameter may incur a noticeable performance penalty.

or

     Enabling this parameter may incur a noticeable performance penalty,
     especially when a fewer kinds of queries are executed on many
     concurrent connections.

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

On 2020/07/03 16:02, Pavel Stehule wrote:
> 
> 
> pá 3. 7. 2020 v 8:57 odesílatel Fujii Masao <masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>>
napsal:
> 
> 
> 
>     On 2020/07/03 13:05, Pavel Stehule wrote:
>      > Hi
>      >
>      > pá 3. 7. 2020 v 4:39 odesílatel Fujii Masao <masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>
<mailto:masao.fujii@oss.nttdata.com<mailto:masao.fujii@oss.nttdata.com>>> napsal:
 
>      >
>      >
>      >
>      >     On 2020/07/01 7:37, Peter Geoghegan wrote:
>      >      > On Tue, Jun 30, 2020 at 6:40 AM Fujii Masao <masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com><mailto:masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>>> wrote:
 
>      >      >> Ants and Andres suggested to replace the spinlock used in pgss_store() with
>      >      >> LWLock. I agreed with them and posted the POC patch doing that. But I think
>      >      >> the patch is an item for v14. The patch may address the reported performance
>      >      >> issue, but may cause other performance issues in other workloads. We would
>      >      >> need to measure how the patch affects the performance in various workloads.
>      >      >> It seems too late to do that at this stage of v13. Thought?
>      >      >
>      >      > I agree that it's too late for v13.
>      >
>      >     Thanks for the comment!
>      >
>      >     So I pushed the patch and changed default of track_planning to off.
>      >
>      >
>      > Maybe there can be documented so enabling this option can have a negative impact on performance.
> 
>     Yes. What about adding either of the followings into the doc?
> 
>           Enabling this parameter may incur a noticeable performance penalty.
> 
>     or
> 
>           Enabling this parameter may incur a noticeable performance penalty,
>           especially when a fewer kinds of queries are executed on many
>           concurrent connections.
> 
> 
> This second variant looks perfect for this case.

Ok, so patch attached.

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

On 2020/07/04 12:22, Pavel Stehule wrote:
> 
> 
> pá 3. 7. 2020 v 13:02 odesílatel Fujii Masao <masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>>
napsal:
> 
> 
> 
>     On 2020/07/03 16:02, Pavel Stehule wrote:
>      >
>      >
>      > pá 3. 7. 2020 v 8:57 odesílatel Fujii Masao <masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>
<mailto:masao.fujii@oss.nttdata.com<mailto:masao.fujii@oss.nttdata.com>>> napsal:
 
>      >
>      >
>      >
>      >     On 2020/07/03 13:05, Pavel Stehule wrote:
>      >      > Hi
>      >      >
>      >      > pá 3. 7. 2020 v 4:39 odesílatel Fujii Masao <masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com><mailto:masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>>
<mailto:masao.fujii@oss.nttdata.com<mailto:masao.fujii@oss.nttdata.com> <mailto:masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com>>>>napsal:
 
>      >      >
>      >      >
>      >      >
>      >      >     On 2020/07/01 7:37, Peter Geoghegan wrote:
>      >      >      > On Tue, Jun 30, 2020 at 6:40 AM Fujii Masao <masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com><mailto:masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>>
<mailto:masao.fujii@oss.nttdata.com<mailto:masao.fujii@oss.nttdata.com> <mailto:masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com>>>>wrote:
 
>      >      >      >> Ants and Andres suggested to replace the spinlock used in pgss_store() with
>      >      >      >> LWLock. I agreed with them and posted the POC patch doing that. But I think
>      >      >      >> the patch is an item for v14. The patch may address the reported performance
>      >      >      >> issue, but may cause other performance issues in other workloads. We would
>      >      >      >> need to measure how the patch affects the performance in various workloads.
>      >      >      >> It seems too late to do that at this stage of v13. Thought?
>      >      >      >
>      >      >      > I agree that it's too late for v13.
>      >      >
>      >      >     Thanks for the comment!
>      >      >
>      >      >     So I pushed the patch and changed default of track_planning to off.
>      >      >
>      >      >
>      >      > Maybe there can be documented so enabling this option can have a negative impact on performance.
>      >
>      >     Yes. What about adding either of the followings into the doc?
>      >
>      >           Enabling this parameter may incur a noticeable performance penalty.
>      >
>      >     or
>      >
>      >           Enabling this parameter may incur a noticeable performance penalty,
>      >           especially when a fewer kinds of queries are executed on many
>      >           concurrent connections.
>      >
>      >
>      > This second variant looks perfect for this case.
> 
>     Ok, so patch attached.
> 
> 
> +1

Thanks for the review! Pushed.

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

On 2020/07/31 21:40, Hamid Akhtar wrote:
> <https://commitfest.postgresql.org/29/2634/>
> 
> On Mon, Jul 6, 2020 at 10:29 AM Fujii Masao <masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>>
wrote:
> 
> 
> 
>     On 2020/07/04 12:22, Pavel Stehule wrote:
>      >
>      >
>      > pá 3. 7. 2020 v 13:02 odesílatel Fujii Masao <masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>
<mailto:masao.fujii@oss.nttdata.com<mailto:masao.fujii@oss.nttdata.com>>> napsal:
 
>      >
>      >
>      >
>      >     On 2020/07/03 16:02, Pavel Stehule wrote:
>      >      >
>      >      >
>      >      > pá 3. 7. 2020 v 8:57 odesílatel Fujii Masao <masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com><mailto:masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>>
<mailto:masao.fujii@oss.nttdata.com<mailto:masao.fujii@oss.nttdata.com> <mailto:masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com>>>>napsal:
 
>      >      >
>      >      >
>      >      >
>      >      >     On 2020/07/03 13:05, Pavel Stehule wrote:
>      >      >      > Hi
>      >      >      >
>      >      >      > pá 3. 7. 2020 v 4:39 odesílatel Fujii Masao <masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com><mailto:masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>>
<mailto:masao.fujii@oss.nttdata.com<mailto:masao.fujii@oss.nttdata.com> <mailto:masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com>>><mailto:masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>
<mailto:masao.fujii@oss.nttdata.com<mailto:masao.fujii@oss.nttdata.com>> <mailto:masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com><mailto:masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>>>>>
napsal:
>      >      >      >
>      >      >      >
>      >      >      >
>      >      >      >     On 2020/07/01 7:37, Peter Geoghegan wrote:
>      >      >      >      > On Tue, Jun 30, 2020 at 6:40 AM Fujii Masao <masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com><mailto:masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>>
<mailto:masao.fujii@oss.nttdata.com<mailto:masao.fujii@oss.nttdata.com> <mailto:masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com>>><mailto:masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>
<mailto:masao.fujii@oss.nttdata.com<mailto:masao.fujii@oss.nttdata.com>> <mailto:masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com><mailto:masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>>>>>
wrote:
>      >      >      >      >> Ants and Andres suggested to replace the spinlock used in pgss_store() with
>      >      >      >      >> LWLock. I agreed with them and posted the POC patch doing that. But I think
>      >      >      >      >> the patch is an item for v14. The patch may address the reported performance
>      >      >      >      >> issue, but may cause other performance issues in other workloads. We would
>      >      >      >      >> need to measure how the patch affects the performance in various workloads.
>      >      >      >      >> It seems too late to do that at this stage of v13. Thought?
>      >      >      >      >
>      >      >      >      > I agree that it's too late for v13.
>      >      >      >
>      >      >      >     Thanks for the comment!
>      >      >      >
>      >      >      >     So I pushed the patch and changed default of track_planning to off.
>      >      >      >
>      >      >      >
>      >      >      > Maybe there can be documented so enabling this option can have a negative impact on
performance.
>      >      >
>      >      >     Yes. What about adding either of the followings into the doc?
>      >      >
>      >      >           Enabling this parameter may incur a noticeable performance penalty.
>      >      >
>      >      >     or
>      >      >
>      >      >           Enabling this parameter may incur a noticeable performance penalty,
>      >      >           especially when a fewer kinds of queries are executed on many
>      >      >           concurrent connections.
>      >      >
>      >      >
>      >      > This second variant looks perfect for this case.
>      >
>      >     Ok, so patch attached.
>      >
>      >
>      > +1
> 
>     Thanks for the review! Pushed.
> 
>     Regards,
> 
>     -- 
>     Fujii Masao
>     Advanced Computing Technology Center
>     Research and Development Headquarters
>     NTT DATA CORPORATION
> 
> 
> 
> You might also want to update this patch's status in the commitfest:
> https://commitfest.postgresql.org/29/2634/

The patch added into this CF entry has not been committed yet.
So I was thinking that there is no need to update the status yet. No?

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

On 2020/08/17 18:34, Hamid Akhtar wrote:
> 
> 
> On Mon, Aug 17, 2020 at 2:21 PM Fujii Masao <masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>>
wrote:
> 
> 
> 
>     On 2020/07/31 21:40, Hamid Akhtar wrote:
>      > <https://commitfest.postgresql.org/29/2634/>
>      >
>      > On Mon, Jul 6, 2020 at 10:29 AM Fujii Masao <masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>
<mailto:masao.fujii@oss.nttdata.com<mailto:masao.fujii@oss.nttdata.com>>> wrote:
 
>      >
>      >
>      >
>      >     On 2020/07/04 12:22, Pavel Stehule wrote:
>      >      >
>      >      >
>      >      > pá 3. 7. 2020 v 13:02 odesílatel Fujii Masao <masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com><mailto:masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>>
<mailto:masao.fujii@oss.nttdata.com<mailto:masao.fujii@oss.nttdata.com> <mailto:masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com>>>>napsal:
 
>      >      >
>      >      >
>      >      >
>      >      >     On 2020/07/03 16:02, Pavel Stehule wrote:
>      >      >      >
>      >      >      >
>      >      >      > pá 3. 7. 2020 v 8:57 odesílatel Fujii Masao <masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com><mailto:masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>>
<mailto:masao.fujii@oss.nttdata.com<mailto:masao.fujii@oss.nttdata.com> <mailto:masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com>>><mailto:masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>
<mailto:masao.fujii@oss.nttdata.com<mailto:masao.fujii@oss.nttdata.com>> <mailto:masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com><mailto:masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>>>>>
napsal:
>      >      >      >
>      >      >      >
>      >      >      >
>      >      >      >     On 2020/07/03 13:05, Pavel Stehule wrote:
>      >      >      >      > Hi
>      >      >      >      >
>      >      >      >      > pá 3. 7. 2020 v 4:39 odesílatel Fujii Masao <masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com><mailto:masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>>
<mailto:masao.fujii@oss.nttdata.com<mailto:masao.fujii@oss.nttdata.com> <mailto:masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com>>><mailto:masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>
<mailto:masao.fujii@oss.nttdata.com<mailto:masao.fujii@oss.nttdata.com>> <mailto:masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com><mailto:masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>>>>
<mailto:masao.fujii@oss.nttdata.com<mailto:masao.fujii@oss.nttdata.com> <mailto:masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com>><mailto:masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>
<mailto:masao.fujii@oss.nttdata.com<mailto:masao.fujii@oss.nttdata.com>>> <mailto:masao.fujii@oss.nttdata.com
 
>     <mailto:masao.fujii@oss.nttdata.com> <mailto:masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>>
<mailto:masao.fujii@oss.nttdata.com<mailto:masao.fujii@oss.nttdata.com> <mailto:masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com>>>>>>napsal:
 
>      >      >      >      >
>      >      >      >      >
>      >      >      >      >
>      >      >      >      >     On 2020/07/01 7:37, Peter Geoghegan wrote:
>      >      >      >      >      > On Tue, Jun 30, 2020 at 6:40 AM Fujii Masao <masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com><mailto:masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>>
<mailto:masao.fujii@oss.nttdata.com<mailto:masao.fujii@oss.nttdata.com> <mailto:masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com>>><mailto:masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>
<mailto:masao.fujii@oss.nttdata.com<mailto:masao.fujii@oss.nttdata.com>> <mailto:masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com><mailto:masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>>>>
<mailto:masao.fujii@oss.nttdata.com<mailto:masao.fujii@oss.nttdata.com> <mailto:masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com>><mailto:masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>
<mailto:masao.fujii@oss.nttdata.com<mailto:masao.fujii@oss.nttdata.com>>>
 
>     <mailto:masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com> <mailto:masao.fujii@oss.nttdata.com
<mailto:masao.fujii@oss.nttdata.com>><mailto:masao.fujii@oss.nttdata.com <mailto:masao.fujii@oss.nttdata.com>
<mailto:masao.fujii@oss.nttdata.com<mailto:masao.fujii@oss.nttdata.com>>>>>> wrote:
 
>      >      >      >      >      >> Ants and Andres suggested to replace the spinlock used in pgss_store() with
>      >      >      >      >      >> LWLock. I agreed with them and posted the POC patch doing that. But I think
>      >      >      >      >      >> the patch is an item for v14. The patch may address the reported performance
>      >      >      >      >      >> issue, but may cause other performance issues in other workloads. We would
>      >      >      >      >      >> need to measure how the patch affects the performance in various workloads.
>      >      >      >      >      >> It seems too late to do that at this stage of v13. Thought?
>      >      >      >      >      >
>      >      >      >      >      > I agree that it's too late for v13.
>      >      >      >      >
>      >      >      >      >     Thanks for the comment!
>      >      >      >      >
>      >      >      >      >     So I pushed the patch and changed default of track_planning to off.
>      >      >      >      >
>      >      >      >      >
>      >      >      >      > Maybe there can be documented so enabling this option can have a negative impact on
performance.
>      >      >      >
>      >      >      >     Yes. What about adding either of the followings into the doc?
>      >      >      >
>      >      >      >           Enabling this parameter may incur a noticeable performance penalty.
>      >      >      >
>      >      >      >     or
>      >      >      >
>      >      >      >           Enabling this parameter may incur a noticeable performance penalty,
>      >      >      >           especially when a fewer kinds of queries are executed on many
>      >      >      >           concurrent connections.
>      >      >      >
>      >      >      >
>      >      >      > This second variant looks perfect for this case.
>      >      >
>      >      >     Ok, so patch attached.
>      >      >
>      >      >
>      >      > +1
>      >
>      >     Thanks for the review! Pushed.
>      >
>      >     Regards,
>      >
>      >     --
>      >     Fujii Masao
>      >     Advanced Computing Technology Center
>      >     Research and Development Headquarters
>      >     NTT DATA CORPORATION
>      >
>      >
>      >
>      > You might also want to update this patch's status in the commitfest:
>      > https://commitfest.postgresql.org/29/2634/
> 
>     The patch added into this CF entry has not been committed yet.
>     So I was thinking that there is no need to update the status yet. No?
> 
> 
> Your previous email suggested that it's been pushed, hence my comment. Checking the git log, I see a commit was
pushedon July 6 (321fa6a) with the changes that match the latest patch.
 

Yes, I pushed the document_overhead_by_track_planning.patch, but this
CF entry is for pgss_lwlock_v1.patch which replaces spinlocks with lwlocks
in pg_stat_statements. The latter patch has not been committed yet.
Probably attachding the different patches in the same thread would cause
this confusing thing... Anyway, thanks for your comment!

Regards,


-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

> Yes, I pushed the document_overhead_by_track_planning.patch, but this
> CF entry is for pgss_lwlock_v1.patch which replaces spinlocks with lwlocks
> in pg_stat_statements. The latter patch has not been committed yet.
> Probably attachding the different patches in the same thread would cause
> this confusing thing... Anyway, thanks for your comment!

To avoid further confusion, I attached the rebased version of
the patch that was registered at CF. I'd appreciate it if
you review this version.

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

The following review has been posted through the commitfest application:
make installcheck-world:  tested, passed
Implements feature:       not tested
Spec compliant:           not tested
Documentation:            not tested

Overall, the patch works fine. However, I have a few observations:

(1) Code Comments:
- The code comments should be added for the 2 new macros, in particular for PGSS_NUM_LOCK_PARTITIONS. As you explained
inyour email, this may be used to limit the number of locks if a very large value for pgss_max is specified.
 
- From the code I inferred that the number of locks can in future be less than pgss_max (per your email where in future
thismacro could be used to limit the number of locks). I suggest to perhaps add some notes helping future changes in
thiscode area.
 

(2) It seems like that "pgss->lock = &(pgss->base + pgss_max)->lock;" statement should not use pgss_max directly and
insteaduse PGSS_NUM_LOCK_PARTITIONS macro, as when a limit is imposed on number of locks, this statement will cause an
overrun.


-- 
Highgo Software (Canada/China/Pakistan)
URL : www.highgo.ca
ADDR: 10318 WHALLEY BLVD, Surrey, BC
CELL：+923335449950  EMAIL: mailto:hamid.akhtar@highgo.ca
SKYPE: engineeredvirus

The new status of this patch is: Waiting on Author

Hi,

On 2020-08-19 00:43, Fujii Masao wrote:
>> Yes, I pushed the document_overhead_by_track_planning.patch, but this
>> CF entry is for pgss_lwlock_v1.patch which replaces spinlocks with 
>> lwlocks
>> in pg_stat_statements. The latter patch has not been committed yet.
>> Probably attachding the different patches in the same thread would 
>> cause
>> this confusing thing... Anyway, thanks for your comment!
> 
> To avoid further confusion, I attached the rebased version of
> the patch that was registered at CF. I'd appreciate it if
> you review this version.

I tested pgss_lwlock_v2.patch with 3 workloads. And I couldn't observe 
performance
improvement in our environment and I'm afraid to say that even worser in 
some case.
  - Workload1: pgbench select-only mode
  - Workload2: pgbench custom scripts which run "SELECT 1;"
  - Workload3: pgbench custom scripts which run 1000 types of different 
simple queries

- Workload1
First we set the pg_stat_statements.track_planning to on/off and run the 
fully-cached pgbench
select-only mode on pg14head which is installed in on-premises 
server(32CPU, 256GB mem).
However in this enveronment we couldn't reproduce 45% performance drop 
due to s_lock conflict
(Tharakan-san mentioned in his post on 
2895b53b033c47ccb22972b589050dd9@EX13D05UWC001.ant.amazon.com).

- Workload2
Then we adopted pgbench custom script "SELECT 1;" which supposed to 
increase the s_lock and
make it easier to reproduce the issue. In this case around 10% of 
performance decrease
which also shows slightly increase in s_lock (~10%). With this senario, 
despite a s_lock
absence, the patch shows more than 50% performance degradation 
regardless of track_planning.
And also we couldn't see performance improvement in this workload.

pgbench:
  initialization: pgbench -i -s 100
  benchmarking  : pgbench -j16 -c128 -T180 -r -n -f <script> -h <address> 
-U <user> -p <port> -d <db>
   # VACUUMed and pg_prewarmed manually before run the benchmark
query：SELECT 1;
>   pgss_lwlock_v2.patch  track_planning  TPS         decline rate   
> s_lock   CPU usage
>   -                     OFF             810509.4    standard       
> 0.17%    98.8％（sys24.9%,user73.9%）
>   -                     ON              732823.1    -9.6%          
> 1.94%    95.1％（sys22.8%,user72.3%）
>   +                     OFF             371035.0    -49.4%         -    
>     65.2％（sys20.6%,user44.6%）
>   +                     ON              193965.2    -47.7%         -    
>     41.8％（sys12.1%,user29.7%）
   # "-" is showing that s_lock was not reported from the perf.

- Workload3
Next, there is concern that replacement of LWLock may reduce performance 
in some other workloads.
(Fujii-san mentioned in his post on 
42a13b4c-e60c-c6e7-3475-8eff8050bed4@oss.nttdata.com).
To clarify this, we prepared 1000 simple queries which is supposed to 
prevent the conflict of
s_lock and may expect to see the behavior without s_lock. In this case, 
no performance decline
was observed and also we couldn't see any additional memory consumption 
or cpu usage.

pgbench:
  initialization: pgbench -n -i -s 100 --partitions=1000 
--partition-method=range
  benchmarking  : command is same as (Workload1)
query: SELECT abalance FROM pgbench_accounts_xxxx WHERE aid = :aid + 
(10000 * :random_num - 10000);
>   pgss_lwlock_v2.patch  track_planning  TPS       decline rate   CPU 
> usage
>   -                     OFF             88329.1   standard       
> 82.1%（sys6.5%,user75.6%）
>   -                     ON              88015.3   -0.36%         
> 82.6%（sys6.5%,user76.1%）
>   +                     OFF             88177.5    0.18%         
> 82.2%（sys6.5%,user75.7%）
>   +                     ON              88079.1   -0.11%         
> 82.5%（sys6.5%,user76.0%）

(Environment)
machine:
  server/client - 32 CPUs / 256GB  # used same machine as server & client
postgres:
  version: v14 (6eee73e)
  configure: '--prefix=/usr/pgsql-14a' 'CFLAGS=-O2'
GUC param (changed from defaults):
  shared_preload_libraries = 'pg_stat_statements, pg_prewarm'
  autovacuum = off
  checkpoint = 120min
  max_connections=300
  listen_address='*'
  shared_buffers=64GB


Regards,

-- 
Hibiki Tanaka

On 2020/09/11 16:23, bttanakahbk wrote:
> Hi,
> 
> On 2020-08-19 00:43, Fujii Masao wrote:
>>> Yes, I pushed the document_overhead_by_track_planning.patch, but this
>>> CF entry is for pgss_lwlock_v1.patch which replaces spinlocks with lwlocks
>>> in pg_stat_statements. The latter patch has not been committed yet.
>>> Probably attachding the different patches in the same thread would cause
>>> this confusing thing... Anyway, thanks for your comment!
>>
>> To avoid further confusion, I attached the rebased version of
>> the patch that was registered at CF. I'd appreciate it if
>> you review this version.
> 
> I tested pgss_lwlock_v2.patch with 3 workloads. And I couldn't observe performance
> improvement in our environment and I'm afraid to say that even worser in some case.
>   - Workload1: pgbench select-only mode
>   - Workload2: pgbench custom scripts which run "SELECT 1;"
>   - Workload3: pgbench custom scripts which run 1000 types of different simple queries

Thanks for running the benchmarks!


> 
> - Workload1
> First we set the pg_stat_statements.track_planning to on/off and run the fully-cached pgbench
> select-only mode on pg14head which is installed in on-premises server(32CPU, 256GB mem).
> However in this enveronment we couldn't reproduce 45% performance drop due to s_lock conflict
> (Tharakan-san mentioned in his post on 2895b53b033c47ccb22972b589050dd9@EX13D05UWC001.ant.amazon.com).
> 
> - Workload2
> Then we adopted pgbench custom script "SELECT 1;" which supposed to increase the s_lock and
> make it easier to reproduce the issue. In this case around 10% of performance decrease
> which also shows slightly increase in s_lock (~10%). With this senario, despite a s_lock
> absence, the patch shows more than 50% performance degradation regardless of track_planning.
> And also we couldn't see performance improvement in this workload.
> 
> pgbench:
>   initialization: pgbench -i -s 100
>   benchmarking  : pgbench -j16 -c128 -T180 -r -n -f <script> -h <address> -U <user> -p <port> -d <db>
>    # VACUUMed and pg_prewarmed manually before run the benchmark
> query：SELECT 1;
>>   pgss_lwlock_v2.patch  track_planning  TPS         decline rate s_lock   CPU usage
>>   -                     OFF             810509.4    standard 0.17%    98.8％（sys24.9%,user73.9%）
>>   -                     ON              732823.1    -9.6% 1.94%    95.1％（sys22.8%,user72.3%）
>>   +                     OFF             371035.0    -49.4%         -     65.2％（sys20.6%,user44.6%）
>>   +                     ON              193965.2    -47.7%         -     41.8％（sys12.1%,user29.7%）
>    # "-" is showing that s_lock was not reported from the perf.

Ok, so my proposed patch degrated the performance in this case :(
This means that replacing spinlock with lwlock in pgss is not proper
approach for the lock contention issue on pgss...

I proposed to split the spinlock for each pgss entry into two
to reduce the lock contention, upthread. One is for planner stats,
and the other is for executor stats. Is it worth working on
this approach as an alternative idea? Or does anyone have any better idea?

Regards,


-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

On Fri, Sep 11, 2020 at 4:04 PM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
>
> On 2020/09/11 16:23, bttanakahbk wrote:
> >
> > pgbench:
> >   initialization: pgbench -i -s 100
> >   benchmarking  : pgbench -j16 -c128 -T180 -r -n -f <script> -h <address> -U <user> -p <port> -d <db>
> >    # VACUUMed and pg_prewarmed manually before run the benchmark
> > query：SELECT 1;
> >>   pgss_lwlock_v2.patch  track_planning  TPS         decline rate s_lock   CPU usage
> >>   -                     OFF             810509.4    standard 0.17%    98.8％（sys24.9%,user73.9%）
> >>   -                     ON              732823.1    -9.6% 1.94%    95.1％（sys22.8%,user72.3%）
> >>   +                     OFF             371035.0    -49.4%         -     65.2％（sys20.6%,user44.6%）
> >>   +                     ON              193965.2    -47.7%         -     41.8％（sys12.1%,user29.7%）
> >    # "-" is showing that s_lock was not reported from the perf.
>
> Ok, so my proposed patch degrated the performance in this case :(
> This means that replacing spinlock with lwlock in pgss is not proper
> approach for the lock contention issue on pgss...
>
> I proposed to split the spinlock for each pgss entry into two
> to reduce the lock contention, upthread. One is for planner stats,
> and the other is for executor stats. Is it worth working on
> this approach as an alternative idea? Or does anyone have any better idea?

For now only calls and [min|max|mean|total]_time are split between
planning and execution, so we'd have to do the same for the rest of
the counters to be able to have 2 different spinlocks.  That'll
increase the size of the struct quite a lot, and we'd also have to
change the SRF output, which is already quite wide.

Hi,

> During fully-cached SELECT-only test using pgbench, Postgres v13Beta1
> shows ~45% performance drop [2] at high DB connection counts (when
> compared with v12.3)

It'd be great if we could also give credit to "Sean Massey" for this find.

This study on performance regression was done after his (internal) triaging
that the v13 regression disappeared before this commit.

I regret the omission in the original post.
-
thanks
robins

Hi,

Attached is a patch that adds Sean's name to the Release 13 Docs.

> It'd be great if we could also give credit to "Sean Massey" for this
> find.
>
> This study on performance regression was done after his (internal)
> triaging that the v13 regression disappeared before this commit.
>
> I regret the omission in the original post.

Once again regret mentioning this earlier, but hope this is in time before GA :(

-
thanks
robins

"Tharakan, Robins" <tharar@amazon.com> writes:
>> During fully-cached SELECT-only test using pgbench, Postgres v13Beta1
>> shows ~45% performance drop [2] at high DB connection counts (when
>> compared with v12.3)

> It'd be great if we could also give credit to "Sean Massey" for this find.

I poked through my Postgres inbox, and couldn't find any previous
mail from or mentioning a Sean Massey.

While there's not much of a formal policy around this, we usually limit
ourselves to crediting people who have directly interacted with the PG
community.  I'm aware that there's more than a few cases where someone
talks to the community on behalf of a company-internal team ... but
since we have little visibility into such situations, whoever's doing
the talking gets all the credit.

Given that background, it seems like crediting Sean here would be
slightly unfair special treatment.  I'd encourage him to join the
mailing lists and be part of the community directly --- then we'll
definitely know what he's contributed.

            regards, tom lane

On 2021/04/19 8:36, Justin Pryzby wrote:
> Reviewing this change which was committed last year as
> 321fa6a4a26c9b649a0fbec9fc8b019f19e62289
> 
> On Fri, Jul 03, 2020 at 03:57:38PM +0900, Fujii Masao wrote:
>> On 2020/07/03 13:05, Pavel Stehule wrote:
>>> pá 3. 7. 2020 v 4:39 odesílatel Fujii Masao <masao.fujii@oss.nttdata.com> napsal:
>>>
>>> Maybe there can be documented so enabling this option can have a negative impact on performance.
>>
>> Yes. What about adding either of the followings into the doc?
>>
>>      Enabling this parameter may incur a noticeable performance penalty.
>>
>> or
>>
>>      Enabling this parameter may incur a noticeable performance penalty,
>>      especially when a fewer kinds of queries are executed on many
>>      concurrent connections.
> 
> Something seems is wrong with this sentence, and I'm not sure what it's trying
> to say.  Is this right ?

pg_stat_statements users different spinlock for each kind of query.
So fewer kinds of queries many sessions execute, fewer spinlocks
they try to acquire. This may lead to spinlock contention and
significant performance degration. This is what the statement is
trying to say.

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

On 2021/04/19 23:55, Justin Pryzby wrote:
> What does "kind" mean ?  I think it means a "normalized" query or a "query
> structure".
> 
> "a fewer kinds" is wrong, so I think the docs should say "a small number of
> queries" or maybe:

Okay, I agree to update the description.

>>>>       Enabling this parameter may incur a noticeable performance penalty,
>>>>       especially similar queries are run by many concurrent connections and
>>>>       compete to update the same pg_stat_statements entry

"a small number of" is better than "similar" at the above because
"similar" sounds a bit unclear in this case?

It's better to use "entries" rather than "entry" at the above?

Regards,
  

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

On 2021/04/21 23:53, Justin Pryzby wrote:
> Or:
> 
>         Enabling this parameter may incur a noticeable performance penalty,
>         especially similar queries are executed by many concurrent connections
>         and compete to update a small number of pg_stat_statements entries.

I prefer this. But what about using "identical" instead of "similar"
because pg_stat_statements docs already uses "identical" in some places?

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

On Tue, Jun 29, 2021 at 10:09 AM Justin Pryzby <pryzby@telsasoft.com> wrote:
>
> Checking back - here's the latest patch.
>
> diff --git a/doc/src/sgml/pgstatstatements.sgml b/doc/src/sgml/pgstatstatements.sgml
> index 930081c429..9e98472c5c 100644
> --- a/doc/src/sgml/pgstatstatements.sgml
> +++ b/doc/src/sgml/pgstatstatements.sgml
> @@ -696,8 +696,9 @@
>        <varname>pg_stat_statements.track_planning</varname> controls whether
>        planning operations and duration are tracked by the module.
>        Enabling this parameter may incur a noticeable performance penalty,
> -      especially when queries with the same queryid are executed on many
> -      concurrent connections.
> +      especially when queries with identical structure are executed by many
> +      concurrent connections which compete to update a small number of
> +      pg_stat_statements entries.
>        The default value is <literal>off</literal>.
>        Only superusers can change this setting.
>       </para>

Is "identical structure" really accurate here?  For instance a multi
tenant application could rely on the search_path and only use
unqualified relation name.  So while they have queries with identical
structure, those will generate a large number of different query_id.

On Tue, Jun 29, 2021 at 10:45 AM Justin Pryzby <pryzby@telsasoft.com> wrote:
>
> We borrowed that language from the previous text:
>
> | Plannable queries (that is, SELECT, INSERT, UPDATE, and DELETE) are combined into a single pg_stat_statements entry
wheneverthey have identical query structures according to an internal hash calculation
 

Yes, but here's it's "identical query structure", which seems less
ambiguous than "identical structure" as iI think one could think it
refer to internal representation as much as as the query text.  And
it's also removing any doubt with the final "internal hash
calculation".

> Really, I'm only trying to fix where it currently says "a fewer kinds".

I agree that "fewer kinds" should be improved.

>        Enabling this parameter may incur a noticeable performance penalty,
> -      especially when a fewer kinds of queries are executed on many
> -      concurrent connections.
> +      especially when queries with identical structure are executed by many
> +      concurrent connections which compete to update a small number of
> +      pg_stat_statements entries.
>
> It could say "identical structure" or "the same queryid" or "identical queryid".

I think we should try to reuse the previous formulation.  How about
"statements with identical query structure"?  Or replace query
structure with "internal representation", in both places?

On 2021/06/30 0:12, Julien Rouhaud wrote:
>>         Enabling this parameter may incur a noticeable performance penalty,
>> -      especially when a fewer kinds of queries are executed on many
>> -      concurrent connections.
>> +      especially when queries with identical structure are executed by many
>> +      concurrent connections which compete to update a small number of
>> +      pg_stat_statements entries.
>>
>> It could say "identical structure" or "the same queryid" or "identical queryid".
> 
> I think we should try to reuse the previous formulation.  How about
> "statements with identical query structure"?

I'm fine with this. So what about the following diff? I added <structname> tag.

        <varname>pg_stat_statements.track_planning</varname> controls whether
        planning operations and duration are tracked by the module.
        Enabling this parameter may incur a noticeable performance penalty,
-      especially when a fewer kinds of queries are executed on many
-      concurrent connections.
+      especially when statements with identical query structure are executed
+      by many concurrent connections which compete to update a small number of
+      <structname>pg_stat_statements</structname> entries.
        The default value is <literal>off</literal>.
        Only superusers can change this setting.

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

On Thu, Jul 1, 2021 at 4:28 PM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
>
> I'm fine with this. So what about the following diff? I added <structname> tag.
>
>         <varname>pg_stat_statements.track_planning</varname> controls whether
>         planning operations and duration are tracked by the module.
>         Enabling this parameter may incur a noticeable performance penalty,
> -      especially when a fewer kinds of queries are executed on many
> -      concurrent connections.
> +      especially when statements with identical query structure are executed
> +      by many concurrent connections which compete to update a small number of
> +      <structname>pg_stat_statements</structname> entries.
>         The default value is <literal>off</literal>.
>         Only superusers can change this setting.

It seems perfect, thanks!

On 2021/07/07 18:09, Julien Rouhaud wrote:
> On Thu, Jul 1, 2021 at 4:28 PM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
>>
>> I'm fine with this. So what about the following diff? I added <structname> tag.
>>
>>          <varname>pg_stat_statements.track_planning</varname> controls whether
>>          planning operations and duration are tracked by the module.
>>          Enabling this parameter may incur a noticeable performance penalty,
>> -      especially when a fewer kinds of queries are executed on many
>> -      concurrent connections.
>> +      especially when statements with identical query structure are executed
>> +      by many concurrent connections which compete to update a small number of
>> +      <structname>pg_stat_statements</structname> entries.
>>          The default value is <literal>off</literal>.
>>          Only superusers can change this setting.
> 
> It seems perfect, thanks!

Pushed. Thanks!

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

On Wed, Jul 7, 2021 at 8:57 PM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
>
> Pushed. Thanks!

Thanks!