Обсуждение: pgbench tps drop from 5000 to 37 going from localhost to a server 13ms away

Hi all,

I've been doing some lightweight load testing with
“pgbench -c8 -j8 -T10”

When run locally on the postgres server I've testing, this gives around
5000tps

When I do it from a server that has a 13ms ping latency, it drops to 37tps.

This is using the default pgbench script, is it to be expected?
If so, why?

cheers,

Chris

That seems to be a large drop. On the other hand 13 ms is also like a very large network latency. On LAN your usually
inthe sub ms area. So going from e.g. 0.2 ms to 13ms is 65 fold decrease. What is the network toplogy like? 

Jan

Von meinem iPad gesendet

> Am 24.07.2015 um 18:59 schrieb Chris Withers <chris@simplistix.co.uk>:
>
> Hi all,
>
> I've been doing some lightweight load testing with
> “pgbench -c8 -j8 -T10”
>
> When run locally on the postgres server I've testing, this gives around 5000tps
>
> When I do it from a server that has a 13ms ping latency, it drops to 37tps.
>
> This is using the default pgbench script, is it to be expected?
> If so, why?
>
> cheers,
>
> Chris
>
>
> --
> Sent via pgsql-general mailing list (pgsql-general@postgresql.org)
> To make changes to your subscription:
> http://www.postgresql.org/mailpref/pgsql-general

Am 24.07.2015 um 18:59 schrieb Chris Withers <chris@simplistix.co.uk>:

Hi all,

I've been doing some lightweight load testing with

“pgbench -c8 -j8 -T10”

When run locally on the postgres server I've testing, this gives around 5000tps

When I do it from a server that has a 13ms ping latency, it drops to 37tps.

This is using the default pgbench script, is it to be expected?

If so, why?

On 24/07/2015 19:21, Jan Lentfer wrote:
>
>>> I've been doing some lightweight load testing with
>>> “pgbench -c8 -j8 -T10”
>>>
>>> When run locally on the postgres server I've testing, this gives
>>> around 5000tps
>>>
>>> When I do it from a server that has a 13ms ping latency, it drops to
>>> 37tps.
>>>
>>> This is using the default pgbench script, is it to be expected?
>>> If so, why?
>>>
>
>
>> That seems to be a large drop. On the other hand 13 ms is also like a
>> very large network latency. On LAN your usually in the sub ms area. So
>> going from e.g. 0.2 ms to 13ms is 65 fold decrease. What is the
>> network toplogy like?

10G between two colos, a switch at each end.

What's interesting is how it goes when adjusting the number for
threads/connections. As a baseline, I did one of each. As expected, this
gave around 10 tps, (7 statements in the standard file * 13ms latency
gives around 0.1s per transaction). This behaviour continued up to -c3
-j3 linearly achieving roughly 10tps per client/thread:

starting vacuum...end.
transaction type: TPC-B (sort of)
scaling factor: 1
query mode: simple
number of clients: 3
number of threads: 3
duration: 10 s
number of transactions actually processed: 317
latency average: 94.637 ms
tps = 31.494417 (including connections establishing)
tps = 31.668794 (excluding connections establishing)
statement latencies in milliseconds:
0.002016 \set nbranches 1 * :scale
0.000438 \set ntellers 10 * :scale
0.000379 \set naccounts 100000 * :scale
0.000489 \setrandom aid 1 :naccounts
0.000404 \setrandom bid 1 :nbranches
0.000413 \setrandom tid 1 :ntellers
0.000470 \setrandom delta -5000 5000
13.061975 BEGIN;
13.174287 UPDATE pgbench_accounts SET abalance = abalance + :delta WHERE
aid = :aid;
13.127691 SELECT abalance FROM pgbench_accounts WHERE aid = :aid;
14.552413 UPDATE pgbench_tellers SET tbalance = tbalance + :delta WHERE
tid = :tid;
14.109375 UPDATE pgbench_branches SET bbalance = bbalance + :delta WHERE
bid = :bid;
13.113028 INSERT INTO pgbench_history (tid, bid, aid, delta, mtime)
VALUES (:tid, :bid, :aid, :delta, CURRENT_TIMESTAMP);
13.256063 END;

Now, at -c4 and -j4 the problem becomes apparent:

starting vacuum...end.
transaction type: TPC-B (sort of)
scaling factor: 1
query mode: simple
number of clients: 4
number of threads: 4
duration: 10 s
number of transactions actually processed: 362
latency average: 110.497 ms
tps = 35.912951 (including connections establishing)
tps = 36.111849 (excluding connections establishing)
statement latencies in milliseconds:
0.001917 \set nbranches 1 * :scale
0.000511 \set ntellers 10 * :scale
0.000384 \set naccounts 100000 * :scale
0.000525 \setrandom aid 1 :naccounts
0.000406 \setrandom bid 1 :nbranches
0.000472 \setrandom tid 1 :ntellers
0.000483 \setrandom delta -5000 5000
13.063624 BEGIN;
13.170928 UPDATE pgbench_accounts SET abalance = abalance + :delta WHERE
aid = :aid;
13.122450 SELECT abalance FROM pgbench_accounts WHERE aid = :aid;
16.532138 UPDATE pgbench_tellers SET tbalance = tbalance + :delta WHERE
tid = :tid;
28.090450 UPDATE pgbench_branches SET bbalance = bbalance + :delta WHERE
bid = :bid;
13.112207 INSERT INTO pgbench_history (tid, bid, aid, delta, mtime)
VALUES (:tid, :bid, :aid, :delta, CURRENT_TIMESTAMP);
13.229887 END;

We've dropped down to 9 tps per client/thread, and it's the update
statements that are growing in time, here's the worst case:

starting vacuum...end.
transaction type: TPC-B (sort of)
scaling factor: 1
query mode: simple
number of clients: 10
number of threads: 10
duration: 10 s
number of transactions actually processed: 382
latency average: 261.780 ms
tps = 37.310918 (including connections establishing)
tps = 37.517192 (excluding connections establishing)
statement latencies in milliseconds:
0.001798 \set nbranches 1 * :scale
0.000437 \set ntellers 10 * :scale
0.000385 \set naccounts 100000 * :scale
0.000597 \setrandom aid 1 :naccounts
0.000369 \setrandom bid 1 :nbranches
0.000437 \setrandom tid 1 :ntellers
0.000401 \setrandom delta -5000 5000
13.064963 BEGIN;
13.192241 UPDATE pgbench_accounts SET abalance = abalance + :delta WHERE
aid = :aid;
13.121914 SELECT abalance FROM pgbench_accounts WHERE aid = :aid;
83.994516 UPDATE pgbench_tellers SET tbalance = tbalance + :delta WHERE
tid = :tid;
113.638228 UPDATE pgbench_branches SET bbalance = bbalance + :delta
WHERE bid = :bid;
13.133390 INSERT INTO pgbench_history (tid, bid, aid, delta, mtime)
VALUES (:tid, :bid, :aid, :delta, CURRENT_TIMESTAMP);
13.288079 END;

What on earth could be causing those updates to now take getting on for
an order of magnitude more than the latency to the server?

cheers,

Chris

starting vacuum...end.

transaction type: TPC-B (sort of)
scaling factor: 1

This is your problem.  There is only one row in the pgbench_branch table, and every transaction has to update that one row.  This is inherently a seriaized event.

One solution is to just use a large scale on the benchmark so that they upate random pgbench_branch rows, rather than all updating the same row:

pgbench -i -s50

Alternatively, you could write a custom file so that all 7 commands are sent down in one packet.