Re: [HACKERS] Block level parallel vacuum

On Mon, Oct 28, 2019 at 2:13 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
> On Thu, Oct 24, 2019 at 4:33 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> >
> > On Thu, Oct 24, 2019 at 4:21 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > >
> > > On Thu, Oct 24, 2019 at 11:51 AM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> > > >
> > > > On Fri, Oct 18, 2019 at 12:18 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> > > > >
> > > > > On Fri, Oct 18, 2019 at 11:25 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > > > > >
> > > > > > I am thinking if we can write the patch for both the approaches (a.
> > > > > > compute shared costs and try to delay based on that, b. try to divide
> > > > > > the I/O cost among workers as described in the email above[1]) and do
> > > > > > some tests to see the behavior of throttling, that might help us in
> > > > > > deciding what is the best strategy to solve this problem, if any.
> > > > > > What do you think?
> > > > >
> > > > > I agree with this idea.  I can come up with a POC patch for approach
> > > > > (b).  Meanwhile, if someone is interested to quickly hack with the
> > > > > approach (a) then we can do some testing and compare.  Sawada-san,
> > > > > by any chance will you be interested to write POC with approach (a)?
> > > > > Otherwise, I will try to write it after finishing the first one
> > > > > (approach b).
> > > > >
> > > > I have come up with the POC for approach (a).
>
> > > Can we compute the overall throttling (sleep time) in the operation
> > > separately for heap and index, then divide the index's sleep_time with
> > > a number of workers and add it to heap's sleep time?  Then, it will be
> > > a bit easier to compare the data between parallel and non-parallel
> > > case.
> I have come up with a patch to compute the total delay during the
> vacuum.  So the idea of computing the total cost delay is
>
> Total cost delay = Total dealy of heap scan + Total dealy of
> index/worker;  Patch is attached for the same.
>
> I have prepared this patch on the latest patch of the parallel
> vacuum[1].  I have also rebased the patch for the approach [b] for
> dividing the vacuum cost limit and done some testing for computing the
> I/O throttling.  Attached patches 0001-POC-compute-total-cost-delay
> and 0002-POC-divide-vacuum-cost-limit can be applied on top of
> v31-0005-Add-paralell-P-option-to-vacuumdb-command.patch.  I haven't
> rebased on top of v31-0006, because v31-0006 is implementing the I/O
> throttling with one approach and 0002-POC-divide-vacuum-cost-limit is
> doing the same with another approach.   But,
> 0001-POC-compute-total-cost-delay can be applied on top of v31-0006 as
> well (just 1-2 lines conflict).
>
> Testing:  I have performed 2 tests, one with the same size indexes and
> second with the different size indexes and measured total I/O delay
> with the attached patch.
>
> Setup:
> VacuumCostDelay=10ms
> VacuumCostLimit=2000
>
> Test1 (Same size index):
> create table test(a int, b varchar, c varchar);
> create index idx1 on test(a);
> create index idx2 on test(b);
> create index idx3 on test(c);
> insert into test select i, repeat('a',30)||i, repeat('a',20)||i from
> generate_series(1,500000) as i;
> delete from test where a < 200000;
>
>                       Vacuum (Head)                   Parallel Vacuum
>            Vacuum Cost Divide Patch
> Total Delay        1784 (ms)                           1398(ms)
>                  1938(ms)
>
>
> Test2 (Variable size dead tuple in index)
> create table test(a int, b varchar, c varchar);
> create index idx1 on test(a);
> create index idx2 on test(b) where a > 100000;
> create index idx3 on test(c) where a > 150000;
>
> insert into test select i, repeat('a',30)||i, repeat('a',20)||i from
> generate_series(1,500000) as i;
> delete from test where a < 200000;
>
> Vacuum (Head)                                   Parallel Vacuum
>               Vacuum Cost Divide Patch
> Total Delay 1438 (ms)                               1029(ms)
>                    1529(ms)
>
>
> Conclusion:
> 1. The tests prove that the total I/O delay is significantly less with
> the parallel vacuum.
> 2. With the vacuum cost divide the problem is solved but the delay bit
> more compared to the non-parallel version.  The reason could be the
> problem discussed at[2], but it needs further investigation.
>
> Next, I will test with the v31-0006 (shared vacuum cost) patch.  I
> will also try to test different types of indexes.
>

Thank you for testing!

I realized that v31-0006 patch doesn't work fine so I've attached the
updated version patch that also incorporated some comments I got so
far. Sorry for the inconvenience. I'll apply your 0001 patch and also
test the total delay time.

Regards,

--
Masahiko Sawada