Re: [HACKERS] Block level parallel vacuum

On Tue, Oct 29, 2019 at 1:59 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> On Tue, Oct 29, 2019 at 4:06 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >
> > 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.
> >
>
> FWIW I'd like to share the results of total delay time evaluation of
> approach (a) (shared cost balance). I used the same workloads that
> Dilip shared and set vacuum_cost_delay to 10. The results of two test
> cases are here:
>
> * Test1
> normal      : 12656 ms (hit 50594, miss 5700, dirty 7258, total 63552)
> 2 workers : 17149 ms (hit 47673, miss 8647, dirty 9157, total 65477)
> 1 worker   : 19498 ms (hit 45954, miss 10340, dirty 10517, total 66811)
>
> * Test2
> normal      : 1530 ms (hit 30645, miss 2, dirty 3, total 30650)
> 2 workers : 1538 ms (hit 30645, miss 2, dirty 3, total 30650)
> 1 worker   : 1538 ms (hit 30645, miss 2, dirty 3, total 30650)
>
> 'hit', 'miss' and 'dirty' are the total numbers of buffer hits, buffer
> misses and flushing dirty buffer, respectively. 'total' is the sum of
> these three values.
>
> In this evaluation I expect that parallel vacuum cases delay time as
> much as the time of normal vacuum because the total number of pages to
> vacuum is the same and we have the shared cost balance value and each
> workers decide to sleep based on that value. According to the above
> Test1 results, we can see that there is a big difference in the total
> delay time among  these cases (normal vacuum case is shortest), but
> the cause of this is that parallel vacuum had to to flush more dirty
> pages. Actually after increased shared_buffer I got expected results:
>
> * Test1 (after increased shared_buffers)
> normal      : 2807 ms (hit 56295, miss 2, dirty 3, total 56300)
> 2 workers : 2840 ms (hit 56295, miss 2, dirty 3, total 56300)
> 1 worker   : 2841 ms (hit 56295, miss 2, dirty 3, total 56300)
>
> I updated the patch that computes the total cost delay shared by
> Dilip[1] so that it collects the number of buffer hits and so on, and
> have attached it. It can be applied on top of my latest patch set[1].

Thanks, Sawada-san.  In my next test, I will use this updated patch.

-- 
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com