Re: autovac issue with large number of tables - Mailing list pgsql-hackers

Hi,

On Thu, Dec 3, 2020 at 9:09 PM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
>
>
>
> On 2020/12/03 11:46, Kasahara Tatsuhito wrote:
> > On Wed, Dec 2, 2020 at 7:11 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >>
> >> On Wed, Dec 2, 2020 at 3:33 PM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
> >>>
> >>>
> >>>
> >>> On 2020/12/02 12:53, Masahiko Sawada wrote:
> >>>> On Tue, Dec 1, 2020 at 5:31 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >>>>>
> >>>>> On Tue, Dec 1, 2020 at 4:32 PM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>> On 2020/12/01 16:23, Masahiko Sawada wrote:
> >>>>>>> On Tue, Dec 1, 2020 at 1:48 PM Kasahara Tatsuhito
> >>>>>>> <kasahara.tatsuhito@gmail.com> wrote:
> >>>>>>>>
> >>>>>>>> Hi,
> >>>>>>>>
> >>>>>>>> On Mon, Nov 30, 2020 at 8:59 PM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>> On 2020/11/30 10:43, Masahiko Sawada wrote:
> >>>>>>>>>> On Sun, Nov 29, 2020 at 10:34 PM Kasahara Tatsuhito
> >>>>>>>>>> <kasahara.tatsuhito@gmail.com> wrote:
> >>>>>>>>>>>
> >>>>>>>>>>> Hi, Thanks for you comments.
> >>>>>>>>>>>
> >>>>>>>>>>> On Fri, Nov 27, 2020 at 9:51 PM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
> >>>>>>>>>>>>
> >>>>>>>>>>>>
> >>>>>>>>>>>>
> >>>>>>>>>>>> On 2020/11/27 18:38, Kasahara Tatsuhito wrote:
> >>>>>>>>>>>>> Hi,
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> On Fri, Nov 27, 2020 at 1:43 AM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>> On 2020/11/26 10:41, Kasahara Tatsuhito wrote:
> >>>>>>>>>>>>>>> On Wed, Nov 25, 2020 at 8:46 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>> On Wed, Nov 25, 2020 at 4:18 PM Kasahara Tatsuhito
> >>>>>>>>>>>>>>>> <kasahara.tatsuhito@gmail.com> wrote:
> >>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>> Hi,
> >>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>> On Wed, Nov 25, 2020 at 2:17 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>> On Fri, Sep 4, 2020 at 7:50 PM Kasahara Tatsuhito
> >>>>>>>>>>>>>>>>>> <kasahara.tatsuhito@gmail.com> wrote:
> >>>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>>> Hi,
> >>>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>>> On Wed, Sep 2, 2020 at 2:10 AM Kasahara Tatsuhito
> >>>>>>>>>>>>>>>>>>> <kasahara.tatsuhito@gmail.com> wrote:
> >>>>>>>>>>>>>>>>>>>>> I wonder if we could have table_recheck_autovac do two probes of the stats
> >>>>>>>>>>>>>>>>>>>>> data.  First probe the existing stats data, and if it shows the table to
> >>>>>>>>>>>>>>>>>>>>> be already vacuumed, return immediately.  If not, *then* force a stats
> >>>>>>>>>>>>>>>>>>>>> re-read, and check a second time.
> >>>>>>>>>>>>>>>>>>>> Does the above mean that the second and subsequent table_recheck_autovac()
> >>>>>>>>>>>>>>>>>>>> will be improved to first check using the previous refreshed statistics?
> >>>>>>>>>>>>>>>>>>>> I think that certainly works.
> >>>>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>>>> If that's correct, I'll try to create a patch for the PoC
> >>>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>>> I still don't know how to reproduce Jim's troubles, but I was able to reproduce
> >>>>>>>>>>>>>>>>>>> what was probably a very similar problem.
> >>>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>>> This problem seems to be more likely to occur in cases where you have
> >>>>>>>>>>>>>>>>>>> a large number of tables,
> >>>>>>>>>>>>>>>>>>> i.e., a large amount of stats, and many small tables need VACUUM at
> >>>>>>>>>>>>>>>>>>> the same time.
> >>>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>>> So I followed Tom's advice and created a patch for the PoC.
> >>>>>>>>>>>>>>>>>>> This patch will enable a flag in the table_recheck_autovac function to use
> >>>>>>>>>>>>>>>>>>> the existing stats next time if VACUUM (or ANALYZE) has already been done
> >>>>>>>>>>>>>>>>>>> by another worker on the check after the stats have been updated.
> >>>>>>>>>>>>>>>>>>> If the tables continue to require VACUUM after the refresh, then a refresh
> >>>>>>>>>>>>>>>>>>> will be required instead of using the existing statistics.
> >>>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>>> I did simple test with HEAD and HEAD + this PoC patch.
> >>>>>>>>>>>>>>>>>>> The tests were conducted in two cases.
> >>>>>>>>>>>>>>>>>>> (I changed few configurations. see attached scripts)
> >>>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>>> 1. Normal VACUUM case
> >>>>>>>>>>>>>>>>>>>         - SET autovacuum = off
> >>>>>>>>>>>>>>>>>>>         - CREATE tables with 100 rows
> >>>>>>>>>>>>>>>>>>>         - DELETE 90 rows for each tables
> >>>>>>>>>>>>>>>>>>>         - SET autovacuum = on and restart PostgreSQL
> >>>>>>>>>>>>>>>>>>>         - Measure the time it takes for all tables to be VACUUMed
> >>>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>>> 2. Anti wrap round VACUUM case
> >>>>>>>>>>>>>>>>>>>         - CREATE brank tables
> >>>>>>>>>>>>>>>>>>>         - SELECT all of these tables (for generate stats)
> >>>>>>>>>>>>>>>>>>>         - SET autovacuum_freeze_max_age to low values and restart PostgreSQL
> >>>>>>>>>>>>>>>>>>>         - Consumes a lot of XIDs by using txid_curent()
> >>>>>>>>>>>>>>>>>>>         - Measure the time it takes for all tables to be VACUUMed
> >>>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>>> For each test case, the following results were obtained by changing
> >>>>>>>>>>>>>>>>>>> autovacuum_max_workers parameters to 1, 2, 3(def) 5 and 10.
> >>>>>>>>>>>>>>>>>>> Also changing num of tables to 1000, 5000, 10000 and 20000.
> >>>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>>> Due to the poor VM environment (2 VCPU/4 GB), the results are a little unstable,
> >>>>>>>>>>>>>>>>>>> but I think it's enough to ask for a trend.
> >>>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>>> ===========================================================================
> >>>>>>>>>>>>>>>>>>> [1.Normal VACUUM case]
> >>>>>>>>>>>>>>>>>>>        tables:1000
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 1:   (HEAD) 20 sec VS (with patch)  20 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 2:   (HEAD) 18 sec VS (with patch)  16 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 3:   (HEAD) 18 sec VS (with patch)  16 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 5:   (HEAD) 19 sec VS (with patch)  17 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 10:  (HEAD) 19 sec VS (with patch)  17 sec
> >>>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>>>        tables:5000
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 1:   (HEAD) 77 sec VS (with patch)  78 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 2:   (HEAD) 61 sec VS (with patch)  43 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 3:   (HEAD) 38 sec VS (with patch)  38 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 5:   (HEAD) 45 sec VS (with patch)  37 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 10:  (HEAD) 43 sec VS (with patch)  35 sec
> >>>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>>>        tables:10000
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 1:   (HEAD) 152 sec VS (with patch)  153 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 2:   (HEAD) 119 sec VS (with patch)   98 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 3:   (HEAD)  87 sec VS (with patch)   78 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 5:   (HEAD) 100 sec VS (with patch)   66 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 10:  (HEAD)  97 sec VS (with patch)   56 sec
> >>>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>>>        tables:20000
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 1:   (HEAD) 338 sec VS (with patch)  339 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 2:   (HEAD) 231 sec VS (with patch)  229 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 3:   (HEAD) 220 sec VS (with patch)  191 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 5:   (HEAD) 234 sec VS (with patch)  147 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 10:  (HEAD) 320 sec VS (with patch)  113 sec
> >>>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>>> [2.Anti wrap round VACUUM case]
> >>>>>>>>>>>>>>>>>>>        tables:1000
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 1:   (HEAD) 19 sec VS (with patch) 18 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 2:   (HEAD) 14 sec VS (with patch) 15 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 3:   (HEAD) 14 sec VS (with patch) 14 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 5:   (HEAD) 14 sec VS (with patch) 16 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 10:  (HEAD) 16 sec VS (with patch) 14 sec
> >>>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>>>        tables:5000
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 1:   (HEAD) 69 sec VS (with patch) 69 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 2:   (HEAD) 66 sec VS (with patch) 47 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 3:   (HEAD) 59 sec VS (with patch) 37 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 5:   (HEAD) 39 sec VS (with patch) 28 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 10:  (HEAD) 39 sec VS (with patch) 29 sec
> >>>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>>>        tables:10000
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 1:   (HEAD) 139 sec VS (with patch) 138 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 2:   (HEAD) 130 sec VS (with patch)  86 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 3:   (HEAD) 120 sec VS (with patch)  68 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 5:   (HEAD)  96 sec VS (with patch)  41 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 10:  (HEAD)  90 sec VS (with patch)  39 sec
> >>>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>>>        tables:20000
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 1:   (HEAD) 313 sec VS (with patch) 331 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 2:   (HEAD) 209 sec VS (with patch) 201 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 3:   (HEAD) 227 sec VS (with patch) 141 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 5:   (HEAD) 236 sec VS (with patch)  88 sec
> >>>>>>>>>>>>>>>>>>>         autovacuum_max_workers 10:  (HEAD) 309 sec VS (with patch)  74 sec
> >>>>>>>>>>>>>>>>>>> ===========================================================================
> >>>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>>> The cases without patch, the scalability of the worker has decreased
> >>>>>>>>>>>>>>>>>>> as the number of tables has increased.
> >>>>>>>>>>>>>>>>>>> In fact, the more workers there are, the longer it takes to complete
> >>>>>>>>>>>>>>>>>>> VACUUM to all tables.
> >>>>>>>>>>>>>>>>>>> The cases with patch, it shows good scalability with respect to the
> >>>>>>>>>>>>>>>>>>> number of workers.
> >>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>> It seems a good performance improvement even without the patch of
> >>>>>>>>>>>>>>>>>> shared memory based stats collector.
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>> Sounds great!
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>>> Note that perf top results showed that hash_search_with_hash_value,
> >>>>>>>>>>>>>>>>>>> hash_seq_search and
> >>>>>>>>>>>>>>>>>>> pgstat_read_statsfiles are dominant during VACUUM in all patterns,
> >>>>>>>>>>>>>>>>>>> with or without the patch.
> >>>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>>> Therefore, there is still a need to find ways to optimize the reading
> >>>>>>>>>>>>>>>>>>> of large amounts of stats.
> >>>>>>>>>>>>>>>>>>> However, this patch is effective in its own right, and since there are
> >>>>>>>>>>>>>>>>>>> only a few parts to modify,
> >>>>>>>>>>>>>>>>>>> I think it should be able to be applied to current (preferably
> >>>>>>>>>>>>>>>>>>> pre-v13) PostgreSQL.
> >>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>> +1
> >>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>> +
> >>>>>>>>>>>>>>>>>> +       /* We might be better to refresh stats */
> >>>>>>>>>>>>>>>>>> +       use_existing_stats = false;
> >>>>>>>>>>>>>>>>>>           }
> >>>>>>>>>>>>>>>>>> +   else
> >>>>>>>>>>>>>>>>>> +   {
> >>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>> -   heap_freetuple(classTup);
> >>>>>>>>>>>>>>>>>> +       heap_freetuple(classTup);
> >>>>>>>>>>>>>>>>>> +       /* The relid has already vacuumed, so we might be better to
> >>>>>>>>>>>>>>>>>> use exiting stats */
> >>>>>>>>>>>>>>>>>> +       use_existing_stats = true;
> >>>>>>>>>>>>>>>>>> +   }
> >>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>> With that patch, the autovacuum process refreshes the stats in the
> >>>>>>>>>>>>>>>>>> next check if it finds out that the table still needs to be vacuumed.
> >>>>>>>>>>>>>>>>>> But I guess it's not necessarily true because the next table might be
> >>>>>>>>>>>>>>>>>> vacuumed already. So I think we might want to always use the existing
> >>>>>>>>>>>>>>>>>> for the first check. What do you think?
> >>>>>>>>>>>>>>>>> Thanks for your comment.
> >>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>> If we assume the case where some workers vacuum on large tables
> >>>>>>>>>>>>>>>>> and a single worker vacuum on small tables, the processing
> >>>>>>>>>>>>>>>>> performance of the single worker will be slightly lower if the
> >>>>>>>>>>>>>>>>> existing statistics are checked every time.
> >>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>> In fact, at first I tried to check the existing stats every time,
> >>>>>>>>>>>>>>>>> but the performance was slightly worse in cases with a small number of workers.
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>> Do you have this benchmark result?
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>> (Checking the existing stats is lightweight , but at high frequency,
> >>>>>>>>>>>>>>>>>        it affects processing performance.)
> >>>>>>>>>>>>>>>>> Therefore, at after refresh statistics, determine whether autovac
> >>>>>>>>>>>>>>>>> should use the existing statistics.
> >>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>> Yeah, since the test you used uses a lot of small tables, if there are
> >>>>>>>>>>>>>>>> a few workers, checking the existing stats is unlikely to return true
> >>>>>>>>>>>>>>>> (no need to vacuum). So the cost of existing stats check ends up being
> >>>>>>>>>>>>>>>> overhead. Not sure how slow always checking the existing stats was,
> >>>>>>>>>>>>>>>> but given that the shared memory based stats collector patch could
> >>>>>>>>>>>>>>>> improve the performance of refreshing stats, it might be better not to
> >>>>>>>>>>>>>>>> check the existing stats frequently like the patch does. Anyway, I
> >>>>>>>>>>>>>>>> think it’s better to evaluate the performance improvement with other
> >>>>>>>>>>>>>>>> cases too.
> >>>>>>>>>>>>>>> Yeah, I would like to see how much the performance changes in other cases.
> >>>>>>>>>>>>>>> In addition, if the shared-based-stats patch is applied, we won't need to reload
> >>>>>>>>>>>>>>> a huge stats file, so we will just have to check the stats on
> >>>>>>>>>>>>>>> shared-mem every time.
> >>>>>>>>>>>>>>> Perhaps the logic of table_recheck_autovac could be simpler.
> >>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>> BTW, I found some typos in comments, so attache a  fixed version.
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>> The patch adds some duplicated codes into table_recheck_autovac().
> >>>>>>>>>>>>>> It's better to make the common function performing them and make
> >>>>>>>>>>>>>> table_recheck_autovac() call that common function, to simplify the code.
> >>>>>>>>>>>>> Thanks for your comment.
> >>>>>>>>>>>>> Hmm.. I've cut out the duplicate part.
> >>>>>>>>>>>>> Attach the patch.
> >>>>>>>>>>>>> Could you confirm that it fits your expecting?
> >>>>>>>>>>>>
> >>>>>>>>>>>> Yes, thanks for updataing the patch! Here are another review comments.
> >>>>>>>>>>>>
> >>>>>>>>>>>> +       shared = pgstat_fetch_stat_dbentry(InvalidOid);
> >>>>>>>>>>>> +       dbentry = pgstat_fetch_stat_dbentry(MyDatabaseId);
> >>>>>>>>>>>>
> >>>>>>>>>>>> When using the existing stats, ISTM that these are not necessary and
> >>>>>>>>>>>> we can reuse "shared" and "dbentry" obtained before. Right?
> >>>>>>>>>>> Yeah, but unless autovac_refresh_stats() is called, these functions
> >>>>>>>>>>> read the information from the
> >>>>>>>>>>> local hash table without re-read the stats file, so the process is very light.
> >>>>>>>>>>> Therefore, I think, it is better to keep the current logic to keep the
> >>>>>>>>>>> code simple.
> >>>>>>>>>>>
> >>>>>>>>>>>>
> >>>>>>>>>>>> +               /* We might be better to refresh stats */
> >>>>>>>>>>>> +               use_existing_stats = false;
> >>>>>>>>>>>>
> >>>>>>>>>>>> I think that we should add more comments about why it's better to
> >>>>>>>>>>>> refresh the stats in this case.
> >>>>>>>>>>>>
> >>>>>>>>>>>> +               /* The relid has already vacuumed, so we might be better to use existing stats */
> >>>>>>>>>>>> +               use_existing_stats = true;
> >>>>>>>>>>>>
> >>>>>>>>>>>> I think that we should add more comments about why it's better to
> >>>>>>>>>>>> reuse the stats in this case.
> >>>>>>>>>>> I added  comments.
> >>>>>>>>>>>
> >>>>>>>>>>> Attache the patch.
> >>>>>>>>>>>
> >>>>>>>>>>
> >>>>>>>>>> Thank you for updating the patch. Here are some small comments on the
> >>>>>>>>>> latest (v4) patch.
> >>>>>>>>>>
> >>>>>>>>>> +    * So if the last time we checked a table that was already vacuumed after
> >>>>>>>>>> +    * refres stats, check the current statistics before refreshing it.
> >>>>>>>>>> +    */
> >>>>>>>>>>
> >>>>>>>>>> s/refres/refresh/
> >>>>>>>> Thanks! fixed.
> >>>>>>>> Attached the patch.
> >>>>>>>>
> >>>>>>>>>>
> >>>>>>>>>> -----
> >>>>>>>>>> +/* Counter to determine if statistics should be refreshed */
> >>>>>>>>>> +static bool use_existing_stats = false;
> >>>>>>>>>> +
> >>>>>>>>>>
> >>>>>>>>>> I think 'use_existing_stats' can be declared within table_recheck_autovac().
> >>>>>>>>>>
> >>>>>>>>>> -----
> >>>>>>>>>> While testing the performance, I realized that the statistics are
> >>>>>>>>>> reset every time vacuumed one table, leading to re-reading the stats
> >>>>>>>>>> file even if 'use_existing_stats' is true. Please refer that vacuum()
> >>>>>>>>>> eventually calls AtEOXact_PgStat() which calls to
> >>>>>>>>>> pgstat_clear_snapshot().
> >>>>>>>>>
> >>>>>>>>> Good catch!
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>>> I believe that's why the performance of the
> >>>>>>>>>> method of always checking the existing stats wasn’t good as expected.
> >>>>>>>>>> So if we save the statistics somewhere and use it for rechecking, the
> >>>>>>>>>> results of the performance benchmark will differ between these two
> >>>>>>>>>> methods.
> >>>>>>>> Thanks for you checks.
> >>>>>>>> But, if a worker did vacuum(), that means this worker had determined
> >>>>>>>> need vacuum in the
> >>>>>>>> table_recheck_autovac(). So, use_existing_stats set to false, and next
> >>>>>>>> time, refresh stats.
> >>>>>>>> Therefore I think the current patch is fine, as we want to avoid
> >>>>>>>> unnecessary refreshing of
> >>>>>>>> statistics before the actual vacuum(), right?
> >>>>>>>
> >>>>>>> Yes, you're right.
> >>>>>>>
> >>>>>>> When I benchmarked the performance of the method of always checking
> >>>>>>> existing stats I edited your patch so that it sets use_existing_stats
> >>>>>>> = true even if the second check is false (i.g., vacuum is needed).
> >>>>>>> And the result I got was worse than expected especially in the case of
> >>>>>>> a few autovacuum workers. But it doesn't evaluate the performance of
> >>>>>>> that method rightly as the stats snapshot is cleared every time
> >>>>>>> vacuum. Given you had similar results, I guess you used a similar way
> >>>>>>> when evaluating it, is it right? If so, it’s better to fix this issue
> >>>>>>> and see how the performance benchmark results will differ.
> >>>>>>>
> >>>>>>> For example, the results of the test case with 10000 tables and 1
> >>>>>>> autovacuum worker I reported before was:
> >>>>>>>
> >>>>>>> 10000 tables:
> >>>>>>>       autovac_workers 1  : 158s,157s, 290s
> >>>>>>>
> >>>>>>> But after fixing that issue in the third method (always checking the
> >>>>>>> existing stats), the results are:
> >>>>>>
> >>>>>> Could you tell me how you fixed that issue? You copied the stats to
> >>>>>> somewhere as you suggested or skipped pgstat_clear_snapshot() as
> >>>>>> I suggested?
> >>>>>
> >>>>> I used the way you suggested in this quick test; skipped
> >>>>> pgstat_clear_snapshot().
> >>>>>
> >>>>>>
> >>>>>> Kasahara-san seems not to like the latter idea because it might
> >>>>>> cause bad side effect. So we should use the former idea?
> >>>>>
> >>>>> Not sure. I'm also concerned about the side effect but I've not checked yet.
> >>>>>
> >>>>> Since probably there is no big difference between the two ways in
> >>>>> terms of performance I'm going to see how the performance benchmark
> >>>>> result will change first.
> >>>>
> >>>> I've tested performance improvement again. From the left the execution
> >>>> time of the current HEAD, Kasahara-san's patch, the method of always
> >>>> checking the existing stats (using approach suggested by Fujii-san),
> >>>> in seconds.
> >>>>
> >>>> 1000 tables:
> >>>>      autovac_workers 1  : 13s, 13s, 13s
> >>>>      autovac_workers 2  : 6s, 4s, 4s
> >>>>      autovac_workers 3  : 3s, 4s, 3s
> >>>>      autovac_workers 5  : 3s, 3s, 2s
> >>>>      autovac_workers 10: 2s, 3s, 2s
> >>>>
> >>>> 5000 tables:
> >>>>      autovac_workers 1  : 71s, 71s, 72s
> >>>>      autovac_workers 2  : 37s, 32s, 32s
> >>>>      autovac_workers 3  : 29s, 26s, 26s
> >>>>      autovac_workers 5  : 20s, 19s, 18s
> >>>>      autovac_workers 10: 13s, 8s, 8s
> >>>>
> >>>> 10000 tables:
> >>>>      autovac_workers 1  : 158s,157s, 159s
> >>>>      autovac_workers 2  : 80s, 53s, 78s
> >>>>      autovac_workers 3  : 75s, 67s, 67s
> >>>>      autovac_workers 5  : 61s, 42s, 42s
> >>>>      autovac_workers 10: 69s, 26s, 25s
> >>>>
> >>>> 20000 tables:
> >>>>      autovac_workers 1  : 379s, 380s, 389s
> >>>>      autovac_workers 2  : 236s, 232s, 233s
> >>>>      autovac_workers 3  : 222s, 181s, 182s
> >>>>      autovac_workers 5  : 212s, 132s, 139s
> >>>>      autovac_workers 10: 317s, 91s, 89s
> >>>>
> >>>> I don't see a big difference between Kasahara-san's patch and the
> >>>> method of always checking the existing stats.
> >>>
> >>> Thanks for doing the benchmark!
> >>>
> >>> This benchmark result makes me think that we don't need to tweak
> >>> AtEOXact_PgStat() and can use Kasahara-san approach.
> >>> That's good news :)
> >>
> >> Yeah, given that all autovaucum workers have the list of tables to
> >> vacuum in the same order in most cases, the assumption in
> >> Kasahara-san’s patch that if a worker needs to vacuum a table it’s
> >> unlikely that it will be able to skip the next table using the current
> >> snapshot of stats makes sense to me.
> >>
> >> One small comment on v6 patch:
> >>
> >> + /* When we decide to do vacuum or analyze, the existing stats cannot
> >> + * be reused in the next cycle because it's cleared at the end of vacuum
> >> + * or analyze (by AtEOXact_PgStat()).
> >> + */
> >> + use_existing_stats = false;
> >>
> >> I think the comment should start on the second line (i.g., \n is
> >> needed after /*).
> > Oops, thanks.
> > Fixed.
>
> Thanks for updating the patch!
>
> I applied the following cosmetic changes to the patch.
> Attached is the updated version of the patch.
> Coud you review this version?
Thanks for tweaking the patch.

> - Ran pgindent to fix some warnings that "git diff --check"
>    reported on the patch.
> - Made the order of arguments consistent between
>    recheck_relation_needs_vacanalyze and relation_needs_vacanalyze.
> - Renamed the variable use_existing_stats to reuse_stats for simplicity.
> - Added more comments.
I think it's no problem.
The patch passed makecheck, and  I benchmarked "Anti wrap round VACUUM
case" (only 20000 tables) just in case.

From the left the execution time of the current HEAD, v8 patch.
tables 20000:
  autovac workers  1: 319sec, 315sec
  autovac workers  2: 301sec, 190sec
  autovac workers  3: 270sec, 133sec
  autovac workers  5: 211sec,  86sec
  autovac workers 10: 376sec,  68sec

It's as expected.

> Barring any objection, I'm thinking to commit this version.
+1

Best regards,

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



--
Tatsuhito Kasahara
kasahara.tatsuhito _at_ gmail.com