Re: pg_stat_statements issue with parallel maintenance (Was Re: WAL usage calculation patch)

On Sun, 29 Mar 2020 at 20:44, Masahiko Sawada
<masahiko.sawada@2ndquadrant.com> wrote:
>
> On Sun, 29 Mar 2020 at 20:15, Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > On Sun, Mar 29, 2020 at 1:44 PM Julien Rouhaud <rjuju123@gmail.com> wrote:
> > >
> > > On Sun, Mar 29, 2020 at 9:52 AM Masahiko Sawada
> > > <masahiko.sawada@2ndquadrant.com> wrote:
> > > >
> > > > I've run vacuum with/without parallel workers on the table having 5
> > > > indexes. The vacuum reads all blocks of table and indexes.
> > > >
> > > > * VACUUM command with no parallel workers
> > > > =# select total_time, shared_blks_hit, shared_blks_read,
> > > > shared_blks_hit + shared_blks_read as total_read_blks,
> > > > shared_blks_dirtied, shared_blks_written from pg_stat_statements where
> > > > query ~ 'vacuum';
> > > >
> > > >   total_time  | shared_blks_hit | shared_blks_read | total_read_blks |
> > > > shared_blks_dirtied | shared_blks_written
> > > > --------------+-----------------+------------------+-----------------+---------------------+---------------------
> > > >  19857.217207 |           45238 |           226944 |          272182 |
> > > >              225943 |              225894
> > > > (1 row)
> > > >
> > > > * VACUUM command with 4 parallel workers
> > > > =# select total_time, shared_blks_hit, shared_blks_read,
> > > > shared_blks_hit + shared_blks_read as total_read_blks,
> > > > shared_blks_dirtied, shared_blks_written from pg_stat_statements where
> > > > query ~ 'vacuum';
> > > >
> > > >  total_time  | shared_blks_hit | shared_blks_read | total_read_blks |
> > > > shared_blks_dirtied | shared_blks_written
> > > > -------------+-----------------+------------------+-----------------+---------------------+---------------------
> > > >  6932.117365 |           45205 |            73079 |          118284 |
> > > >              72403 |               72365
> > > > (1 row)
> > > >
> > > > The total number of blocks of table and indexes are about 182243
> > > > blocks. As Julien reported, obviously the total number of read blocks
> > > > during parallel vacuum is much less than single process vacuum's
> > > > result.
> > > >
> > > > Parallel create index has the same issue but it doesn't exist in
> > > > parallel queries for SELECTs.
> > > >
> > > > I think we need to change parallel maintenance commands so that they
> > > > report buffer usage like what ParallelQueryMain() does; prepare to
> > > > track buffer usage during query execution by
> > > > InstrStartParallelQuery(), and report it by InstrEndParallelQuery()
> > > > after parallel maintenance command. To report buffer usage of parallel
> > > > maintenance command correctly, I'm thinking that we can (1) change
> > > > parallel create index and parallel vacuum so that they prepare
> > > > gathering buffer usage, or (2) have a common entry point for parallel
> > > > maintenance commands that is responsible for gathering buffer usage
> > > > and calling the entry functions for individual maintenance command.
> > > > I'll investigate it more in depth.
> > >
> > > As I just mentioned, (2) seems like a better design as it's quite
> > > likely that the number of parallel-aware utilities will probably
> > > continue to increase.  One problem also is that parallel CREATE INDEX
> > > has been introduced in pg11, so (2) probably won't be packpatchable
> > > (and (1) seems problematic too).
> > >
> >
> > I am not sure if we can decide at this stage whether it is
> > back-patchable or not.  Let's first see the patch and if it turns out
> > to be complex, then we can try to do some straight-forward fix for
> > back-branches.
>
> Agreed.
>
> > In general, I don't see why the fix here should be
> > complex?
>
> Yeah, particularly the approach (1) will not be complex. I'll write a
> patch tomorrow.
>

I've attached two patches fixing this issue for parallel index
creation and parallel vacuum. These approaches take the same approach;
we allocate DSM to share buffer usage and the leader gathers them,
described as approach (1) above. I think this is a straightforward
approach for this issue. We can create a common entry point for
parallel maintenance command that is responsible for gathering buffer
usage as well as sharing query text etc. But it will accompany
relatively big change and it might be overkill at this stage. We can
discuss that and it will become an item for PG14.

Regards,

-- 
Masahiko Sawada            http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services