Re: new vacuum is slower for small tables

2008/12/8 Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>:
> Pavel Stehule wrote:
>>
>> I did small tests and I found so for small tables (less 1000 rows)
>> VACUUM based on visibility maps are slower than old implementation
>>
>> it is about 5ms X 20ms
>
> How did you measure that?

it's simple test

create table x(a integer,  b integer);
insert into x select i, i from generate_series(1,1000) g(i);

and then vacuum on 8.3.5 and vacuum on current CVS HEAD.

both cases are reading from cache.

>
> I tried to reproduce that here, and indeed it seems to be much slower on CVS
> HEAD than on PG 8.3. I tried to short-circuit the vacuum completely, making
> it a no-op:
>
> diff --git a/src/backend/commands/vacuumlazy.c
> b/src/backend/commands/vacuumlazy
> index 475c38a..c31917d 100644
> --- a/src/backend/commands/vacuumlazy.c
> +++ b/src/backend/commands/vacuumlazy.c
> @@ -275,6 +275,7 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
>
>        lazy_space_alloc(vacrelstats, nblocks);
>
> +       nblocks = 0;
>        for (blkno = 0; blkno < nblocks; blkno++)
>        {
>                Buffer          buf;
>
> but that made no difference at all; vacuuming a one page table on CVS HEAD
> with that hack is still slower than PG 8.3 without that hack. Which suggests
> that the slowdown is not related to visibility map.
>
> Oprofile suggests that most of the time is actually spent in
> pgstat_vacuum_stat. And more precisely in pstat_collect_oids, which is
> called by pgstat_vacuum_stat.
>
> We added support for tracking call counts and elapsed runtime of
> user-defined functions back in May. That added the code to
> pgstat_vacuum_stat to tell stats collector about deleted functions, which
> involves populating a hash table of all functions in the database. It looks
> like *that's* what's causing the slowdown.
>
> I think we can live with the extra overhead.
>
> --
>  Heikki Linnakangas
>  EnterpriseDB   http://www.enterprisedb.com
>