From: | Ranier Vilela <ranier(dot)vf(at)gmail(dot)com> |
---|---|
To: | Thomas Munro <thomas(dot)munro(at)gmail(dot)com> |
Cc: | Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Parallel Seq Scan vs kernel read ahead |
Date: | 2020-05-20 23:14:12 |
Message-ID: | CAEudQApf+Q0kde4MVZJG6piJEezvDOc-g4uVMT2cTc8qzg9spg@mail.gmail.com |
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Lists: | pgsql-hackers |
Em qua., 20 de mai. de 2020 às 18:49, Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
escreveu:
> On Wed, May 20, 2020 at 11:03 PM Ranier Vilela <ranier(dot)vf(at)gmail(dot)com>
> wrote:
> > Time: 47767,916 ms (00:47,768)
> > Time: 32645,448 ms (00:32,645)
>
> Just to make sure kernel caching isn't helping here, maybe try making
> the table 2x or 4x bigger? My test was on a virtual machine with only
> 4GB RAM, so the table couldn't be entirely cached.
>
4x bigger.
Postgres defaults settings.
postgres=# create table t as select generate_series(1, 800000000)::int i;
SELECT 800000000
postgres=# \timing
Timing is on.
postgres=# set max_parallel_workers_per_gather = 0;
SET
Time: 8,622 ms
postgres=# select count(*) from t;
count
-----------
800000000
(1 row)
Time: 227238,445 ms (03:47,238)
postgres=# set max_parallel_workers_per_gather = 1;
SET
Time: 20,975 ms
postgres=# select count(*) from t;
count
-----------
800000000
(1 row)
Time: 138027,351 ms (02:18,027)
regards,
Ranier Vilela
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