| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Andres Freund <andres(at)2ndquadrant(dot)com> |
| Cc: | Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>, Claudio Freire <klaussfreire(at)gmail(dot)com>, Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr>, Josh Berkus <josh(at)agliodbs(dot)com>, PostgreSQL Developers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: postgresql latency & bgwriter not doing its job |
| Date: | 2014-08-30 17:50:40 |
| Message-ID: | 6599.1409421040@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Andres Freund <andres(at)2ndquadrant(dot)com> writes:
> On 2014-08-27 19:23:04 +0300, Heikki Linnakangas wrote:
>> A long time ago, Itagaki Takahiro wrote a patch sort the buffers and write
>> them out in order (http://www.postgresql.org/message-id/flat/20070614153758(dot)6A62(dot)ITAGAKI(dot)TAKAHIRO(at)oss(dot)ntt(dot)co(dot)jp).
>> The performance impact of that was inconclusive, but one thing that it
>> allows nicely is to interleave the fsyncs, so that you write all the buffers
>> for one file, then fsync it, then next file and so on.
> ...
> So, *very* clearly sorting is a benefit.
pg_bench alone doesn't convince me on this. The original thread found
cases where it was a loss, IIRC; you will need to test many more than
one scenario to prove the point.
Also, it does not matter how good it looks in test cases if it causes
outright failures due to OOM; unlike you, I am not prepared to just "wave
away" that risk. A possible compromise is to sort a limited number of
buffers ---- say, collect a few thousand dirty buffers then sort, dump and
fsync them, repeat as needed.
regards, tom lane
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