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Re: Group commit, revised

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: Peter Geoghegan <peter(at)2ndquadrant(dot)com>, PG Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Group commit, revised
Date: 2012-01-18 01:23:23
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Lists: pgsql-hackers
On Tue, Jan 17, 2012 at 12:37 PM, Heikki Linnakangas
<heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
> I found it very helpful to reduce wal_writer_delay in pgbench tests, when
> running with synchronous_commit=off. The reason is that hint bits don't get
> set until the commit record is flushed to disk, so making the flushes more
> frequent reduces the contention on the clog. However, Simon made async
> commits nudge WAL writer if the WAL page fills up, so I'm not sure how
> relevant that experience is anymore.

There's still a small but measurable effect there in master.  I think
we might be able to make it fully auto-tuning, but I don't think we're
fully there yet (not sure how much this patch changes that equation).

I suggested a design similar to the one you just proposed to Simon
when he originally suggested this feature.  It seems that if the WAL
writer is the only one doing WAL flushes, then there must be some IPC
overhead - and context switching - involved whenever WAL is flushed.
But clearly we're saving something somewhere else, on the basis of
Peter's results, so maybe it's not worth worrying about.  It does seem
pretty odd to have all the regular backends going through the WAL
writer and the auxiliary processes using a different mechanism,
though.  If we got rid of that, maybe WAL writer wouldn't even require
a lock, if there's only one process that can be doing it at a time.

What happens in standalone mode?

Robert Haas
The Enterprise PostgreSQL Company

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