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Re: Group commit, revised

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Peter Geoghegan <peter(at)2ndquadrant(dot)com>, PG Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Group commit, revised
Date: 2012-01-31 08:02:24
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Lists: pgsql-hackers
On Tue, Jan 31, 2012 at 7:43 AM, Heikki Linnakangas
<heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:

> That seems like a pretty marginal gain. If you're bound by the speed of
> fsyncs, this will reduce the latency by the time it takes to mark the clog,
> which is tiny in comparison to all the other stuff that needs to happen,
> like, flushing the WAL. And that's ignoring any additional overhead caused
> by the signaling between processes. If you're bound by CPU capacity, this
> doesn't help at all because it just moves the work around.

We're not bound by CPU capacity. Latency is an issue, especially when
contention drives it higher with occasional spikes.

I expect this to have a good measurable impact, as well as a
stabilising effect on the latency.

> Anyway, this is quite different from the original goal and patch for group
> commit, so can we please leave this for 9.3, and move on with the review of
> pending 9.2 patches.

Actually, there is very little change here from the original patch.

But I would note that your own changes were also quite different, and
had no noticeable gain. They were also based on a brand new and
radical set of thoughts.

 Simon Riggs         
 PostgreSQL Development, 24x7 Support, Training & Services

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