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Re: Spread checkpoint sync

From: Jeff Janes <jeff(dot)janes(at)gmail(dot)com>
To: Greg Smith <greg(at)2ndquadrant(dot)com>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Spread checkpoint sync
Date: 2010-11-30 22:56:46
Message-ID: AANLkTin5nDz41WE7DratHNj6V9XeuygLap5xaNrGj=89@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Sun, Nov 14, 2010 at 3:48 PM, Greg Smith <greg(at)2ndquadrant(dot)com> wrote:

...

> One change that turned out be necessary rather than optional--to get good
> performance from the system under tuning--was to make regular background
> writer activity, including fsync absorb checks, happen during these sync
> pauses.  The existing code ran the checkpoint sync work in a pretty tight
> loop, which as I alluded to in an earlier patch today can lead to the
> backends competing with the background writer to get their sync calls
> executed.  This squashes that problem if the background writer is setup
> properly.

Have you tested out this "absorb during syncing phase" code without
the sleep between the syncs?
I.e. so that it still a tight loop, but the loop alternates between
sync and absorb, with no intentional pause?

I wonder if all the improvement you see might not be due entirely to
the absorb between syncs, and none or very little from
the sleep itself.

I ask because I don't have a mental model of how the pause can help.
Given that this dirty data has been hanging around for many minutes
already, what is a 3 second pause going to heal?

The healing power of clearing out the absorb queue seems much more obvious.

Cheers,

Jeff

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