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Re: Spread checkpoint sync

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Greg Smith <greg(at)2ndquadrant(dot)com>
Cc: Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, Ron Mayer <rm_pg(at)cheapcomplexdevices(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Spread checkpoint sync
Date: 2011-01-17 03:37:33
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Lists: pgsql-hackers
On Sun, Jan 16, 2011 at 10:13 PM, Greg Smith <greg(at)2ndquadrant(dot)com> wrote:
> I have finished a first run of benchmarking the current 9.1 code at various
> sizes.  See for many
> details.  The interesting stuff is in Test Set 3, near the bottom.  That's
> the first one that includes buffer_backend_fsync data.  This iall on ext3 so
> far, but is using a newer 2.6.32 kernel, the one from Ubuntu 10.04.
> The results are classic Linux in 2010:  latency pauses from checkpoint sync
> will easily leave the system at a dead halt for a minute, with the worst one
> observed this time dropping still for 108 seconds.

I wish I understood better what makes Linux systems "freeze up" under
heavy I/O load.  Linux - like other UNIX-like systems - generally has
reasonably effective mechanisms for preventing a single task from
monopolizing the (or a) CPU in the presence of other processes that
also wish to be time-sliced, but the same thing doesn't appear to be
true of I/O.

> I think a helpful next step here would be to put Robert's fsync compaction
> patch into here and see if that helps.  There are enough backend syncs
> showing up in the difficult workloads (scale>=1000, clients >=32) that its
> impact should be obvious.

Thanks for doing this work.  I look forward to the results.

Robert Haas
The Enterprise PostgreSQL Company

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