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Re: Initial 9.2 pgbench write results

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Ants Aasma <ants(dot)aasma(at)eesti(dot)ee>
Cc: Greg Smith <greg(at)2ndquadrant(dot)com>, simon(at)2ndquadrant(dot)com, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Initial 9.2 pgbench write results
Date: 2012-02-28 16:46:39
Message-ID: CA+TgmoZQ9HpHi9UXYOCZ71dwyZj=Vtb6DsGgw2K0gcUgVKnBDw@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Tue, Feb 28, 2012 at 1:15 AM, Ants Aasma <ants(dot)aasma(at)eesti(dot)ee> wrote:
> My hypothesis for the TPS regression is that it is due to write combining.
> When the workload is mainly bound by I/O, every little bit that can be saved
> helps the bottomline. Larger scalefactors don't get the benefit because
> there is less write combining going on overall.

This is an interesting hypothesis which I think we can test.  I'm
thinking of writing a quick patch (just for testing, not for commit)
to set a new buffer flag BM_BGWRITER_CLEANED to every buffer the
background writer cleans.   Then we can keep a count of how often such
buffers are dirtied before they're evicted, vs. how often they're
evicted before they're dirtied.  If any significant percentage of them
are redirtied before they're evicted, that would confirm this
hypothesis.  At any rate I think the numbers would be interesting to
see.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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