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Re: Initial 9.2 pgbench write results

From: Ants Aasma <ants(dot)aasma(at)eesti(dot)ee>
To: Greg Smith <greg(at)2ndquadrant(dot)com>
Cc: simon(at)2ndquadrant(dot)com, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Robert Haas <robertmhaas(at)gmail(dot)com>
Subject: Re: Initial 9.2 pgbench write results
Date: 2012-02-28 06:15:11
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Lists: pgsql-hackers
On Feb 27, 2012 10:36 PM, "Greg Smith" <greg(at)2ndquadrant(dot)com> wrote:
> One of the reasons I drilled right into this spot is because of fears
that running the writer more often would sprout regressions in TPS.  I
can't explain exactly why exactly having backends write their own buffers
out at the latest possible moment works significantly better in some cases
here.  But that fact isn't new to 9.2; it's just has a slightly higher
potential to get in the way, now that the writing happens during the sync

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.

Anyway, most people don't run their databases at 100% load. At lesser loads
bgwriter should help end user latency. Is there a standard benchmark to
measure that?

Ants Aasma

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