linux-kernel - Re: Poor PostgreSQL scaling on Linux 2.6.25-rc5 (vs 2.6.22)

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Date:	Mon, 17 Mar 2008 06:21:34 +0100
From:	Willy Tarreau <w@....eu>
To:	Ray Lee <ray-lk@...rabbit.org>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	"LKML," <linux-kernel@...r.kernel.org>
Subject: Re: Poor PostgreSQL scaling on Linux 2.6.25-rc5 (vs 2.6.22)

On Sun, Mar 16, 2008 at 10:16:36PM -0700, Ray Lee wrote:
> On Sun, Mar 16, 2008 at 5:44 PM, Nick Piggin <nickpiggin@...oo.com.au> wrote:
> >  I don't see how it is really helpful for interactive processes either.
> >  By definition, if they are not CPU bound, then they should be run
> >  quite soon after waking up; if they are CPU bound, then reducing
> >  efficiency by increasing context switches is effectively going to
> >  increase their latency anyway.
> 
> How? Are you saying that switching the granularity to, say, 25ms, will
> *decrease* the latency of interactive tasks?
> 
> And the efficiency we're talking about reducing here is due to the
> fact that tasks are hitting cold caches more times per second when the
> granularity is smaller, correct? Or are you concerned by another
> issue?
> 
> > Can this be changed by default, please?
> 
> Not without benchmarks of interactivity, please. There are far, far
> more linux desktops than there are servers. People expect to have to
> tune servers (I do, for the servers I maintain). People don't expect
> to have to tune a desktop to make it run well.

Oh and even on servers, when your anti-virus proxy reaches a load of 800,
you're happy no to have too large a time-slice so that you regularly get
a chance of being allowed to type in commands over SSH.

Large time-slices are needed only in HPC environments IMHO, where only
one task runs.

Willy

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