linux-kernel - Re: [PATCH 00/13] IO-less dirty throttling v2

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Message-Id: <20101117233350.321f9935.akpm@linux-foundation.org>
Date:	Wed, 17 Nov 2010 23:33:50 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	Christoph Hellwig <hch@....de>,
	"Theodore Ts'o" <tytso@....edu>,
	Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mel Gorman <mel@....ul.ie>, Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-mm <linux-mm@...ck.org>, linux-fsdevel@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 00/13] IO-less dirty throttling v2

On Thu, 18 Nov 2010 18:27:06 +1100 Dave Chinner <david@...morbit.com> wrote:

> > > Indeed, nobody has
> > > realised (until now) just how inefficient it really is because of
> > > the fact that the overhead is mostly hidden in user process system
> > > time.
> > 
> > "hidden"?  You do "time dd" and look at the output!
> > 
> > _now_ it's hidden.  You do "time dd" and whee, no system time!
> 
> What I meant is that the cost of foreground writeback was hidden in
> the process system time. Now we have separated the two of them, we
> can see exactly how much it was costing us because it is no longer
> hidden inside the process system time.

About a billion years ago I wrote the "cyclesoak" thingy which measures
CPU utilisation the other way around: run a lowest-priority process on
each CPU in the background, while running your workload, then find out
how much CPU time cyclesoak *didn't* consume.  That way you account for
everything: user time, system time, kernel threads, interrupts,
softirqs, etc.  It turned out to be pretty accurate, despite the
then-absence of SCHED_IDLE.

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