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Date: Wed, 17 Nov 2010 23:33:50 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Dave Chinner <david@fromorbit.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>, Jan Kara <jack@suse.cz>,
        Christoph Hellwig <hch@lst.de>, "Theodore Ts'o" <tytso@mit.edu>,
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        linux-mm <linux-mm@kvack.org>, linux-fsdevel@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 00/13] IO-less dirty throttling v2
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On Thu, 18 Nov 2010 18:27:06 +1100 Dave Chinner <david@fromorbit.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.