linux-kernel - Re: Unpredictable performance

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Message-Id: <200801260100.24597.nickpiggin@yahoo.com.au>
Date:	Sat, 26 Jan 2008 01:00:24 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Asbjorn Sannes <asbjorsa@....uio.no>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Unpredictable performance

On Friday 25 January 2008 22:32, Asbjorn Sannes wrote:
> Hi,
>
> I am experiencing unpredictable results with the following test
> without other processes running (exception is udev, I believe):
> cd /usr/src/test
> tar -jxf ../linux-2.6.22.12
> cp ../working-config linux-2.6.22.12/.config
> cd linux-2.6.22.12
> make oldconfig
> time make -j3 > /dev/null # This is what I note down as a "test" result
> cd /usr/src ; umount /usr/src/test ; mkfs.ext3 /dev/cc/test
> and then reboot
>
> The kernel is booted with the parameter mem=81920000
>
> For 2.6.23.14 the results vary from (real time) 33m30.551s to 45m32.703s
> (30 runs)
> For 2.6.23.14 with nop i/o scheduler from 29m8.827s to 55m36.744s (24 runs)
> For 2.6.22.14 also varied a lot.. but, lost results :(
> For 2.6.20.21 only vary from 34m32.054s to 38m1.928s (10 runs)
>
> Any idea of what can cause this? I have tried to make the runs as equal
> as possible, rebooting between each run.. i/o scheduler is cfq as default.
>
> sys and user time only varies a couple of seconds.. and the order of
> when it is "fast" and when it is "slow" is completly random, but it
> seems that the results are mostly concentrated around the mean.

Hmm, lots of things could cause it. With such big variations in
elapsed time, and small variations on CPU time, I guess the fs/IO
layers are the prime suspects, although it could also involve the
VM if you are doing a fair amount of page reclaim.

Can you boot with enough memory such that it never enters page
reclaim? `grep scan /proc/vmstat` to check.

Otherwise you could mount the working directory as tmpfs to
eliminate IO.

bisecting it down to a single patch would be really helpful if you
can spare the time.

Thanks,
Nick
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