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From: "Asbjørn Sannes" <>
To: Nick Piggin <>
Cc: Linux Kernel Mailing List <>
Subject: Re: Unpredictable performance
Date: Fri, 25 Jan 2008 16:03:24 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

Asbjørn Sannes wrote:
> Nick Piggin wrote:
>> 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-
>>> cp ../working-config linux-
>>> cd linux-
>>> 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 the results vary from (real time) 33m30.551s to 45m32.703s
>>> (30 runs)
>>> For with nop i/o scheduler from 29m8.827s to 55m36.744s (24 runs)
>>> For also varied a lot.. but, lost results :(
>>> For 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.
> I'm going to run some tests without limiting the memory to 80 megabytes
> (so that it is 2 gigabyte) and see how much it varies then, but iff I
> recall correctly it did not vary much. I'll reply to this e-mail with
> the results.
5 runs gives me:
real    5m58.626s
real    5m57.280s
real    5m56.584s
real    5m57.565s
real    5m56.613s

Should I test with tmpfs aswell?

Asbjorn Sannes

  reply	other threads:[~2008-01-25 15:02 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-01-25 11:32 Asbjorn Sannes
2008-01-25 14:00 ` Nick Piggin
2008-01-25 14:31   ` Asbjørn Sannes
2008-01-25 15:03     ` Asbjørn Sannes [this message]
2008-01-26  0:38       ` Nick Piggin
2008-01-28  9:12         ` Asbjørn Sannes
2008-01-25 17:16 ` Ray Lee
2008-01-25 20:49   ` Asbjørn Sannes
     [not found]     ` <>
2008-01-28  9:02       ` Asbjørn Sannes

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