linux-kernel - Re: Unpredictable performance

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Date:	Fri, 25 Jan 2008 16:03:24 +0100
From:	Asbjørn Sannes <ace@...nes.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Unpredictable performance

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-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.
>>   
>>     
> 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

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