Re: Unpredictable performance

["Asbjørn Sannes" <ace@sannes.org>]

Nick Piggin wrote:
> On Saturday 26 January 2008 02:03, Asbjørn Sannes wrote:
>   
>> 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?
>>     
>
> I wouldn't worry about it. It seems like it might be due to page reclaim
> (fs / IO can't be ruled out completely though). Hmm, I haven't been following
> reclaim so closely lately; you say it started going bad around 2.6.22? It
> may be lumpy reclaim patches?
>   
Going to bisect it soon, but I suspect it will take some time
(considering how many runs I need to make any sense of the results).

--
Asbjorn Sannes