Messages in this thread | | | Date | Mon, 28 Jan 2008 10:12:24 +0100 | From | Asbjørn Sannes <> | Subject | Re: Unpredictable performance |
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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
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