Re: Unpredictable performance

[Nick Piggin]

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?
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