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    SubjectRe: 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-
    >>> 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

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      Last update: 2008-01-25 16:05    [W:0.023 / U:15.252 seconds]
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