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    SubjectRe: 2.6.19-rc5: known regressions
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    On Wed, 2006-11-08 at 15:11 -0800, Tim Chen wrote:
    > On Wed, 2006-11-08 at 17:22 +0100, Adrian Bunk wrote:
    >
    > > There's perhaps one thing that might help us to see whether it's just a
    > > benchmark effekt or a real problem:
    > >
    > > With Tim's CONFIG_NR_CPUS=8, NR_IRQS only increases from 224 in 2.6.18
    > > to 512 in 2.6.19-rc.
    > >
    > > With CONFIG_NR_CPUS=255, NR_IRQS increases from 224 in 2.6.18
    > > to 8416 in 2.6.19-rc.
    > >
    > > @Tim:
    > > Can you try CONFIG_NR_CPUS=255 with both 2.6.18 and 2.6.19-rc5?
    > >
    >
    > With CONFIG_NR_CPUS increased from 8 to 64:
    > 2.6.18 see no change in fork time measured.
    > 2.6.19-rc5 see a 138% increase in fork time.
    >

    Lmbench is broken in its fork time measurement.
    It includes overhead time when it is pinning processes onto
    specific cpu. The actual fork time is not affected by NR_IRQS.

    Lmbench calls the following C library function to determine the
    number of processors online before it pin the processes:
    sysconf(_SC_NPROCESSORS_ONLN);

    This function takes the same order of time to run as
    fork itself. In addition, runtime of this function
    increases with NR_IRQS. This resulted in the change in
    time measured.

    After hardcoding the number of online processors in lmbench,
    the fork time measured now does not change with CONFIG_NR_CPUS
    for both 2.6.18 and 2.6.19-rc5. So we can now conclude that
    NR_IRQS does not affect fork. We can remove this particular
    issue from the known regression.

    Tim
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