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    SubjectRe: [PATCH RFC/RFB] x86_64, i386: interrupt dispatch changes
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    On Mon, 10 Nov 2008 09:58:46 +0100, "Ingo Molnar" <mingo@elte.hu> said:
    > * Alexander van Heukelum <heukelum@fastmail.fm> wrote:
    > > Hi all,
    > >
    > > I have spent some time trying to find out how expensive the
    > > segment-switching patch was. I have only one computer available at
    > > the time: a "Sempron 2400+", 32-bit-only machine.
    > >
    > > Measured were timings of "hackbench 10" in a loop. The average was
    > > taken of more than 100 runs. Timings were done for two seperate
    > > boots of the system.

    Hi Ingo,

    I guess you just stopped reading here?

    > hackbench is _way_ too noisy to measure such cycle-level differences
    > as irq entry changes cause. It also does not really stress interrupts
    > - it only stresses networking, the VFS and the scheduler.
    >
    > a better test might have been to generate a ton of interrupts, but
    > even then it's _very_ hard to measure it properly.

    I should have presented the second benchmark as the first I
    guess. I really just used hackbench as a workload. I gathered
    it would give a good amount of exceptions like page faults and
    maybe others. It would be nice to have a simple debug switch in
    the kernel to make it generate a lot of interrupts, though ;).

    > The best method is
    > what i've suggested to you early on: run a loop in user-space and
    > observe irq costs via RDTSC, as they happen. Then build a histogram
    > and compare the before/after histogram. Compare best-case results as
    > well (the first slot of the histogram), as those are statistically
    > much more significant than a noisy average.

    See the rest of the mail you replied to and its attachment. I've put
    the programs I used and the histogram in

    http://heukelum.fastmail.fm/irqstubs/

    I think rdtsctest.c is pretty much what you describe.

    Greetings,
    Alexander

    > Measuring such things in a meaningful way is really tricky business.
    > Using hackbench to measure IRQ entry micro-costs is like trying to
    > take a photo of a delicate flower at night, by using an atomic bomb as
    > the flash-light: you certainly get some sort of effect to report, but
    > there's not many nuances left in the picture to really look at ;-)
    >
    > Ingo
    --
    Alexander van Heukelum
    heukelum@fastmail.fm

    --
    http://www.fastmail.fm - Same, same, but different...



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