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SubjectRe: [PATCH RFC/RFB] x86_64, i386: interrupt dispatch changes

* 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.

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. 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.

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


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