Messages in this thread | | | Date | Mon, 10 Nov 2008 09:58:46 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH RFC/RFB] x86_64, i386: interrupt dispatch changes |
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* 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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