Messages in this thread | | | From | "Alexander van Heukelum" <> | Subject | Re: [PATCH RFC/RFB] x86_64, i386: interrupt dispatch changes | Date | Mon, 10 Nov 2008 13:44:21 +0100 |
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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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