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

["Alexander van Heukelum" <heukelum@fastmail.fm>]

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

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