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

[H. Peter Anvin]

Ingo Molnar wrote:
> * Alexander van Heukelum <heukelum at fastmail.fm> wrote:
> 
> 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.
> 

For what it's worth, I tested this out, and I'm pretty sure you need to
run a uniprocessor configuration (or system) for it to make sense --
otherwise you end up missing too many of the interrupts.  I first tested
this on an 8-processor system and, well, came up with nothing.

I'm going to try this later on a uniprocessor, unless Alexander beats me
to it.

	-hpa