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

[Alexander van Heukelum]

On Tue, 4 Nov 2008 17:36:36 +0100, "Ingo Molnar" <mingo at elte.hu> said:
> 
> * Alexander van Heukelum <heukelum at fastmail.fm> wrote:
> 
> > I wonder how the time needed for reading the GDT segments balances 
> > against the time needed due to the extra redirection due to running 
> > the stubs. I'ld be interested if the difference can be measured with 
> > the current implementation. (I really need to highjack a machine to 
> > do some measurements; I hoped someone would do it before I got to it 
> > ;) )
> > 
> > Even if some CPU's have some internal optimization for the case 
> > where the gate segment is the same as the current one, I wonder if 
> > it is really important... Interrupts that occur while the processor 
> > is running userspace already cause changing segments. They are more 
> > likely to be in cache, maybe.
> 
> there are three main factors:
> 
> - Same-value segment loads are optimized on most modern CPUs and can
>   give a few cycles (2-3) advantage. That might or might not apply to 
>   the microcode that does IRQ entry processing. (A cache miss will 
>   increase the cost much more but that is true in general as well)
> 
> - A second effect is that the changed data structure layout: a more
>   compressed GDT entry (6 bytes) against a more spread out (~7 bytes,
>   not aligned) interrupt trampoline. Note that the first one is data 
>   cache the second one is instruction cache - the two have different 
>   sizes, different implementations and different hit/miss pressures. 
>   Generally the instruction-cache is the more precious resource and we 
>   optimize for that first, for data cache second.
> 
> - A third effect is branch prediction: currently we are fanning 
>   out all the vectors into ~240 branches just to recover a single 
>   constant in essence. That is quite wasteful of instruction cache 
>   resources, because from the logic side it's a data constant, not a 
>   control flow difference. (we demultiplex that number into an 
>   interrupt handler later on, but the CPU has no knowledge of that 
>   relationship)
> 
> ... all in one, the situation is complex enough on the CPU 
> architecture side for it to really necessiate a measurement in 
> practice, and that's why i have asked you to do them: the numbers need 
> to go hand in hand with the patch submission.
> 
> My estimation is that if we do it right, your approach will behave 
> better on modern CPUs (which is what matters most for such things), 
> especially on real workloads where there's a considerable 
> instruction-cache pressure. But it should be measured in any case.

Fully agreed. I will do some measurements in the near future, maybe
next week. At least noone came up with an absolutely blocking problem
with this approach ;).

Greetings,
    Alexander

> 	Ingo
-- 
  Alexander van Heukelum
  heukelum at fastmail.fm

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