Re: [Qemu-devel] performance monitor

[Clemens Kolbitsch]

On Thursday 03 January 2008 23:07:07 you wrote:
> > Does anyone have an idea on how I can measure performance in qemu to a
> > somewhat accurate level? I have modified qemu (the memory handling) and
> > the linux kernel and want to find out the penalty this introduced... does
> > anyone have any comments / ideas on this?
>
> Short answer is you probably can't. And even if you can I won't believe
> tyour results unless you've verified them on real hardware :-)
>
> With the exception of some very small embedded cores, Modern CPUs have
> complex out of order execution pipelines and multi-level cache hierarchies.
> It's common for performance to be dominated by these secondary factors
> rather than raw instruction throughput.
>
> Exactly what features dominate performance is very application specific.
> Determining which factor dominates is unlikely to be something qemu can
> help with.
>
> However if e.g. you know that for your application there's a good
> correlation was between performance and L2 cache misses you could
> instrument qemu to and a L1/L2 cache model. The overhead will be fairly
> severe (easily 10x slower), and completely screw up any realtime
> measurements. However it would produce some useful cache use statistics
> that you could use to guesstimate actual performance. This is similar to
> how cachegrind works. Obviously if your application isn't cache bound then
> these figures will be meaningless.

Well, the measuring I had in mind partly concentrats on TLB misses, page 
faults, etc. (in addition to the cycle measuring). guess i'll have to 
implement something for myself in qemu :-/

But thanks a lot for helping me out!