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From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] time inside qemu
Date: Tue, 17 Apr 2007 15:22:16 +0100
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To: qemu-devel@nongnu.org
Cc: Marius Monton <Marius.Monton@uab.cat>

> code (a) : for (int i = 0; i< 1000000; i++) c[i] = a[i] * b[i];
>
> code (b) : for (int i = 0; i< 1000; i++) for(int  j = 0; j < b[i]; j++)
> c[i] += a[i];
>
> code (c) : for (int i = 0; i< 1000; i++) c[i] = HW_MUL(a[i], b[i]);
>
> I'm sure that code (b) will execute much longer that code (a) inside
> qemu (sure that different that in real platform), and I'd like to
> compute executing time for code (c) in some way.
> So, how can I trap time information/calculation inside qemu?

You can't.

As I've said before, any performance measurements you make inside qemu are 
completely meaningless.

You may be able to say that executing 1000 iterations takes longer than 10 
iterations of the same loop, but you didn't need a simulator to tell you 
that. Also, there's a good chance qemu will take almost exactly the same time 
to execute them because the execution time will be dominated by the 
translation time for the first iteration.

You can not compare the cost of (say) add v.s. multiply, or of two different 
multiply instructions. The timings for the individual instructions (or any 
particular sequence of instructions) bear no relation whatsoever to the 
timings for the same sequence on real hardware.

I'm repeating myself now, so I intend this to my my last response to this 
thread.

Paul