Re: [Qemu-devel] time inside qemu

[Paul Brook <paul@codesourcery.com>]

On Monday 16 April 2007 15:41, Marius Monton wrote:
> > Any benchmark/performance measurements you make inside qemu are
> > meaningless. qemu performance bears no relation whatsoever to the
> > performance characteristics of real hardware.
>
> That's true, and I don't care about it. I'd like to get a method to
> stop/start time inside qemu in order to simulate execution of large
> pieces of hw out of qemu (look at qemu-systemc project).
> If qemu is freeze  meanwhile a systemc simulation is in progress
> (simulating a HW device of system), time should be freeze also.
> In this way, execution time of a program inside qemu should appear
> shorter when using accelerator HW than only SW application. I know these
> times are not reals, but it should be enough to estimate correctness and
> execution time on real platforms.

You're deluding yourself. I simply don't believe you can get any meaningful 
performance measurements out of qemu. You certainly can't use it to evaluate 
the correctness of time sensitive algorithms.

qemu execution times can easily be orders of magnitude different from real 
hardware. i.e. if you have two operations that take the same amount of time 
to execute on real hardware, one of those operations may take many times 
longer than the other inside qemu.

If nothing else you're entirely at the mercy of the host OS process scheduler 
and signal delivery. The emulated CPU may stall for an arbitrary time at any 
point.

Paul