Re: [Qemu-devel] Reporting Heisenbugs in qemu

[Paolo Bonzini <pbonzini@redhat.com>]

> Paolo Bonzini <pbonzini@redhat.com> writes:
> 
>   I guess that's the register windows.  There's only so much you can do to
>   optimize them, and heavily recursive workloads (like Perl, or the RTL
>   half of GCC) pay a hefty price.
>   
> Two qemu targets stand out for slowness, sparc (32 and 64) and mips (64,
> don't know about 32).
> 
> x86 (32 and 64), arm, and ppc run with a slowdown of < 30 for my bogus
> benchmark of GMP configure+make.
> 
> With FreeBSD x86_64 I see a slowdown of just 13.  (My reference system
> runs FreeBSD, so running FreeBSD under qemu is only far.)
> 
> My claimed slowdown factors are affected by kernel, libraries, and
> unfortunately very much by gcc speed, which vary with target.
> 
> If the sparc emulation speed is due to register windows, then why does
> mips seem just as slow?

No idea. :)  Of all the architectures you listed, MIPS is really the one
that I have no inkling of...

> If register windows shortage is a problem, it should be easy to pretend
> to have lots of them, right?

That can help to avoid trapping to the kernel.  But you still have to
spill the whole window to the stack on every call to a non-leaf function,
which can be expensive even if you do it in the translated code.

Paolo