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Re: [Qemu-devel] Reporting Heisenbugs in qemu

From: Paolo Bonzini
Subject: Re: [Qemu-devel] Reporting Heisenbugs in qemu
Date: Wed, 8 May 2013 06:18:40 -0400 (EDT)

> Paolo Bonzini <address@hidden> 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.


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