Re: [Qemu-devel] Reporting Heisenbugs in qemu

[Torbjorn Granlund <tg@gmplib.org>]

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?

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

-- 
Torbjörn