Re: [Qemu-devel] [PATCH for-1.5 0/9] Disable expensive QOM cast debugging for official releases

[Aurelien Jarno]

On Fri, May 10, 2013 at 12:41:07PM -0500, Anthony Liguori wrote:
> Paolo Bonzini <address@hidden> writes:
> 
> > Il 10/05/2013 16:39, Anthony Liguori ha scritto:
> >> I just oppose the notion of disabling casts and *especially* only
> >> disabling casts for official builds.
> >
> > This actually happens all the time.  Exactly this kind of type-safe cast
> > is disabled in releases of GCC, but enabled when building from svn trunk.
> 
> Let's assume for a moment that you are right and this behavior is what
> we should have.  Let's also assume there is a real regression here
> which has yet to have been established.

I have pointed out in another mail of this thread, that this type
checking account for about 9% of slowdown. Isn't that enough to say it's
a regression?

I still have to compare to 1.4, but it's not very fair, as a
significant work has been but on TCG to improve the performances,
especially on the target-ppc code.

I am currently preparing an image so that people can test that
themselves.

> As soon as we open up 1.6 for development, we face an immediate
> regression in performance.  So we need to fix the real problem here
> anyway.
> 
> I strongly suspect that if there is a problem, that optimizing
> leaf/concrete casts will take care of it.  Otherwise, a small lookup
> cache ought to do the trick too.  We can even use pointer comparisons
> for the lookup cache which should make it extremely cheap.
>
>
> Disabling casts doesn't give us a long term fix.  One of the solutions
> above does and patches also exist.

Why do you insist on using dynamic cast, on a hot path? It was using a
simple pointer before QOM in 1.4, it won't be less safe with if we use
a static cast instead of a dynamic one in 1.5.

With your arguments, we should drop all dataplane code, as it doesn't
use the QEMU core code. People should just not complain about the
performance.

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
address@hidden                 http://www.aurel32.net