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Date: Thu, 20 Mar 2014 13:45:56 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] QOM cast debug
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To: Laurent Desnogues <laurent.desnogues@gmail.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>

Il 20/03/2014 11:52, Laurent Desnogues ha scritto:
> Hello,
>
> while looking at some perf results, I saw object_dynamic_cast_assert
> taking more than 3% of the run time.
>
> After some digging I found out that this time can be cut by passing
> --disable-qom-cast-debug to configure.  This was added by Paolo:
>
> commit 3556c233d931ad5ffa46a35cb25cfc057732ebb8
> Author: Paolo Bonzini <pbonzini@redhat.com>
> Date:   Fri May 10 14:16:40 2013 +0200
>
>     qom: allow turning cast debugging off
>
>     Cast debugging can have a substantial cost (20% or more).  Instead of adding
>     special-cased "fast casts" in the hot paths, we can just disable it in
>     releases.  The tracing facilities we just added make it easier to analyze
>     those problems that cast debugging would reveal.
>
> I find it odd that the default is to have this debug flag on by
> default while the other such debug options are off.  Wouldn't it make
> more sense to have it off by default and let devs turn it on?

I agree, but Anthony (and Andreas?) did not.

Which path is calling object_dynamic_cast_assert so much?  It was agreed 
to just use C casts in hot code.

Paolo