From: Paolo Bonzini <pbonzini@redhat.com>
To: Laurent Desnogues <laurent.desnogues@gmail.com>,
"qemu-devel@nongnu.org" <qemu-devel@nongnu.org>
Subject: Re: [Qemu-devel] QOM cast debug
Date: Thu, 20 Mar 2014 13:45:56 +0100 [thread overview]
Message-ID: <532AE304.8060904@redhat.com> (raw)
In-Reply-To: <CABoDooODBhUuVSzueGk1=uu2Gbvhy=RSBdxmgXiRmPcfXBLrEA@mail.gmail.com>
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
next prev parent reply other threads:[~2014-03-20 12:46 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-03-20 10:52 [Qemu-devel] QOM cast debug Laurent Desnogues
2014-03-20 12:45 ` Paolo Bonzini [this message]
2014-03-20 13:24 ` Laurent Desnogues
2014-03-20 13:27 ` Paolo Bonzini
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