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Re: [Qemu-trivial] [Qemu-devel] [PATCH] Do not enable QOM debugging by d

From: BALATON Zoltan
Subject: Re: [Qemu-trivial] [Qemu-devel] [PATCH] Do not enable QOM debugging by default
Date: Mon, 16 Jul 2018 23:08:01 +0200 (CEST)
User-agent: Alpine 2.21 (BSF 202 2017-01-01)

On Mon, 16 Jul 2018, Peter Maydell wrote:
On 16 July 2018 at 20:47, BALATON Zoltan <address@hidden> wrote:
Commit 3556c233d 5 years ago added an option to disable QOM debugging,
noting that it can have high performance cost but left the default to on.
Change the default to off and only enable it when debugging is requested
to avoid the performance penalty for those compiling without debug options.

Signed-off-by: BALATON Zoltan <address@hidden>

Looking at the code, doesn't this disable all the type checking
on the QOM casts? That is not something I think we should drop,
in the same way that we do not disable assert()s even in the
non-debug build.

Is this coming up as significant in profiling? In the past we've
done some special casing if there are particular QOM casts in
hot paths that use significant runtime.

My profiling results are not very accurate (could only gather sample based results with oprofile as --enable-gprof haven't worked for me) so I don't have numbers but I did see object_class_dynamic_cast_assert coming out among the top few without disabling this option but I don't know which call sites are responsible.

Are there any recommendations for profiling QEMU (especially TCG) somewhere that I should be aware of?


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