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

From: Aurelien Jarno
Subject: Re: [Qemu-devel] [PATCH for-1.5 0/9] Disable expensive QOM cast debugging for official releases
Date: Fri, 10 May 2013 17:56:32 +0200
User-agent: Mutt/1.5.20 (2009-06-14)

On Fri, May 10, 2013 at 02:16:34PM +0200, Paolo Bonzini wrote:
> Cast debugging can have a substantial cost (20% or more, measured by
> Aurelien on qemu-system-ppc64).  Instead of adding special-cased "fast
> casts" in the hot paths, we can just disable it in releases.  At the
> same time, add tracing facilities that simplify the analysys of those
> problems that cast debugging would reveal.
> At least patches 1-7 are for 1.5.
> Paolo Bonzini (9):
>   qom: improve documentation of cast functions
>   qom: allow casting of a NULL class
>   qom: add a fast path to object_class_dynamic_cast
>   qom: pass file/line/function to asserting casts
>   qom: trace asserting casts
>   qom: allow turning cast debugging off
>   build: disable QOM cast debugging for official releases
>   qom: simplify object_class_dynamic_cast, part 1
>   qom: simplify object_class_dynamic_cast, part 2
>  configure            | 20 ++++++++------
>  include/qom/object.h | 40 ++++++++++++++++++++++-----
>  qom/object.c         | 77 
> ++++++++++++++++++++++++++++++++++------------------
>  trace-events         |  3 ++
>  4 files changed, 99 insertions(+), 41 deletions(-)

I have tested this series with qemu-system-ppc64, on a Core i7 2600 CPU.
The process was set to a single core using taskset. Inside the guest
(Debian ppc64 from debian-ports), I ran the command three times:

  lintian g++-4.8_4.8.0-6_ppc64.deb

I used lintian as it's a perl code, that trigger the discussion about
sparc/ppc comparison.

First of all with this patch series, the object_class_dynamic_cast calls
went down to below 0.1% when using perf top. Before the patch series was
applied, the command took in average on 3 runs 142.4s. With the patch
series, it went down to 129.8s, so almost 9% faster.

To improve the performance a bit more, and come back to the same kind of
code as before, we should move simple accessors from qom/*.c to
include/qom/*.h and mark them as inline, so that they can be removed by
the compiler. Currently, even if the function is simple it's still a
call/ret in the hot path instead of a simple pointer addition.

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

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