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

[Paolo Bonzini]

Il 10/05/2013 15:01, Anthony Liguori ha scritto:
> Paolo Bonzini <address@hidden> writes:
> 
>> Cast debugging can have a substantial cost (20% or more, measured by
>> Aurelien on qemu-system-ppc64).
> 
> [Needs citation]

Sure: http://permalink.gmane.org/gmane.comp.emulators.qemu/210830

 49,73%  perf-10672.map           [.] 0x7f7853ab4e0f
 13,23%  qemu-system-ppc64        [.] cpu_ppc_exec
 13,16%  libglib-2.0.so.0.3200.4  [.] g_hash_table_lookup
  8,18%  libglib-2.0.so.0.3200.4  [.] g_str_hash
  2,47%  qemu-system-ppc64        [.] object_class_dynamic_cast
  1,97%  qemu-system-ppc64        [.] type_is_ancestor
  1,05%  libglib-2.0.so.0.3200.4  [.] g_str_equal

That's ~27%.

>> 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.
> 
> I'd prefer not to disable but instead focus on improving performance.

For 1.5?  This is a regression in 1.5 due to more and more usage of
foo_env_on_cpu.

Paolo

> I suspect any performance overhead is resolving the type string to
> typeimpl.  One work around is to have a dynamic cast that takes a
> typeimpl and then use a function that returns a static similar to how
> glib works.
> 
> If you've got a reproducible case where the overhead is high, it should
> be easy to check.
> 
> Regards,
> 
> Anthony Liguori
> 
>>
>> 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(-)
>>
>> -- 
>> 1.8.1.4
>