* [Qemu-devel] QOM cast debug
@ 2014-03-20 10:52 Laurent Desnogues
2014-03-20 12:45 ` Paolo Bonzini
0 siblings, 1 reply; 4+ messages in thread
From: Laurent Desnogues @ 2014-03-20 10:52 UTC (permalink / raw)
To: qemu-devel@nongnu.org, Paolo Bonzini
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?
Thanks,
Laurent
^ permalink raw reply [flat|nested] 4+ messages in thread* Re: [Qemu-devel] QOM cast debug
2014-03-20 10:52 [Qemu-devel] QOM cast debug Laurent Desnogues
@ 2014-03-20 12:45 ` Paolo Bonzini
2014-03-20 13:24 ` Laurent Desnogues
0 siblings, 1 reply; 4+ messages in thread
From: Paolo Bonzini @ 2014-03-20 12:45 UTC (permalink / raw)
To: Laurent Desnogues, 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
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [Qemu-devel] QOM cast debug
2014-03-20 12:45 ` Paolo Bonzini
@ 2014-03-20 13:24 ` Laurent Desnogues
2014-03-20 13:27 ` Paolo Bonzini
0 siblings, 1 reply; 4+ messages in thread
From: Laurent Desnogues @ 2014-03-20 13:24 UTC (permalink / raw)
To: Paolo Bonzini; +Cc: qemu-devel@nongnu.org
On Thu, Mar 20, 2014 at 1:45 PM, Paolo Bonzini <pbonzini@redhat.com> wrote:
> 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.
The culprit seems to be ENV_GET_CPU:
#define ENV_GET_CPU(e) CPU(arm_env_get_cpu(e))
#define CPU(obj) OBJECT_CHECK(CPUState, (obj), TYPE_CPU)
#define OBJECT_CHECK(type, obj, name) \
((type *)object_dynamic_cast_assert(OBJECT(obj), (name), \
__FILE__, __LINE__, __func__))
0.00 0.00 1391069/289549110 io_writel [46]
0.00 0.00 2038947/289549110 helper_le_stl_mmu [32]
0.00 0.00 3986870/289549110 helper_ret_ldub_mmu [31]
0.00 0.00 8131587/289549110 helper_ret_ldb_cmmu [28]
0.00 0.00 17849223/289549110 helper_le_ldul_mmu [18]
0.00 0.00 32244969/289549110 tb_find_slow [26]
0.00 0.00 32347716/289549110
arm_cpu_handle_mmu_fault [11]
0.00 0.00 32347737/289549110 get_phys_addr_v6 [17]
0.00 0.00 32598025/289549110 get_page_addr_code [27]
0.00 0.00 124705628/289549110 tb_find_fast [25]
[106] 0.0 0.00 0.00 289549110 object_dynamic_cast_assert [106]
Laurent
^ permalink raw reply [flat|nested] 4+ messages in thread* Re: [Qemu-devel] QOM cast debug
2014-03-20 13:24 ` Laurent Desnogues
@ 2014-03-20 13:27 ` Paolo Bonzini
0 siblings, 0 replies; 4+ messages in thread
From: Paolo Bonzini @ 2014-03-20 13:27 UTC (permalink / raw)
To: Laurent Desnogues; +Cc: qemu-devel@nongnu.org
Il 20/03/2014 14:24, Laurent Desnogues ha scritto:
> On Thu, Mar 20, 2014 at 1:45 PM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>> 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.
>
> The culprit seems to be ENV_GET_CPU:
>
> #define ENV_GET_CPU(e) CPU(arm_env_get_cpu(e))
> #define CPU(obj) OBJECT_CHECK(CPUState, (obj), TYPE_CPU)
> #define OBJECT_CHECK(type, obj, name) \
> ((type *)object_dynamic_cast_assert(OBJECT(obj), (name), \
> __FILE__, __LINE__, __func__))
>
> 0.00 0.00 1391069/289549110 io_writel [46]
> 0.00 0.00 2038947/289549110 helper_le_stl_mmu [32]
> 0.00 0.00 3986870/289549110 helper_ret_ldub_mmu [31]
> 0.00 0.00 8131587/289549110 helper_ret_ldb_cmmu [28]
> 0.00 0.00 17849223/289549110 helper_le_ldul_mmu [18]
> 0.00 0.00 32244969/289549110 tb_find_slow [26]
> 0.00 0.00 32347716/289549110
> arm_cpu_handle_mmu_fault [11]
> 0.00 0.00 32347737/289549110 get_phys_addr_v6 [17]
> 0.00 0.00 32598025/289549110 get_page_addr_code [27]
> 0.00 0.00 124705628/289549110 tb_find_fast [25]
> [106] 0.0 0.00 0.00 289549110 object_dynamic_cast_assert [106]
Andreas, why can't we just do (CPUState *) in the actual ENV_GET_CPU
macro? It's pretty much assured to be called by hot spots.
Paolo
^ permalink raw reply [flat|nested] 4+ messages in thread
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2014-03-20 10:52 [Qemu-devel] QOM cast debug Laurent Desnogues
2014-03-20 12:45 ` Paolo Bonzini
2014-03-20 13:24 ` Laurent Desnogues
2014-03-20 13:27 ` Paolo Bonzini
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