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Re: [Qemu-devel] [PATCH 4/6] trace: Add per-vCPU tracing states for even

From: Lluís Vilanova
Subject: Re: [Qemu-devel] [PATCH 4/6] trace: Add per-vCPU tracing states for events with the 'vcpu' property
Date: Tue, 14 Jun 2016 14:17:23 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux)

Paolo Bonzini writes:

> On 14/06/2016 10:39, Stefan Hajnoczi wrote:
>> On Mon, Jun 13, 2016 at 06:39:46PM +0200, Lluís Vilanova wrote:
>>> Paolo Bonzini writes:
>>>> On 13/06/2016 14:15, Lluís Vilanova wrote:
>>>>>> That said, I am skeptical about the benefit of the interfaces you are
>>>>>> adding.  They add a lot of complication and overhead (especially
>>>>>> regarding the memory/cache overhead of the dstate array) without a clear
>>>>>> use case, in my opinion; all the processing you do at run-time is just
>>>>>> as well suited for later filtering.
>>>>> This should make tracing faster on the future with multi-threaded TCG, as 
>>>>> well
>>>>> as trace files much smaller if you're tracing something like memory
>>>>> accesses. Also, bear in mind this series was split from a much larger one 
>>>>> for
>>>>> simplicity. The follow-up one provides much larger performance benefits by
>>>>> avoiding the generation of TCG code to call the tracing backend when a 
>>>>> vCPU is
>>>>> not traced.
>>>> This still assumes that tracing only some VCPUs is a common use case.
>>>> Is it?...
>>> I use it for code profiling by sampling across vCPUs, or only on vCPUs I 
>>> know
>>> run processes of my interest. The profiles can then be used for analyzing 
>>> the
>>> application/system behaviour.
>>> Also, with the fast-path checks already in place 
>>> ('trace_events_enabled_count'),
>>> performance when not tracing should never be worse with this series.
>>> If this feature does not look useful to overall QEMU I will fold it into my
>>> other out-of-tree patches.
>> I think the per-vcpu tracing feature is reasonable for qemu.git as long
>> as it does not introduce performance regressions for existing users.

> I'm okay with it if the dstate array is changed to uint16_t.

That should be fine. I don't think QEMU is gonna run more than 2**16 vCPUs
anytime soon :)


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