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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: Mon, 13 Jun 2016 18:39:46 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux)

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.


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