Re: [Qemu-devel] [PATCH 4/6] trace: Add per-vCPU tracing states for events with the 'vcpu' property

[Stefan Hajnoczi]

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.

Stefan

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