Re: [Qemu-devel] [PATCH v2 3/3] trace: enable all events by default

[Lluís]

Blue Swirl writes:

> On Tue, Nov 9, 2010 at 2:08 PM, Lluís <address@hidden> wrote:
>> Enable all trace events by default, assuming their frequency is relatively 
>> low,
>> so there will be no measurable performace impact.

> Even if the impact for one tracepoint is low, if all tracepoints are
> enabled, eventually (considering that the number of tracepoints will
> increase) just the data flow to the trace file will start to slow down
> the guest.

> But I'm not completely opposed to removing 'disable' keyword, for
> example by turning all tracepoints off initially. Though then there
> would be no way to trace the initialization activities.

Well, the default trace backend is 'nop' so there's no overhead at all
on default compilations.

Even more, if you enable a backend other than 'nop', right now you still
have to manually (de)activate the trace-events you want, depending on
the backend you choose.

For example, right now backends behave:
    * nop
      No overhead regardless of the "disable" property/keyword
    * simple
      "disable" sets the initial event state, but they're always able to
      trace (compiled-in).
    * ust
      If "disable" is present the event is not compiled-in, but by
      default events are not tracing from the start.
    * dtrace
      Similar to "simple" (don't remember if "disable" has any effect at
      all).

Then, with the patch I sent, "disable" always uses the "nop" backend
with that event, and otherwise follows through the default per-backend
state (all of which should have an inactive state by default).

So, my patch is just a matter of having all events available _only_ when
you use a backend other than "nop".

Lluis

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