Messages in this thread | | | Date | Tue, 4 Nov 2008 09:44:06 -0500 (EST) | From | Steven Rostedt <> | Subject | Re: [PATCH 3/3] ftrace: function tracer with irqs disabled |
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On Tue, 4 Nov 2008, Ingo Molnar wrote:
> > * Ingo Molnar <mingo@elte.hu> wrote: > > > * Steven Rostedt <rostedt@goodmis.org> wrote: > > > > > Running hackbench 3 times with the irqs disabled and 3 times with > > > the preempt disabled function tracer yielded: > > > > > > tracing type times entries recorded > > > ------------ -------- ---------------- > > > irq disabled 43.393 166433066 > > > 43.282 166172618 > > > 43.298 166256704 > > > > > > preempt disabled 38.969 159871710 > > > 38.943 159972935 > > > 39.325 161056510 > > > > your numbers might be correct, but i found that hackbench is not > > reliable boot-to-boot - it can easily produce 10% systematic noise > > or more. (perhaps depending on how the various socket data > > structures happen to be allocated) > > > > the really conclusive way to test this would be to add a hack that > > either does preempt disable or irqs disable, depending on a runtime > > flag - and then observe how hackbench performance reacts to the > > value of that flag. > > ... which is exactly what your patch implements :-)
Yep ;-)
Those numbers were done without any reboots in between. I even tried it several times, randomly picking to use irqs_disabled and preempt_disabled, and everytime preempt_disabled was around 39 secs, and irqs disabled was around 43.
> > > note that preempt-disable will also produce less trace entries, > > especially in very irq-rich workloads. Hence it will be "faster". > > this point still holds. Do we have any good guess about the 'captured > trace events per second' rate in the two cases, are they the same?
If you look at the end of my change log, I printed stats from a patch I added that counted the times that ftrace recursed, but did not record. Those numbers were quite big with preempt_disabled.
>> With irq disabled: 1,150 times the function tracer did not trace due to >> recursion. >> with preempt disabled: 5,117,718 times.
When we used the preempt disabled version, we lost 5 million traces, as suppose to the irq disabled which was only 1,150 traces lost.
Considering that we had 166,256,704 traces total, that 5 million is only 4% lost of traces. Still quite a lot. But again, this is an extreme, because we are tracing hackbench.
-- Steve
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