linux-kernel - Re: [PATCH 3/3] ftrace: function tracer with irqs disabled

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Date:	Tue, 4 Nov 2008 10:04:05 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 3/3] ftrace: function tracer with irqs disabled


* Ingo Molnar <mingo@...e.hu> wrote:

> * Steven Rostedt <rostedt@...dmis.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 :-)

> 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?

	Ingo
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