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

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Date:	Tue, 04 Nov 2008 16:17:07 +0800
From:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Steven Rostedt <rostedt@...dmis.org>, 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


On Tue, 2008-11-04 at 09:07 +0100, Ingo Molnar 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 
I found that, too. But if I kill most background processes before testing,
hackbench result looks quite stable.

> - 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.
> 
> note that preempt-disable will also produce less trace entries, 
> especially in very irq-rich workloads. Hence it will be "faster".
> 
> 	Ingo
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