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

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Message-ID: <20081104080747.GA27075@elte.hu>
Date:	Tue, 4 Nov 2008 09:07:47 +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

* 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.

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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