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

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Date:	Tue, 4 Nov 2008 17:43:58 +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:

> > > > 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

> 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.

there's about 10% difference between the two hackbench results - so 
the lack of 5% of the traces could make up for about half of that 
overhead.

anyway, that still leaves the other 5% as the _true_ overhead of IRQ 
disable.

is there some other workload that does not lose this many trace 
entries, making it easier to compare irqs-off against preempt-off?

	Ingo
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