linux-kernel - Re: [PATCH 1/4] tracing: move __DO

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Date:	Sat, 18 Apr 2009 10:16:23 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
cc:	Jeremy Fitzhardinge <jeremy@...p.org>, Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: Re: [PATCH 1/4] tracing: move __DO_TRACE out of line


On Sat, 18 Apr 2009, Mathieu Desnoyers wrote:
> 
> tbench test
> 
> kernel : 2.6.30-rc1
> 
> running on a 8-cores x86_64, localhost server
> 
> tracepoints inactive :
> 
> 2051.20 MB/sec

Is this with or without inlined trace points?

It would be interesting to see:

time with tracepoints not configured at all

time with inline tracepoints (inactive)

time with out-of-line tracepoints (inactive)

Because if inline trace points affect the use of normal operations when 
inactive, that would be a cause against trace points all together.

-- Steve

> 
> "google" tracepoints activated, flight recorder mode (overwrite) tracing
> 
> inline tracepoints
> 
> 1704.70 MB/sec (16.9 % slower than baseline)
> 
> out-of-line tracepoints
> 
> 1635.14 MB/sec (20.3 % slower than baseline)
> 
> So the overall tracer impact is 20 % bigger just by making the
> tracepoints out-of-line. This is going to add up quickly if we add as
> much function calls as we currently find in the event tracer fast path,
> but LTTng, OTOH, has been designed to minimize the number of such
> function calls, and you see a good example of why it's been such an
> important design goal above.
> 
> About cache-line usage, I agree that in some cases gcc does not seem
> intelligent enough to move those code paths away from the fast path.
> What we would really whant there is -freorder-blocks-and-partition, but
> I doubt we want this for the whole kernel, as it makes some jumps
> slightly larger. One thing we should maybe look into is to add some kind
> of "very unlikely" builtin expect to gcc that would teach it to really
> put the branch in a cache-cold location, no matter what.
> 
> Mathieu
> 
> -- 
> Mathieu Desnoyers
> OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
> 
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