linux-kernel - Re: [patch 09/17] LTTng instrumentation

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Date:	Thu, 17 Jul 2008 16:25:24 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Masami Hiramatsu <mhiramat@...hat.com>, linux-mm@...ck.org,
	Dave Hansen <haveblue@...ibm.com>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Hideo AOKI <haoki@...hat.com>,
	Takashi Nishiie <t-nishiie@...css.fujitsu.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>
Subject: Re: [patch 09/17] LTTng instrumentation - filemap

On Wednesday 16 July 2008 08:26, Mathieu Desnoyers wrote:
> Instrumentation of waits caused by memory accesses on mmap regions.
>
> Those tracepoints are used by LTTng.
>
> About the performance impact of tracepoints (which is comparable to
> markers), even without immediate values optimizations, tests done by Hideo
> Aoki on ia64 show no regression. His test case was using hackbench on a
> kernel where scheduler instrumentation (about 5 events in code scheduler
> code) was added. See the "Tracepoints" patch header for performance result
> detail.

BTW. this sort of test is practically useless to measure overhead. If
a modern CPU is executing out of primed insn/data and branch prediction
cache, then yes this sort of thing is pretty well free.

I see *real* workloads that have got continually and incrementally slower
eg from 2.6.5 to 2.6.20+ as "features" get added. Surprisingly, none of
them ever showed up individually on a microbenchmark.

OK, for this case if you can configure it out, I guess that's fine. But
let's not pretend that adding code and branches and function calls are
ever free.
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