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From: Nick Piggin <nickpiggin@yahoo.com.au>
To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Subject: Re: [patch 09/17] LTTng instrumentation - filemap
Date: Thu, 17 Jul 2008 16:25:24 +1000
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Cc: akpm@linux-foundation.org, Ingo Molnar <mingo@elte.hu>,
       linux-kernel@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
       Masami Hiramatsu <mhiramat@redhat.com>, linux-mm@kvack.org,
       Dave Hansen <haveblue@us.ibm.com>, "Frank Ch. Eigler" <fche@redhat.com>,
       Hideo AOKI <haoki@redhat.com>,
       Takashi Nishiie <t-nishiie@np.css.fujitsu.com>,
       Steven Rostedt <rostedt@goodmis.org>,
       Eduard - Gabriel Munteanu <eduard.munteanu@linux360.ro>
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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.