linux-kernel - Re: [PATCH 2/2] perf lock: New subcommand "lock" to perf for analyzing lock statistics

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Date:	Mon, 07 Dec 2009 11:38:05 -0500
From:	Steven Rostedt <srostedt@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Hitoshi Mitake <mitake@....info.waseda.ac.jp>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Tom Zanussi <tzanussi@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: Re: [PATCH 2/2] perf lock: New subcommand "lock" to perf for
 analyzing lock statistics

On Mon, 2009-12-07 at 16:38 +0800, Xiao Guangrong wrote:
> 
> Ingo Molnar wrote:
> 
> > Also, i agree that the performance aspect is probably the most pressing 
> > issue. Note that 'perf bench sched messaging' is very locking intense so 
> > a 10x slowdown is not entirely unexpected - we still ought to optimize 
> > it all some more. 'perf lock' is an excellent testcase for this in any 
> > case.
> > 
> 
> Here are some test results to show the overhead of lockdep trace events:
> 
>                    select    pagefault   mmap    Memory par   Cont_SW
>                    latency    latency   latency   R/W BD      latency
> 
> disable ftrace        0         0         0         0          0
> 
> enable all ftrace  -16.65%    -109.80%   -93.62%   0.14%      -6.94%
> 
> enable all ftrace  -2.67%      1.08%     -3.65%   -0.52%      -0.68%
> except lockdep
> 
> 
> We also found big overhead when using kernbench and fio, but we haven't
> verified whether it's caused by lockdep events.

Well, it is expected that recording all locking is going to have a
substantial overhead. In my measurements, a typical event takes around
250ns per event (note, I've gotten this down to 140ns in recent updates,
and even 90ns by disabling integrity checks, but I don't want to disable
those checks in production).

Anyway, if you add just 100ns to every lock taken in the kernel, that
will definitely increase the overhead. Just enable spin_lock() in the
function tracer and watch the performance go down.  This is why, when
using the function tracer I usually add all locking to the notrace
filter. This alone helps tremendously in tracing functions.

-- Steve


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