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SubjectRe: [PATCH 2/2] perf lock: New subcommand "lock" to perf for analyzing lock statistics
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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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