Re: perf for analyzing userspace contention

[David Ahern <dsahern@gmail.com>]

On 10/17/2011 05:43 PM, Arun Sharma wrote:
> On Thu, Oct 13, 2011 at 11:36:23PM -0700, Roland Dreier wrote:
>> Hi everyone,
>>
>> Hope this question from a kernel hacker about profiling userspace
>> isn't too dumb...
>>
>> Anyway, suppose I have a multithreaded userspace app that uses a bunch of
>> pthread_mutexes, and I want to figure out which locks are hot and/or heavily
>> contended.  What's the best way to do that?  Is perf the right, or is there
>> something better?  (This seems like such an obvious thing to want that there
>> must be some good way to get this data, I hope)
> 
> Sampling on sys_futex entry/exit is a good start. But it doesn't tell
> you if the thread spent 1us or 1ms waiting on the futex. We really need
> to add weights to the profile based on sleep times (or other criteria).

Doesn't the delta tell you the time it spends waiting?

> 
> You might want to follow further discussion in the thread with the
> subject "Profiling sleep times?".
> 
> perf record -ag -e cs -- sleep 1

And augmenting with cs (isn't the -c1 required? I've always added it for
software events) tells you if it was scheduled out.

David


> 
> is a more general version of this. It helps in understanding what's
> causing the thread to give up CPU voluntarily (you might have to filter
> the output a bit).
> 
>  -Arun
> 
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