netdev - Re: Understanding lock contention in __udp4_lib_mcast

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Date:	Thu, 27 Jun 2013 15:20:08 -0500
From:	Shawn Bohrer <sbohrer@...advisors.com>
To:	Rick Jones <rick.jones2@...com>
Cc:	netdev@...r.kernel.org
Subject: Re: Understanding lock contention in __udp4_lib_mcast_deliver

On Thu, Jun 27, 2013 at 12:58:39PM -0700, Rick Jones wrote:
> On 06/27/2013 12:22 PM, Shawn Bohrer wrote:
> >I'm looking for opportunities to improve the multicast receive
> >performance for our application, and I thought I'd spend some time
> >trying to understand what I thought might be a small/simple
> >improvement.  Profiling with perf I see that there is spin_lock
> >contention in __udp4_lib_mcast_deliver:
> >
> >0.68%  swapper  [kernel.kallsyms]               [k] _raw_spin_lock
> >        |
> >        --- _raw_spin_lock
> >           |
> >           |--24.13%-- perf_adjust_freq_unthr_context.part.21
> >           |
> >           |--22.40%-- scheduler_tick
> >           |
> >           |--14.96%-- __udp4_lib_mcast_deliver
> 
> Are there other processes showing _raw_spin_lock time?  It may be
> more clear to add a --sort symbol,dso or some such to your perf
> report command.  Because what you show there suggests less than 1%
> of the active cycles are in _raw_spin_lock.

You think I'm wasting time going after small potatoes huh?  On a
normal system it looks like it is about .12% total which is indeed
small but my thinking was that I should be able to make that go to 0
easily by ensuring we use unique ports and only have one socket per
multicast addr:port.  Now that I've failed at making it go to 0 I
would mostly like to understand what part of my thinking was flawed.
Or perhaps I can make it go to zero if I do ...

--
Shawn

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