netdev - Re: Understanding lock contention in __udp4_lib_mcast

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Date:	Thu, 27 Jun 2013 16:58:13 -0500
From:	Shawn Bohrer <sbohrer@...advisors.com>
To:	Or Gerlitz <or.gerlitz@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: Understanding lock contention in __udp4_lib_mcast_deliver

On Fri, Jun 28, 2013 at 12:39:38AM +0300, Or Gerlitz wrote:
> On Thu, Jun 27, 2013 at 10:22 PM, Shawn Bohrer <sbohrer@...advisors.com> 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
> >
> > The lock appears to be the udp_hslot lock protecting sockets that have
> > been hashed based on the local port.  My application contains multiple
> > processes and each process listens on multiple multicast sockets.  We
> > open one socket per multicast addr:port per machine.  All of the local
> > UDP ports are unique.  The UDP hash table has 65536 entries and it
> > appears the hash function is simply the port number plus a
> > contribution from the network namespace (I believe the namespace part
> > should be constant since we're not not using network namespaces.
> > Perhaps I should disable CONFIG_NET_NS).
> >
> > $ dmesg | grep "UDP hash"
> > [    0.606472] UDP hash table entries: 65536 (order: 9, 2097152 bytes)
> >
> > At this point I'm confused why there is any contention over this
> > spin_lock.  We should have enough hash table entries to avoid
> > collisions and all of our ports are unique.  Assuming that part is
> > true I could imagine contention if the kernel is processing two
> > packets for the same multicast addr:port in parallel but is that
> > possible?  I'm using a Mellanox ConnectX-3 card that has multiple
> > receive queues but I've always thought that all packets for a given
> > srcip:srcport:destip:destport would all be delivered to a single queue
> > and thus not processed in parallel.
> 
> But for a given dest mcast ip / udp port you can have multiple senders
> where each has different source ucast ip / udp port, isn't that?

Sure, in my case it is one sender per multicast group / port.

> anyway if you disable the udp_rss module param you can be sure that
> all traffic to dest mcast group goes to one ring, still this ring will
> be the only one active, so I am not this is wise.

I can still try disabling udp_rss and see what happens.

Thanks,
Shawn

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