netdev - Re: Heavy spin_lock contention in __udp4_lib_mcast

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Date:	Thu, 26 Apr 2012 18:31:28 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Shawn Bohrer <sbohrer@...advisors.com>
Cc:	netdev@...r.kernel.org
Subject: Re: Heavy spin_lock contention in __udp4_lib_mcast_deliver increase

On Thu, 2012-04-26 at 11:28 -0500, Shawn Bohrer wrote:

> No in this case it is 300 unique multicast addresses, and there is one
> socket listening to each multicast address.  So a single message is
> only copied once to a single socket.  The bottle neck appears to be
> that even though a single message is only going to get copied to a
> single socket we still have to walk the list of all 300 sockets while
> holding the spin lock to figure that out.  The incoming packet rate is
> also roughly evenly distributed across all 300 multicast addresses so
> even though we have multiple receive queues they are all contending
> for the same spin lock.
> 

I repeat my question : Are these 300 sockets bound to the same UDP
port ?

If not, they should be spreaded in hash table.

You can make this hash table very big to reduce hash collisions

Boot parameter : uhash_entries=65536



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