netdev - Re: [PATCH net-next] net: ipv6: Add early demux handler for UDP unicast

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Message-Id: <20170418.111633.1943544161712972224.davem@davemloft.net>
Date:   Tue, 18 Apr 2017 11:16:33 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     simon.horman@...ronome.com
Cc:     eric.dumazet@...il.com, subashab@...eaurora.org,
        netdev@...r.kernel.org, netdev-owner@...r.kernel.org
Subject: Re: [PATCH net-next] net: ipv6: Add early demux handler for UDP
 unicast

From: Simon Horman <simon.horman@...ronome.com>
Date: Tue, 18 Apr 2017 17:09:04 +0900

> On Wed, Mar 08, 2017 at 11:22:01AM -0800, Eric Dumazet wrote:
>> On Wed, 2017-03-08 at 12:11 -0700, Subash Abhinov Kasiviswanathan wrote:
>> > On 2017-03-08 11:40, Eric Dumazet wrote:
>> > > Well, this 'optimization' actually hurts when UDP sockets are not
>> > > connected, since this adds an extra cache line miss per incoming 
>> > > packet.
>> > > 
>> > > (DNS servers for example)
>> > 
>> > Hi Eric
>> > 
>> > Thanks for your comments. Would it be preferable to disable early demux 
>> > for the
>> > servers with large unconnected workloads in that case?
>> 
>> Well, many servers handle both TCP and UDP.
>> 
>> For TCP, there is no question about early demux, this is definitely a
>> win.
>> 
>> We probably should have one sysctl to enable TCP early demux, one for
>> UDP early demux.
> 
> If early demux is a clear win for TCP then I wonder if it is
> unnecessary and by some leap also undesirable to have a configuration
> knob for that case.

For forwarding workloads it is pure overhead since the early demux will
never find a local socket, and therefore it is wasted work.