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

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Date:   Wed, 19 Apr 2017 08:06:59 +0900
From:   Simon Horman <simon.horman@...ronome.com>
To:     Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:     David Miller <davem@...emloft.net>, 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

On Tue, Apr 18, 2017 at 08:09:08PM +0200, Hannes Frederic Sowa wrote:
> 
> 
> On Tue, Apr 18, 2017, at 17:16, David Miller wrote:
> > 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.
> 
> Also for some more complicated fib rules setups the early demux logic
> could end up causing doing wrong lookups, because some route might not
> be actually local in one fib rule but it is in another one.

Thanks for the clarification. Knobs for both TCP and UDP now make perfect
sense to me.