netdev - Re: [PATCH v2 net-next] tcp: avoid tx starvation by SYNACK packets

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Date:	Wed, 27 Jun 2012 09:54:30 +0200
From:	Jesper Dangaard Brouer <brouer@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, hans.schillstrom@...csson.com,
	subramanian.vijay@...il.com, dave.taht@...il.com,
	netdev@...r.kernel.org, ncardwell@...gle.com, therbert@...gle.com,
	mph@...h.dk
Subject: Re: [PATCH v2 net-next] tcp: avoid tx starvation by SYNACK packets

On Wed, 2012-06-27 at 09:30 +0200, Eric Dumazet wrote:
> On Wed, 2012-06-27 at 09:24 +0200, Jesper Dangaard Brouer wrote:
> 
> > But, I still believe that we need, to solve this SYN issues by parallel
> > processing of packets. (It seems Eric and Hans are looking at a single
> > core SYN processing scheme, but I might have missed their point).
> 
> Yep
> 
> Parallel processing will only benefit multiqueue setups.
> 
> Many linux servers in colocations are still using a mono queue NIC, and
> default linux configuration is to use a single cpu to handle all
> incoming frames (no RPS/RFS).

I see, your target is different than mine (now I understand you
motivation).  Its good, as optimizing the single queue case, would also
be a benefit once we implement parallel processing / take advantage of
the multi queue devices.


> Sometime the hw IRQ itself is distributed among several cpus, but at one
> single moment, only one cpu is serving the NAPI poll.
> 
> As long as the LISTEN processing is locking the socket, there is no
> point distributing SYN packets to multiple cpus, this only adds
> contention and poor performance because of false sharing.
> 
> My plan is to get rid of the socket lock for LISTEN and use RCU instead.

Well, that would lead to parallel SYN processing, wouldn't it?


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