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

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Date:	Wed, 27 Jun 2012 09:30:16 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jesper Dangaard Brouer <brouer@...hat.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 09:24 +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).

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.

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