netdev - Re: [PATCH net-next] tcp: RFC6298 supersedes RFC2988bis

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Date:	Sun, 15 Apr 2012 17:40:06 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, hkchu@...gle.com, therbert@...gle.com
Subject: Re: [PATCH net-next] tcp: RFC6298 supersedes RFC2988bis

On Sat, 2012-04-14 at 15:47 -0400, David Miller wrote:
> From: Eric Dumazet <eric.dumazet@...il.com>

> > BTW, one side effect of the TCP_TIMEOUT_INIT change (3 -> 1) is
> > inet_csk_reqsk_queue_prune() latency is 200% worse:
> > 
> > It fires every 200ms and scans 40% of hash table each time, listener
> > socket held.
> 
> That's rather unfortunate, but I can't see an easy way around it.  We
> have to process the whole table within the timeout quantum.

I am currently working on this issue (and more generally to provide
better scalable LISTEN/SYN_RECV processing)

1) convert inet_csk_reqsk_queue_prune() to work queue instead of timer
2) use hashed spinlocks to protect syn_table[]
3) use RCU and dont hold parent socket lock to allow parallelism for
multiqueue NICS (or RPS ...)
4) use 32bit instead of 16bit for sk_max_ack_backlog/sk_ack_backlog

It occured to me that on my 12 cores machine (24 threads) and IXGBE card
(24 queues per link), a moderate SYN packets load could basically freeze
whole machine, 23 cpus waiting one cpu is done with the listener lock.

TCP processing on ESTABLISHED/TIME_WAIT sockets has RCU and all goodies,
time has come to address the LISTEN/SYN_RECV states as well.


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