netdev - Re: [RFC] Make TCP prequeue configurable

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Date:	Thu, 27 Sep 2007 15:44:32 -0700
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: [RFC] Make TCP prequeue configurable

On Fri, 28 Sep 2007 00:08:33 +0200
Eric Dumazet <dada1@...mosbay.com> wrote:

> Hi all
> 
> I am sure some of you are going to tell me that prequeue is not
> all black :)
> 
> Thank you
> 
> [RFC] Make TCP prequeue configurable
> 
> The TCP prequeue thing is based on old facts, and has drawbacks.
> 
> 1) It adds 48 bytes per 'struct tcp_sock'
> 2) It adds some ugly code in hot paths
> 3) It has a small hit ratio on typical servers using many sockets
> 4) It may have a high hit ratio on UP machines running one process,
>     where the prequeue adds litle gain. (In fact, letting the user
>     doing the copy after being woke up is better for cache reuse)
> 5) Doing a copy to user in softirq handler is not good, because of
>     potential page faults :(
> 6) Maybe the NET_DMA thing is the only thing that might need prequeue.
> 
> This patch introduces a CONFIG_TCP_PREQUEUE, automatically selected if 
> CONFIG_NET_DMA is on.
> 
> Signed-off-by: Eric Dumazet <dada1@...mosbay.com>
> 

Rather than having a two more compile cases and test cases to deal
with.  If you can prove it is useless, make a case for killing
it completely.

-- 
Stephen Hemminger <shemminger@...ux-foundation.org>
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