netdev - Re: [PATCH v2 net-next 2/2] tcp: reduce out_of

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Date:	Sun, 18 Mar 2012 23:53:08 -0400
From:	Neal Cardwell <ncardwell@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Tom Herbert <therbert@...gle.com>,
	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
	"H.K. Jerry Chu" <hkchu@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH v2 net-next 2/2] tcp: reduce out_of_order memory use

On Sun, Mar 18, 2012 at 5:07 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> With increasing receive window sizes, but speed of light not improved
> that much, out of order queue can contain a huge number of skbs, waiting
> to be moved to receive_queue when missing packets can fill the holes.
>
> Some devices happen to use fat skbs (truesize of 4096 + sizeof(struct
> sk_buff)) to store regular (MTU <= 1500) frames. This makes highly
> probable sk_rmem_alloc hits sk_rcvbuf limit, which can be 4Mbytes in
> many cases.
>
> When limit is hit, tcp stack calls tcp_collapse_ofo_queue(), a true
> latency killer and cpu cache blower.
>
> Doing the coalescing attempt each time we add a frame in ofo queue
> permits to keep memory use tight and in many cases avoid the
> tcp_collapse() thing later.
>
> Tested on various wireless setups (b43, ath9k, ...) known to use big skb
> truesize, this patch removed the "packets collapsed in receive queue due
> to low socket buffer" I had before.
>
> This also reduced average memory used by tcp sockets.
>
> With help from Neal Cardwell.
>
> Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>

Acked-by: Neal Cardwell <ncardwell@...gle.com>

neal
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