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Date:	Mon, 19 Mar 2012 16:57:55 -0400 (EDT)
From:	David Miller <>
Subject: Re: [PATCH v2 net-next 2/2] tcp: reduce out_of_order memory use

From: Eric Dumazet <>
Date: Sun, 18 Mar 2012 14:07:47 -0700

> 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 <>

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