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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 <[email protected]> Cc: Neal Cardwell <[email protected]> Cc: Yuchung Cheng <[email protected]> Cc: H.K. Jerry Chu <[email protected]> Cc: Tom Herbert <[email protected]> Cc: Ilpo Järvinen <[email protected]> Acked-by: Neal Cardwell <[email protected]> Signed-off-by: David S. Miller <[email protected]>
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