netdev - Re: [PATCH net-next] tcp: increase throughput when reordering is high

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Date:	Wed, 21 Aug 2013 22:18:00 -0400
From:	Neal Cardwell <ncardwell@...gle.com>
To:	Yuchung Cheng <ycheng@...gle.com>
Cc:	David Miller <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] tcp: increase throughput when reordering is high

On Wed, Aug 21, 2013 at 8:29 PM, Yuchung Cheng <ycheng@...gle.com> wrote:
> The stack currently detects reordering and avoid spurious
> retransmission very well. However the throughput is sub-optimal under
> high reordering because cwnd is increased only if the data is deliverd
> in order. I.e., FLAG_DATA_ACKED check in tcp_ack().  The more packet
> are reordered the worse the throughput is.
>
> Therefore when reordering is proven high, cwnd should advance whenever
> the data is delivered regardless of its ordering. If reordering is low,
> conservatively advance cwnd only on ordered deliveries in Open state,
> and retain cwnd in Disordered state (RFC5681).
>
> Using netperf on a qdisc setup of 20Mbps BW and random RTT from 45ms
> to 55ms (for reordering effect). This change increases TCP throughput
> by 20 - 25% to near bottleneck BW.
>
> A special case is the stretched ACK with new SACK and/or ECE mark.
> For example, a receiver may receive an out of order or ECN packet with
> unacked data buffered because of LRO or delayed ACK. The principle on
> such an ACK is to advance cwnd on the cummulative acked part first,
> then reduce cwnd in tcp_fastretrans_alert().
>
> Signed-off-by: Yuchung Cheng <ycheng@...gle.com>
> ---
>  net/ipv4/tcp_input.c | 32 ++++++++++++++++++++------------
>  1 file changed, 20 insertions(+), 12 deletions(-)

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

neal
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