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

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Date:	Thu, 22 Aug 2013 14:40:17 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	ycheng@...gle.com
Cc:	ncardwell@...gle.com, edumazet@...gle.com, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] tcp: increase throughput when reordering is
 high

From: Yuchung Cheng <ycheng@...gle.com>
Date: Wed, 21 Aug 2013 17:29:23 -0700

> 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>

Applied, your team is doing great work on the TCP stack!
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