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

[Eric Dumazet <eric.dumazet@gmail.com>]

On Wed, 2013-08-21 at 17:29 -0700, Yuchung Cheng 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@google.com>
> ---
>  net/ipv4/tcp_input.c | 32 ++++++++++++++++++++------------
>  1 file changed, 20 insertions(+), 12 deletions(-)

Acked-by: Eric Dumazet <edumazet@google.com>