On Sun, Dec 10, 2017 at 8:55 PM, Eric Dumazet <eduma...@google.com> wrote:
> Back in linux-3.13 (commit b0983d3c9b13 ("tcp: fix dynamic right sizing"))
> I addressed the pressing issues we had with receiver autotuning.
>
> But DRS suffers from extra latencies caused by rcv_rtt_est.rtt_us
> drifts. One common problem happens during slow start, since the
> apparent RTT measured by the receiver can be inflated by ~50%,
> at the end of one packet train.
>
> Also, a single drop can delay read() calls by one RTT, meaning
> tcp_rcv_space_adjust() can be called one RTT too late.
>
> By replacing the tri-modal heuristic with a continuous function,
> we can offset the effects of not growing 'at the optimal time'.
>
> The curve of the function matches prior behavior if the space
> increased by 25% and 50% exactly.
>
> Cost of added multiply/divide is small, considering a TCP flow
> typically would run this part of the code few times in its life.
>
> I tested this patch with 100 ms RTT / 1% loss link, 100 runs
> of (netperf -l 5), and got an average throughput of 4600 Mbit
> instead of 1700 Mbit.
>
> Signed-off-by: Eric Dumazet <eduma...@google.com>
> Acked-by: Soheil Hassas Yeganeh <soh...@google.com>
> Acked-by: Wei Wang <wei...@google.com>
> ---Acked-by: Neal Cardwell <ncardw...@google.com> Thanks, Eric! neal
