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tcp: smoother receiver autotuning
Back in linux-3.13 (commit b0983d3c9b
("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 <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Wei Wang <weiwan@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
parent
607065bad9
commit
c3916ad932
1 changed files with 5 additions and 14 deletions
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@ -601,26 +601,17 @@ void tcp_rcv_space_adjust(struct sock *sk)
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if (sock_net(sk)->ipv4.sysctl_tcp_moderate_rcvbuf &&
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!(sk->sk_userlocks & SOCK_RCVBUF_LOCK)) {
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int rcvmem, rcvbuf;
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u64 rcvwin;
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u64 rcvwin, grow;
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/* minimal window to cope with packet losses, assuming
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* steady state. Add some cushion because of small variations.
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*/
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rcvwin = ((u64)copied << 1) + 16 * tp->advmss;
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/* If rate increased by 25%,
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* assume slow start, rcvwin = 3 * copied
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* If rate increased by 50%,
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* assume sender can use 2x growth, rcvwin = 4 * copied
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*/
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if (copied >=
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tp->rcvq_space.space + (tp->rcvq_space.space >> 2)) {
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if (copied >=
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tp->rcvq_space.space + (tp->rcvq_space.space >> 1))
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rcvwin <<= 1;
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else
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rcvwin += (rcvwin >> 1);
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}
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/* Accommodate for sender rate increase (eg. slow start) */
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grow = rcvwin * (copied - tp->rcvq_space.space);
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do_div(grow, tp->rcvq_space.space);
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rcvwin += (grow << 1);
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rcvmem = SKB_TRUESIZE(tp->advmss + MAX_TCP_HEADER);
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while (tcp_win_from_space(sk, rcvmem) < tp->advmss)
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