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authorEric Dumazet <edumazet@google.com>2017-12-10 17:55:04 -0800
committerDavid S. Miller <davem@davemloft.net>2017-12-12 10:53:04 -0500
commitc3916ad9320eed8eacd7c0b2cf7f881efceda892 (patch)
treea4fdf9cc629e3e5d300c58b80626059e2ac99437
parent607065bad9931e72207b0cac365d7d4abc06bd99 (diff)
tcp: smoother receiver autotuning
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 <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>
-rw-r--r--net/ipv4/tcp_input.c19
1 files changed, 5 insertions, 14 deletions
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 2900e58738cd..fefb46c16de7 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -601,26 +601,17 @@ void tcp_rcv_space_adjust(struct sock *sk)
if (sock_net(sk)->ipv4.sysctl_tcp_moderate_rcvbuf &&
!(sk->sk_userlocks & SOCK_RCVBUF_LOCK)) {
int rcvmem, rcvbuf;
- u64 rcvwin;
+ u64 rcvwin, grow;
/* minimal window to cope with packet losses, assuming
* steady state. Add some cushion because of small variations.
*/
rcvwin = ((u64)copied << 1) + 16 * tp->advmss;
- /* If rate increased by 25%,
- * assume slow start, rcvwin = 3 * copied
- * If rate increased by 50%,
- * assume sender can use 2x growth, rcvwin = 4 * copied
- */
- if (copied >=
- tp->rcvq_space.space + (tp->rcvq_space.space >> 2)) {
- if (copied >=
- tp->rcvq_space.space + (tp->rcvq_space.space >> 1))
- rcvwin <<= 1;
- else
- rcvwin += (rcvwin >> 1);
- }
+ /* Accommodate for sender rate increase (eg. slow start) */
+ grow = rcvwin * (copied - tp->rcvq_space.space);
+ do_div(grow, tp->rcvq_space.space);
+ rcvwin += (grow << 1);
rcvmem = SKB_TRUESIZE(tp->advmss + MAX_TCP_HEADER);
while (tcp_win_from_space(sk, rcvmem) < tp->advmss)