diff options
| author | Eric Dumazet | 2019-12-23 12:27:53 -0800 |
|---|---|---|
| committer | David S. Miller | 2019-12-27 16:29:14 -0800 |
| commit | 42f3a8aaae66d31d87850fb4b02979a0fc5dc541 (patch) | |
| tree | cc2f39bd39273825a510e8267ac269915b0a16df /net | |
| parent | cff04e2da308c522f654237b45dd64248fe8d1fa (diff) | |
tcp_cubic: tweak Hystart detection for short RTT flows
After switching ca->delay_min to usec resolution, we exit
slow start prematurely for very low RTT flows, setting
snd_ssthresh to 20.
The reason is that delay_min is fed with RTT of small packet
trains. Then as cwnd is increased, TCP sends bigger TSO packets.
LRO/GRO aggregation and/or interrupt mitigation strategies
on receiver tend to inflate RTT samples.
Fix this by adding to delay_min the expected delay of
two TSO packets, given current pacing rate.
Tested:
Sender uses pfifo_fast qdisc
Before :
$ nstat -n;for f in {1..10}; do ./super_netperf 1 -H lpaa24 -l -4000000; done;nstat|egrep "Hystart"
11348
11707
11562
11428
11773
11534
9878
11693
10597
10968
TcpExtTCPHystartTrainDetect 10 0.0
TcpExtTCPHystartTrainCwnd 200 0.0
After :
$ nstat -n;for f in {1..10}; do ./super_netperf 1 -H lpaa24 -l -4000000; done;nstat|egrep "Hystart"
14877
14517
15797
18466
17376
14833
17558
17933
16039
18059
TcpExtTCPHystartTrainDetect 10 0.0
TcpExtTCPHystartTrainCwnd 1670 0.0
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'net')
| -rw-r--r-- | net/ipv4/tcp_cubic.c | 23 |
1 files changed, 21 insertions, 2 deletions
diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c index 068775b91fb5..0e5428ed04fe 100644 --- a/net/ipv4/tcp_cubic.c +++ b/net/ipv4/tcp_cubic.c @@ -436,8 +436,27 @@ static void bictcp_acked(struct sock *sk, const struct ack_sample *sample) delay = 1; /* first time call or link delay decreases */ - if (ca->delay_min == 0 || ca->delay_min > delay) - ca->delay_min = delay; + if (ca->delay_min == 0 || ca->delay_min > delay) { + unsigned long rate = READ_ONCE(sk->sk_pacing_rate); + + /* Account for TSO/GRO delays. + * Otherwise short RTT flows could get too small ssthresh, + * since during slow start we begin with small TSO packets + * and could lower ca->delay_min too much. + * Ideally even with a very small RTT we would like to have + * at least one TSO packet being sent and received by GRO, + * and another one in qdisc layer. + * We apply another 100% factor because @rate is doubled at + * this point. + * We cap the cushion to 1ms. + */ + if (rate) + delay += min_t(u64, USEC_PER_MSEC, + div64_ul((u64)GSO_MAX_SIZE * + 4 * USEC_PER_SEC, rate)); + if (ca->delay_min == 0 || ca->delay_min > delay) + ca->delay_min = delay; + } /* hystart triggers when cwnd is larger than some threshold */ if (!ca->found && hystart && tcp_in_slow_start(tp) && |