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authorEric Dumazet <edumazet@google.com>2014-02-06 15:57:10 -0800
committerDavid S. Miller <davem@davemloft.net>2014-02-06 21:28:06 -0800
commit4a5ab4e224288403b0b4b6b8c4d339323150c312 (patch)
treeafee288be08ddcb19baf988e022cc1394cf9ce83 /net/ipv4/tcp_output.c
parentdbe173079ab58a444e12dbebe96f5aec1e0bed1a (diff)
tcp: remove 1ms offset in srtt computation
TCP pacing depends on an accurate srtt estimation. Current srtt estimation is using jiffie resolution, and has an artificial offset of at least 1 ms, which can produce slowdowns when FQ/pacing is used, especially in DC world, where typical rtt is below 1 ms. We are planning a switch to usec resolution for linux-3.15, but in the meantime, this patch removes the 1 ms offset. All we need is to have tp->srtt minimal value of 1 to differentiate the case of srtt being initialized or not, not 8. The problematic behavior was observed on a 40Gbit testbed, where 32 concurrent netperf were reaching 12Gbps of aggregate speed, instead of line speed. This patch also has the effect of reporting more accurate srtt and send rates to iproute2 ss command as in : $ ss -i dst cca2 Netid State Recv-Q Send-Q Local Address:Port Peer Address:Port tcp ESTAB 0 0 10.244.129.1:56984 10.244.129.2:12865 cubic wscale:6,6 rto:200 rtt:0.25/0.25 ato:40 mss:1448 cwnd:10 send 463.4Mbps rcv_rtt:1 rcv_space:29200 tcp ESTAB 0 390960 10.244.129.1:60247 10.244.129.2:50204 cubic wscale:6,6 rto:200 rtt:0.875/0.75 mss:1448 cwnd:73 ssthresh:51 send 966.4Mbps unacked:73 retrans:0/121 rcv_space:29200 Reported-by: Vytautas Valancius <valas@google.com> Signed-off-by: Eric Dumazet <edumazet@google.com> Cc: Yuchung Cheng <ycheng@google.com> Cc: Neal Cardwell <ncardwell@google.com> Acked-by: Neal Cardwell <ncardwell@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'net/ipv4/tcp_output.c')
-rw-r--r--net/ipv4/tcp_output.c2
1 files changed, 1 insertions, 1 deletions
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 03d26b85eab8..10435b3b9d0f 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1977,7 +1977,7 @@ bool tcp_schedule_loss_probe(struct sock *sk)
/* Schedule a loss probe in 2*RTT for SACK capable connections
* in Open state, that are either limited by cwnd or application.
*/
- if (sysctl_tcp_early_retrans < 3 || !rtt || !tp->packets_out ||
+ if (sysctl_tcp_early_retrans < 3 || !tp->srtt || !tp->packets_out ||
!tcp_is_sack(tp) || inet_csk(sk)->icsk_ca_state != TCP_CA_Open)
return false;