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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>