OK guys
Using a mlx4 testbed I can reproduce the problem by pushing coalescing
settings and disabling SG (thus disabling GSO)
ethtool -K eth0 sg off
Actual changes:
scatter-gather: off
tx-scatter-gather: off
generic-segmentation-offload: off [requested on]
ethtool -C eth0 tx-usecs 1024 tx-frames 64
Meaning that NIC waits one ms before sending the TX IRQ,
and can accumulate 64 frames before forcing the interrupt.
We probably have a bug in cwnd expansion logic :
lpaa23:~# DUMP_TCP_INFO=1 ./netperf -H 10.246.7.152 -Cc
MIGRATED TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 10.246.7.152
() port 0 AF_INET
rto=201000 ato=0 pmtu=1500 rcv_ssthresh=29200 rtt=230 rttvar=30 snd_ssthresh=41
cwnd=59 reordering=3 total_retrans=1 ca_state=0 pacing_rate=5943.1 Mbits
Recv Send Send Utilization Service Demand
Socket Socket Message Elapsed Send Recv Send Recv
Size Size Size Time Throughput local remote local remote
bytes bytes bytes secs. 10^6bits/s % S % S us/KB us/KB
87380 16384 16384 10.00 530.39 0.40 0.32 2.965 2.398
-> final cwnd=59 which is not enough to avoid the 1ms delay between each
burst.
So sender sends ~60 packets, then has to wait 1ms (to get NIC TX IRQ)
before sending the following burst.
I am CCing Neal, he probably can help to root cause the problem.
Thanks
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