netdev - Re: Throughput regression with `tcp: refine TSO autosizing`

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Date:	Wed, 04 Feb 2015 05:16:50 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Michal Kazior <michal.kazior@...to.com>,
	Neal Cardwell <ncardwell@...gle.com>
Cc:	linux-wireless <linux-wireless@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	eyalpe@....mellanox.co.il
Subject: Re: Throughput regression with `tcp: refine TSO autosizing`

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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