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SubjectRe: Raise initial congestion window size / speedup slow start?
On Wed, 14 Jul 2010, David Miller wrote:

> From: Bill Davidsen <davidsen@tmr.com>
> Date: Wed, 14 Jul 2010 11:21:15 -0400
>
> > You may have to go into /proc/sys/net/core and crank up the
> > rmem_* settings, depending on your distribution.
>
> You should never, ever, have to touch the various networking sysctl
> values to get good performance in any normal setup. If you do, it's a
> bug, report it so we can fix it.
>
> I cringe every time someone says to do this, so please do me a favor
> and don't spread this further. :-)
>
> For one thing, TCP dynamically adjusts the socket buffer sizes based
> upon the behavior of traffic on the connection.
>
> And the TCP memory limit sysctls (not the core socket ones) are sized
> based upon available memory. They are there to protect you from
> situations such as having so much memory dedicated to socket buffers
> that there is none left to do other things effectively. It's a
> protective limit, rather than a setting meant to increase or improve
> performance. So like the others, leave these alone too.

What's normal? :-)

netem1% cat /proc/version
Linux version 2.6.30.10-105.2.23.fc11.x86_64 (mockbuild@x86-01.phx2.fedoraproject.org) (gcc version 4.4.1 20090725 (Red Hat 4.4.1-2) (GCC) ) #1 SMP Thu Feb 11 07:06:34 UTC 2010

Linux TCP autotuning across an 80 ms RTT cross country network path:

netem1% nuttcp -T10 -i1 192.168.1.18
14.1875 MB / 1.00 sec = 119.0115 Mbps 0 retrans
558.0000 MB / 1.00 sec = 4680.7169 Mbps 0 retrans
872.8750 MB / 1.00 sec = 7322.3527 Mbps 0 retrans
869.6875 MB / 1.00 sec = 7295.5478 Mbps 0 retrans
858.4375 MB / 1.00 sec = 7201.0165 Mbps 0 retrans
857.3750 MB / 1.00 sec = 7192.2116 Mbps 0 retrans
865.5625 MB / 1.00 sec = 7260.7193 Mbps 0 retrans
872.3750 MB / 1.00 sec = 7318.2095 Mbps 0 retrans
862.7500 MB / 1.00 sec = 7237.2571 Mbps 0 retrans
857.6250 MB / 1.00 sec = 7194.1864 Mbps 0 retrans

7504.2771 MB / 10.09 sec = 6236.5068 Mbps 11 %TX 25 %RX 0 retrans 80.59 msRTT

Manually specified 100 MB TCP socket buffer on the same path:

netem1% nuttcp -T10 -i1 -w100m 192.168.1.18
106.8125 MB / 1.00 sec = 895.9598 Mbps 0 retrans
1092.0625 MB / 1.00 sec = 9160.3254 Mbps 0 retrans
1111.2500 MB / 1.00 sec = 9322.6424 Mbps 0 retrans
1115.4375 MB / 1.00 sec = 9356.2569 Mbps 0 retrans
1116.4375 MB / 1.00 sec = 9365.6937 Mbps 0 retrans
1115.3125 MB / 1.00 sec = 9356.2749 Mbps 0 retrans
1121.2500 MB / 1.00 sec = 9405.6233 Mbps 0 retrans
1125.5625 MB / 1.00 sec = 9441.6949 Mbps 0 retrans
1130.0000 MB / 1.00 sec = 9478.7479 Mbps 0 retrans
1139.0625 MB / 1.00 sec = 9555.8559 Mbps 0 retrans

10258.5120 MB / 10.20 sec = 8440.3558 Mbps 15 %TX 40 %RX 0 retrans 80.59 msRTT

The manually selected TCP socket buffer size both ramps up
quicker and achieves a much higher steady state rate.

-Bill


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