Re: limited network bandwidth with 3.2.x kernels

Eric Dumazet
2012-02-16T19:44:58Z

Le jeudi 16 février 2012 à 19:22 +0100, Eric Dumazet a écrit :

> On sender side, there are also issues, because if I have TSO on or off
> on my 'netem delay 50ms' machine, performance is completely different.

Well, my sender is a 32bit kernel, so tcp_wmem[2] was less than
1Mbytes..

$ cat /proc/sys/net/ipv4/tcp_wmem
4096 16384 903780

Socket cwnd is then capped to ~400 with TSO=off, and ~1500 with TSO=on

TSO off, a 1500 bytes frame now has truesize=2048+192, while in previous
kernel it had truesize=1500+192 before linux 3.2

(903780/2240 = 403)

(TSO permits a better truesize/len ratio, close to 1)

With 50ms RTT, 400 packets in flight means no more than 90Mbit/s