netdev - Re: limited network bandwidth with 3.2.x kernels

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Date:	Thu, 16 Feb 2012 20:44:52 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Neal Cardwell <ncardwell@...gle.com>
Cc:	David Miller <davem@...emloft.net>, bhutchings@...arflare.com,
	netdev@...r.kernel.org, alekcejk@...glemail.com
Subject: Re: limited network bandwidth with 3.2.x kernels

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


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