netdev - A question about MTUs and TCP stack

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Message-ID: <A55791AD2520D54CA92150D16630D03B36116A5B@VA3DIAXVS1F1.RED001.local>
Date:	Tue, 16 Aug 2011 11:33:40 -0700
From:	Pawan Singh <psingh@...ver-peak.com>
To:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: A question about MTUs and TCP stack

Hi

I am posting this question to "netdev" mailing list because I could no longer find "linux-net" mailing list.

I find that the Linux TCP stack consumes huge amount of CPU if the MTU of an interface is set to 2400 and it is receiving 1000 byte Ethernet packets. On the other hand, if the MTU is set to 1500, the CPU consumption is reduced drastically. Increased CPU usage causes network throughput to drop considerably (from 800-900 Mbps to 200 Mbps). My kernel version is fedora core 6 and we are using 1 Gig NICs (Intel 82546GB and Broadcom NetXtreme BCM5721):

Linux he7700-tg 2.6.22.14-72.fc6 #1 SMP Wed Nov 21 14:10:25 EST 2007 x86_64 x86_64 x86_64 GNU/Linux

I do not know the TCP buffer management internals and how they are affected by MTU. Is there some FAQ/information online or do I have to open up the source code and try to identify the source of the problem. I guess I can also try newer versions of the kernel and see if the issue has been resolved.

-Pawan