From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ben Greear Subject: MTU and TCP transmit offload. Date: Wed, 21 Sep 2011 14:06:54 -0700 Message-ID: <4E7A51EE.8010403@candelatech.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit To: netdev Return-path: Received: from mail.candelatech.com ([208.74.158.172]:60951 "EHLO ns3.lanforge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750962Ab1IUVGz (ORCPT ); Wed, 21 Sep 2011 17:06:55 -0400 Received: from [192.168.100.111] (firewall.candelatech.com [70.89.124.249]) (authenticated bits=0) by ns3.lanforge.com (8.14.2/8.14.2) with ESMTP id p8LL6sPO009958 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO) for ; Wed, 21 Sep 2011 14:06:54 -0700 Sender: netdev-owner@vger.kernel.org List-ID: We saw something interesting while doing some testing on 3.0.4. We configured 2 Ethernet NICs with standard 1500 MTU, and added a mac-vlan on each, with MTU of 300. The goal was to generate as many ~300 byte TCP packets as possible, for load testing purposes. We configured our tool to open sockets on the mac-vlans and send/receive TCP (IPv4) traffic. This actually seems to work quite nicely, allowing user-space to do large writes (24k in our case), and it appears have lots of small packets on the wire. We still need to sniff with external system to verify this..but packets-per-second counters look good. Evidently this all works because macvlans know that the NIC can do TSO, and the '300' MTU is passed in the big packet given to the NIC. This got me thinking...at least for my purposes, it would be nice to have a per-socket 'MTU' setting. The idea is that you could ask the NIC to do the TSO at whatever 'mtu' you wanted, without having to resort to mac-vlans with artificially small MTU. So, is there any interest in supporting such a socket option? I can't think of any use besides TCP traffic load testing, but perhaps someone else can think of one? Or, is load-testing enough? Thanks, Ben -- Ben Greear Candela Technologies Inc http://www.candelatech.com