From mboxrd@z Thu Jan 1 00:00:00 1970 From: jamal Subject: Re: [e1000]: flow control on by default - good idea really? Date: Tue, 04 Jul 2006 15:20:39 -0400 Message-ID: <1152040839.5276.25.camel@jzny2> References: <1152033116.5276.22.camel@jzny2> Reply-To: hadi@cyberus.ca Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Cc: netdev@vger.kernel.org, jesse.brandeburg@intel.com, Robert Olsson , john.ronciak@intel.com, Ben Greear , Auke Kok , Jeff Garzik Return-path: Received: from mx03.cybersurf.com ([209.197.145.106]:64162 "EHLO mx03.cybersurf.com") by vger.kernel.org with ESMTP id S932345AbWGDTUx (ORCPT ); Tue, 4 Jul 2006 15:20:53 -0400 Received: from mail.cyberus.ca ([209.197.145.21]) by mx03.cybersurf.com with esmtp (Exim 4.30) id 1FxqS6-0004Px-Kg for netdev@vger.kernel.org; Tue, 04 Jul 2006 15:20:54 -0400 To: "David S. Miller" In-Reply-To: <1152033116.5276.22.camel@jzny2> Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Tue, 2006-04-07 at 13:11 -0400, jamal wrote: > CCing anybody who may have stakes on this. Ignore the email if this > doesnt interest you. > Ok, folks - i had deferred this discussion but it bit me in the ass. > I just spend an hour debugging it (and in the process blew up a gbic i > borrowed, so my day aint going well since i actually have to pay for > this and cant really do the testing i was planning to;-<). > > I have a device connected to a e1000 that was erroneously advertising > both tx/rx flow control but wasnt properly reacting to it. > The default setup on the e1000 has rx flow control turned on. > I was sending at wire rate gige from the device - which is about > 1.48Mpps. The e1000 was in turn sending me flow control packets > as per default/expected behavior. Unfortunately, it was sending > a very large amount of packets. At one point i was seeing upto > 1Mpps and on average, the flow control packets were consuming > 60-70% of the bandwidth. Even when i fixed this behavior to act > properly, allowing flow control on consumed up to 15% of the bandwidth. > Clearly, this is a bad thing. Yes, the device in the first instance was > at fault. But i have argued in the past that NAPI does just fine without > flow control being turned on, so even chewing 5% of bandwidth on flow > control is a bad thing.. > > As a compromise, can we declare flow control as an advanced feature > and turn it off by default? People who feel it is valuable and know > what they are doing can turn it off. I meant turn it on. BTW, As an addendum this default behavior changed around 2.6.16 it seems. cheers, jamal