Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755383Ab0HXPVd (ORCPT ); Tue, 24 Aug 2010 11:21:33 -0400 Received: from smtp.srv.ualberta.ca ([129.128.5.19]:57487 "EHLO mail5.srv.ualberta.ca" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1754315Ab0HXPVa (ORCPT ); Tue, 24 Aug 2010 11:21:30 -0400 X-Greylist: delayed 336 seconds by postgrey-1.27 at vger.kernel.org; Tue, 24 Aug 2010 11:21:30 EDT Date: Tue, 24 Aug 2010 09:14:54 -0600 (Mountain Daylight Time) From: Marc Aurele La France To: Stephen Hemminger cc: linux-kernel@vger.kernel.org, netdev@vger.kernel.org, "David S. Miller" , Alexey Kuznetsov , "Pekka Savola (ipv6)" , James Morris , Hideaki YOSHIFUJI , Patrick McHardy Subject: Re: RFC: MTU for serving NFS on Infiniband In-Reply-To: <20100823080543.319143e3@nehalam> Message-ID: References: <20100823080543.319143e3@nehalam> User-Agent: Alpine 2.00 (WNT 1167 2008-08-23) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2240 Lines: 41 On Mon, 23 Aug 2010, Stephen Hemminger wrote: > On Mon, 23 Aug 2010 08:44:37 -0600 (MDT) > Marc Aurele La France wrote: >> In regrouping for my next tack at this, I noticed that all stack traces go >> through ip_append_data(). This would be ipv6_append_data() in the IPv6 case. >> A _very_ rough draft that would have ip_append_data() temporarily drop down >> to a smaller fake MTU follows ... > Why doesn't NFS generate page size fragments? Does Infiniband or your > device not support this? Any thing that requires higher order allocation > is going to unstable under load. Let's fix the cause not the apply bandaid > solution to the symptom. >From what I can tell, IP fragmentation is done centrally. The MTU is a device attribute, yes. But, here, it is ip_append_data(), not NFS nor the device driver, whose responsibility it is to break up the payload into fragments, either by itself or using any facility supported by the adapter. What I'm saying is that there's no reason to require all fragments, except the last, to be MTU-sized. The RFCs I've looked at allow them to be shorter which can be used to advantage when MTU-sized fragments cannot be allocated in a memory fragmentation scenario, instead of reporting an error. Marc. +----------------------------------+----------------------------------+ | Marc Aurele La France | work: 1-780-492-9310 | | Academic Information and | fax: 1-780-492-1729 | | Communications Technologies | email: tsi@ualberta.ca | | 352 General Services Building +----------------------------------+ | University of Alberta | | | Edmonton, Alberta | Standard disclaimers apply | | T6G 2H1 | | | CANADA | | +----------------------------------+----------------------------------+ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/