Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754803Ab0HZXyG (ORCPT ); Thu, 26 Aug 2010 19:54:06 -0400 Received: from mail.vyatta.com ([76.74.103.46]:36487 "EHLO mail.vyatta.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752015Ab0HZXyE convert rfc822-to-8bit (ORCPT ); Thu, 26 Aug 2010 19:54:04 -0400 Date: Thu, 26 Aug 2010 16:53:59 -0700 From: Stephen Hemminger To: Marc Aurele La France Cc: Eric Dumazet , Ben Hutchings , 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 Message-ID: <20100826165359.3b79b27d@nehalam> In-Reply-To: References: <20100823080543.319143e3@nehalam> <1282672647.2302.15.camel@achroite.uk.solarflarecom.com> <1282688441.22839.34.camel@localhost> <20100824153920.63360072@s6510> <1282823827.2476.663.camel@edumazet-laptop> Organization: Vyatta X-Mailer: Claws Mail 3.7.5 (GTK+ 2.20.1; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1736 Lines: 36 On Thu, 26 Aug 2010 08:43:42 -0600 (Mountain Daylight Time) Marc Aurele La France wrote: > On Thu, 26 Aug 2010, Eric Dumazet wrote: > > Le jeudi 26 ao?t 2010 ? 05:40 -0600, Marc Aurele La France a ?crit : > > >> Steady now. There's no need to YELL nor be arrogant. You and I both know > >> there's a place for NFS over UDP. That's not changing any time soon. While > >> I'm aware of the issue you brought up, it is separate from the one at hand in > >> this discussion. > > >> I do want to thank you, however, for reminding me of TCP. It's something > >> 20/20 hindsight says I should have checked out before starting this thread. > >> Logistically, it'll be a few days before I can do so though. If that allows > >> me to increase the MTU all the way up to 65520, then this UDP thing will > >> likely remain unresolved. > > > Unfortunately, your infiniband device lacks NETIF_F_SG support. > > Oh, the device itself probably has something similar, but ipoib > (IP-over-Infiniband) doesn't export that capability. > > > MTU a bit larger than PAGE_SIZE-overhead will need high order > > allocations ? > > Right. And a 65520 MTU allocates sk_buff's with 128K contiguous payloads. Infiniband device driver needs to be fixed to do SG and checksum offload. Otherwise it is insane to try and run large MTU over it. I even wonder if the dev_change_mtu() function should reject > PAGESIZE mtu for devices that don't do scatter/gather or at least a raise a warning. -- 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/