Return-Path: Received: from bld-mail15.adl6.internode.on.net ([150.101.137.100]:53932 "EHLO mail.internode.on.net" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1755573Ab0BXHjp (ORCPT ); Wed, 24 Feb 2010 02:39:45 -0500 Date: Wed, 24 Feb 2010 18:39:40 +1100 From: Dave Chinner To: Wu Fengguang Cc: Trond Myklebust , "linux-nfs@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , Linux Memory Management List , LKML Subject: Re: [RFC] nfs: use 2*rsize readahead size Message-ID: <20100224073940.GJ16175@discord.disaster> References: <20100224024100.GA17048@localhost> <20100224032934.GF16175@discord.disaster> <20100224041822.GB27459@localhost> <20100224052215.GH16175@discord.disaster> <20100224061247.GA8421@localhost> Content-Type: text/plain; charset=us-ascii In-Reply-To: <20100224061247.GA8421@localhost> Sender: linux-nfs-owner@vger.kernel.org List-ID: MIME-Version: 1.0 On Wed, Feb 24, 2010 at 02:12:47PM +0800, Wu Fengguang wrote: > On Wed, Feb 24, 2010 at 01:22:15PM +0800, Dave Chinner wrote: > > What I'm trying to say is that while I agree with your premise that > > a 7.8MB readahead window is probably far larger than was ever > > intended, I disagree with your methodology and environment for > > selecting a better default value. The default readahead value needs > > to work well in as many situations as possible, not just in perfect > > 1:1 client/server environment. > > Good points. It's imprudent to change a default value based on one > single benchmark. Need to collect more data, which may take time.. Agreed - better to spend time now to get it right... > > > It sounds silly to have > > > > > > client_readahead_size > server_readahead_size > > > > I don't think it is - the client readahead has to take into account > > the network latency as well as the server latency. e.g. a network > > with a high bandwidth but high latency is going to need much more > > client side readahead than a high bandwidth, low latency network to > > get the same throughput. Hence it is not uncommon to see larger > > readahead windows on network clients than for local disk access. > > Hmm I wonder if I can simulate a high-bandwidth high-latency network > with e1000's RxIntDelay/TxIntDelay parameters.. I think netem is the blessed method of emulating different network behaviours. There's a howto+faq for setting it up here: http://www.linuxfoundation.org/collaborate/workgroups/networking/netem Cheers, Dave. -- Dave Chinner david@fromorbit.com