From: Steve Rago Subject: Re: [PATCH] improve the performance of large sequential write NFS workloads Date: Fri, 18 Dec 2009 16:20:11 -0500 Message-ID: <1261171211.1947.135.camel@serenity> References: <1261015420.1947.54.camel@serenity> <1261037877.27920.36.camel@laptop> <1261164799.1947.123.camel@serenity> <20091218194129.GB6153@elte.hu> Mime-Version: 1.0 Content-Type: text/plain Cc: Peter Zijlstra , linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org, Trond.Myklebust@netapp.com, Wu Fengguang , "jens.axboe" To: Ingo Molnar Return-path: Received: from mail.nec-labs.com ([138.15.200.209]:50382 "EHLO mail.nec-labs.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932608AbZLRVUP (ORCPT ); Fri, 18 Dec 2009 16:20:15 -0500 In-Reply-To: <20091218194129.GB6153@elte.hu> Sender: linux-nfs-owner@vger.kernel.org List-ID: On Fri, 2009-12-18 at 20:41 +0100, Ingo Molnar wrote: > * Steve Rago wrote: > > > > Also, I don't think this needs to have a sysctl, it should just work. > > > > The sysctl is a *good thing* in that it allows the eager writeback behavior > > to be tuned and shut off if need be. I can only test the changes on a > > finite set of systems, so better safe than sorry. > > This issue has been settled many years ago and that's not what we do in the > Linux kernel. We prefer patches to core code where we are reasonably sure they > result in good behavior - and then we fix bugs in the new behavior, if any. > > (Otherwise odd sysctls would mushroom quickly and the system would become > untestable in practice.) > > Ingo I don't disagree, but "that's not what we do" hardly provides insight into making the judgment call. In this case, the variety of combinations of NFS server speed, NFS client speed, transmission link speed, client memory size, and server memory size argues for a tunable parameter, because one value probably won't work well in all combinations. Making it change dynamically based on these parameters is more complicated than these circumstances call for, IMHO. Steve