Return-Path: Received: from one.firstfloor.org ([213.235.205.2]:36176 "EHLO one.firstfloor.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752333Ab0HRTZK (ORCPT ); Wed, 18 Aug 2010 15:25:10 -0400 Date: Wed, 18 Aug 2010 21:25:08 +0200 From: Andi Kleen To: "J. Bruce Fields" Cc: Andi Kleen , Neil Brown , Alan Cox , "Patrick J. LoPresti" , linux-fsdevel@vger.kernel.org, linux-nfs@vger.kernel.org, linux-kernel Subject: Re: Proposal: Use hi-res clock for file timestamps Message-ID: <20100818192508.GC6567@basil.fritz.box> References: <87aaolwar8.fsf@basil.nowhere.org> <20100817174134.GA23176@fieldses.org> <20100817182920.GD18161@basil.fritz.box> <20100817190447.GA28049@fieldses.org> <20100817203941.729830b7@lxorguk.ukuu.org.uk> <20100817192937.GD26609@fieldses.org> <20100818155359.66b9ddb6@notabene> <20100818175040.GA6567@basil.fritz.box> <20100818185456.GD13050@fieldses.org> Content-Type: text/plain; charset=us-ascii In-Reply-To: <20100818185456.GD13050@fieldses.org> Sender: linux-nfs-owner@vger.kernel.org List-ID: MIME-Version: 1.0 On Wed, Aug 18, 2010 at 02:54:56PM -0400, J. Bruce Fields wrote: > On Wed, Aug 18, 2010 at 07:50:40PM +0200, Andi Kleen wrote: > > > - nfsd updates it whenever it reads an mtime out of an inode that matches > > > current_fs_time to the granularity of 1/HZ. > > > > That means you have a very very hot cache line on a larger system > > if there are a lot of mtime changes. Probably a bad idea. > > Only if those mtime changes are also followed immediately by nfsd reads > of the mtime. If multiple writers are changing the same location in quick succession you have a hot cache line that gets bounced around. It doesn't need reads, although reads make it even worse. There's a lot of effort currently to make the VFS more parallel and less synchronized and it would be bad again to regress here again. -Andi -- ak@linux.intel.com -- Speaking for myself only.