From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx149.postini.com [74.125.245.149]) by kanga.kvack.org (Postfix) with SMTP id 92A466B0031 for ; Fri, 9 Aug 2013 03:55:31 -0400 (EDT) Date: Fri, 9 Aug 2013 09:55:23 +0200 From: Jan Kara Subject: Re: [RFC 0/3] Add madvise(..., MADV_WILLWRITE) Message-ID: <20130809075523.GA14574@quack.suse.cz> References: <20130807134058.GC12843@quack.suse.cz> <520286A4.1020101@intel.com> <20130808101807.GB4325@quack.suse.cz> <20130808185340.GA13926@quack.suse.cz> <5204229F.8000507@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5204229F.8000507@intel.com> Sender: owner-linux-mm@kvack.org List-ID: To: Dave Hansen Cc: Andy Lutomirski , Jan Kara , linux-mm@kvack.org, linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org On Thu 08-08-13 15:58:39, Dave Hansen wrote: > I was coincidentally tracking down what I thought was a scalability > problem (turned out to be full disks :). I noticed, though, that ext4 > is about 20% slower than ext2/3 at doing write page faults (x-axis is > number of tasks): > > http://www.sr71.net/~dave/intel/page-fault-exts/cmp.html?1=ext3&2=ext4&hide=linear,threads,threads_idle,processes_idle&rollPeriod=5 > > The test case is: > > https://github.com/antonblanchard/will-it-scale/blob/master/tests/page_fault3.c The reason is that ext2/ext3 do almost nothing in their write fault handler - they are about as fast as it can get. ext4 OTOH needs to reserve blocks for delayed allocation, setup buffers under a page etc. This is necessary if you want to make sure that if data are written via mmap, they also have space available on disk to be written to (ext2 / ext3 do not care and will just drop the data on the floor if you happen to hit ENOSPC during writeback). I'm not saying ext4 write fault path cannot possibly be optimized (noone seriously looked into that AFAIK so there may well be some low hanging fruit) but it will always be slower than ext2/3. A more meaningful comparison would be with filesystems like XFS which make similar guarantees regarding data safety. Honza -- Jan Kara SUSE Labs, CR -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org