From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andy Lutomirski Subject: Re: [PATCH v10 00/21] Support ext4 on NV-DIMMs Date: Wed, 27 Aug 2014 18:30:27 -0700 Message-ID: <53FE8633.10305@amacapital.net> References: <20140827130613.c8f6790093d279a447196f17@linux-foundation.org> <20140827211250.GH3285@linux.intel.com> <20140827144622.ed81195a1d94799bb57a3207@linux-foundation.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: Matthew Wilcox , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org To: Andrew Morton , Matthew Wilcox Return-path: Received: from mail-pa0-f45.google.com ([209.85.220.45]:48496 "EHLO mail-pa0-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751086AbaH1Bab (ORCPT ); Wed, 27 Aug 2014 21:30:31 -0400 Received: by mail-pa0-f45.google.com with SMTP id bj1so344791pad.32 for ; Wed, 27 Aug 2014 18:30:30 -0700 (PDT) In-Reply-To: <20140827144622.ed81195a1d94799bb57a3207@linux-foundation.org> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On 08/27/2014 02:46 PM, Andrew Morton wrote: > I assume (because I wasn't told!) that there are two objectives here: > > 1) reduce memory consumption by not maintaining pagecache and > 2) reduce CPU cost by avoiding the double-copies. > > These things are pretty easily quantified. And really they must be > quantified as part of the developer testing, because if you find > they've worsened then holy cow, what went wrong. > There are two more huge ones: 3) Writes via mmap are immediately durable (or at least they're durable after a *very* lightweight flush). 4) No page faults ever once a page is writable (I hope -- I'm not sure whether this series actually achieves that goal). A note on #3: there is ongoing work to enable write-through memory for things like this. Once that's done, then writes via mmap might actually be synchronously durable, depending on chipset details. --Andy