From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pb0-f42.google.com (mail-pb0-f42.google.com [209.85.160.42]) by kanga.kvack.org (Postfix) with ESMTP id 048276B003B for ; Mon, 3 Mar 2014 18:37:10 -0500 (EST) Received: by mail-pb0-f42.google.com with SMTP id rr13so4405587pbb.15 for ; Mon, 03 Mar 2014 15:37:10 -0800 (PST) Received: from mail.linuxfoundation.org (mail.linuxfoundation.org. [140.211.169.12]) by mx.google.com with ESMTP id a3si8449259pay.194.2014.03.03.15.37.09 for ; Mon, 03 Mar 2014 15:37:09 -0800 (PST) Date: Mon, 3 Mar 2014 15:37:07 -0800 From: Andrew Morton Subject: Re: [PATCH 0/1] mm, shmem: map few pages around fault address if they are in page cache Message-Id: <20140303153707.beced5c271179d1b1658a246@linux-foundation.org> In-Reply-To: References: <1393625931-2858-1-git-send-email-quning@google.com> <20140228174150.8ff4edca.akpm@linux-foundation.org> <20140303143834.90ebe8ec5c6a369e54a599ec@linux-foundation.org> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Linus Torvalds Cc: Ning Qu , Mel Gorman , Rik van Riel , "Kirill A. Shutemov" , Hugh Dickins , Andi Kleen , Matthew Wilcox , Dave Hansen , Alexander Viro , Dave Chinner , linux-mm , linux-fsdevel , Linux Kernel Mailing List On Mon, 3 Mar 2014 15:29:00 -0800 Linus Torvalds wrote: > On Mon, Mar 3, 2014 at 2:38 PM, Andrew Morton wrote: > > > > When the file is uncached, results are peculiar: > > > > 0.00user 2.84system 0:50.90elapsed 5%CPU (0avgtext+0avgdata 4198096maxresident)k > > 0inputs+0outputs (1major+49666minor)pagefaults 0swaps > > > > That's approximately 3x more minor faults. > > This is not peculiar. > > When the file is uncached, some pages will obviously be under IO due > to readahead etc. And the fault-around code very much on purpose will > *not* try to wait for those pages, so any busy pages will just simply > not be faulted-around. Of course. > So you should still have fewer minor faults than faulting on *every* > page (ie the non-fault-around case), but I would very much expect that > fault-around will not see the full "one sixteenth" reduction in minor > faults. > > And the order of IO will not matter, since the read-ahead is > asynchronous wrt the page-faults. When a pagefault hits a locked, not-uptodate page it is going to block. Once it wakes up we'd *like* to find lots of now-uptodate pages in that page's vicinity. Obviously, that is happening, but not to the fullest possible extent. We _could_ still achieve the 16x if readahead was cooperating in an ideal fashion. I don't know what's going on in there to produce this consistent 3x factor. -- 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