From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754032Ab1K2Pf4 (ORCPT ); Tue, 29 Nov 2011 10:35:56 -0500 Received: from cantor2.suse.de ([195.135.220.15]:41396 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751672Ab1K2Pfy (ORCPT ); Tue, 29 Nov 2011 10:35:54 -0500 Date: Tue, 29 Nov 2011 16:35:52 +0100 From: Jan Kara To: Wu Fengguang Cc: Andrew Morton , Andi Kleen , Li Shaohua , Linux Memory Management List , linux-fsdevel@vger.kernel.org, LKML Subject: Re: [PATCH 8/9] readahead: basic support for backwards prefetching Message-ID: <20111129153552.GP5635@quack.suse.cz> References: <20111129130900.628549879@intel.com> <20111129131456.925952168@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20111129131456.925952168@intel.com> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 29-11-11 21:09:08, Wu Fengguang wrote: > Add the backwards prefetching feature. It's pretty simple if we don't > support async prefetching and interleaved reads. > > Here is the behavior with an 8-page read sequence from 10000 down to 0. > (The readahead size is a bit large since it's an NFS mount.) > > readahead-random(dev=0:16, ino=3948605, req=10000+8, ra=10000+8-0, async=0) = 8 > readahead-backwards(dev=0:16, ino=3948605, req=9992+8, ra=9968+32-0, async=0) = 32 > readahead-backwards(dev=0:16, ino=3948605, req=9960+8, ra=9840+128-0, async=0) = 128 > readahead-backwards(dev=0:16, ino=3948605, req=9832+8, ra=9584+256-0, async=0) = 256 > readahead-backwards(dev=0:16, ino=3948605, req=9576+8, ra=9072+512-0, async=0) = 512 > readahead-backwards(dev=0:16, ino=3948605, req=9064+8, ra=8048+1024-0, async=0) = 1024 > readahead-backwards(dev=0:16, ino=3948605, req=8040+8, ra=6128+1920-0, async=0) = 1920 > readahead-backwards(dev=0:16, ino=3948605, req=6120+8, ra=4208+1920-0, async=0) = 1920 > readahead-backwards(dev=0:16, ino=3948605, req=4200+8, ra=2288+1920-0, async=0) = 1920 > readahead-backwards(dev=0:16, ino=3948605, req=2280+8, ra=368+1920-0, async=0) = 1920 > readahead-backwards(dev=0:16, ino=3948605, req=360+8, ra=0+368-0, async=0) = 368 > > And a simple 1-page read sequence from 10000 down to 0. > > readahead-random(dev=0:16, ino=3948605, req=10000+1, ra=10000+1-0, async=0) = 1 > readahead-backwards(dev=0:16, ino=3948605, req=9999+1, ra=9996+4-0, async=0) = 4 > readahead-backwards(dev=0:16, ino=3948605, req=9995+1, ra=9980+16-0, async=0) = 16 > readahead-backwards(dev=0:16, ino=3948605, req=9979+1, ra=9916+64-0, async=0) = 64 > readahead-backwards(dev=0:16, ino=3948605, req=9915+1, ra=9660+256-0, async=0) = 256 > readahead-backwards(dev=0:16, ino=3948605, req=9659+1, ra=9148+512-0, async=0) = 512 > readahead-backwards(dev=0:16, ino=3948605, req=9147+1, ra=8124+1024-0, async=0) = 1024 > readahead-backwards(dev=0:16, ino=3948605, req=8123+1, ra=6204+1920-0, async=0) = 1920 > readahead-backwards(dev=0:16, ino=3948605, req=6203+1, ra=4284+1920-0, async=0) = 1920 > readahead-backwards(dev=0:16, ino=3948605, req=4283+1, ra=2364+1920-0, async=0) = 1920 > readahead-backwards(dev=0:16, ino=3948605, req=2363+1, ra=444+1920-0, async=0) = 1920 > readahead-backwards(dev=0:16, ino=3948605, req=443+1, ra=0+444-0, async=0) = 444 > > CC: Andi Kleen > CC: Li Shaohua > Signed-off-by: Wu Fengguang Someone already mentioned this earlier and I don't think I've seen a response: Do you have a realistic usecase for this? I don't think I've ever seen an application reading file backwards... > --- linux-next.orig/include/linux/fs.h 2011-11-29 20:55:27.000000000 +0800 > +++ linux-next/include/linux/fs.h 2011-11-29 20:57:07.000000000 +0800 ... > @@ -676,6 +677,20 @@ ondemand_readahead(struct address_space > } > > /* > + * backwards reading > + */ > + if (offset < ra->start && offset + req_size >= ra->start) { > + ra->pattern = RA_PATTERN_BACKWARDS; > + ra->size = get_next_ra_size(ra, max); > + max = ra->start; > + if (ra->size > max) > + ra->size = max; > + ra->async_size = 0; > + ra->start -= ra->size; IMHO much more obvious way to write this is: ra->size = get_next_ra_size(ra, max); if (ra->size > ra->start) { ra->size = ra->start; ra->start = 0; } else ra->start -= ra->size; Honza -- Jan Kara SUSE Labs, CR