From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx164.postini.com [74.125.245.164]) by kanga.kvack.org (Postfix) with SMTP id 284B76B13F0 for ; Wed, 1 Feb 2012 15:22:43 -0500 (EST) Date: Wed, 1 Feb 2012 12:22:41 -0800 From: Andrew Morton Subject: Re: [PATCH] fix readahead pipeline break caused by block plug Message-Id: <20120201122241.db4056f6.akpm@linux-foundation.org> In-Reply-To: <20120201201017.GC13246@redhat.com> References: <1327996780.21268.42.camel@sli10-conroe> <20120131220333.GD4378@redhat.com> <20120131141301.ba35ffe0.akpm@linux-foundation.org> <20120131222217.GE4378@redhat.com> <20120201033653.GA12092@redhat.com> <20120201091807.GA7451@infradead.org> <20120201201017.GC13246@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Vivek Goyal Cc: Christoph Hellwig , Shaohua Li , lkml , linux-mm , Jens Axboe , Herbert Poetzl , Eric Dumazet , Wu Fengguang On Wed, 1 Feb 2012 15:10:17 -0500 Vivek Goyal wrote: > On Wed, Feb 01, 2012 at 04:18:07AM -0500, Christoph Hellwig wrote: > > On Tue, Jan 31, 2012 at 10:36:53PM -0500, Vivek Goyal wrote: > > > I still see that IO is being submitted one page at a time. The only > > > real difference seems to be that queue unplug happening at random times > > > and many a times we are submitting much smaller requests (40 sectors, 48 > > > sectors etc). > > > > This is expected given that the block device node uses > > block_read_full_page, and not mpage_readpage(s). > > What is the difference between block_read_full_page() and > mpage_readpage(). block_read_full_page() will attach buffer_heads to the page and will perform IO via those buffer_heads. mpage_readpage() feeds the page directly to the BIO layer and leaves it without attached buffer_heads. > IOW, why block device does not use mpage_readpage(s) > interface? We've tried it in the past and problems ensued. A quick google search for blkdev_readpages turns up stuff like http://us.generation-nt.com/answer/patch-add-readpages-support-block-devices-help-201462802.html > Applying following patch improved the speed from 110MB/s to more than > 230MB/s. Yeah. It should be doable - it would be a matter of hunting down and squishing the oddball corner cases. -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org