From mboxrd@z Thu Jan 1 00:00:00 1970 From: Fengguang Wu Subject: Re: [PATCH] mm: readahead: remove redundant ra_pages in file_ra_state Date: Fri, 26 Oct 2012 16:02:40 +0800 Message-ID: <20121026080240.GA13662@localhost> References: <20121025025826.GB23462@localhost> <20121026002544.GI29378@dastard> <20121026012758.GA6282@localhost> <5089F5AD.5040708@gmail.com> <20121026065855.GA9179@localhost> <508A35B0.30106@gmail.com> <20121026070936.GA12282@localhost> <508A399D.6000506@gmail.com> <20121026073630.GA12886@localhost> <508A4007.5080906@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Dave Chinner , YingHang Zhu , akpm@linux-foundation.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org To: Ni zhan Chen Return-path: Content-Disposition: inline In-Reply-To: <508A4007.5080906@gmail.com> Sender: owner-linux-mm@kvack.org List-Id: linux-fsdevel.vger.kernel.org On Fri, Oct 26, 2012 at 03:47:19PM +0800, Ni zhan Chen wrote: > On 10/26/2012 03:36 PM, Fengguang Wu wrote: > >On Fri, Oct 26, 2012 at 03:19:57PM +0800, Ni zhan Chen wrote: > >>On 10/26/2012 03:09 PM, Fengguang Wu wrote: > >>>On Fri, Oct 26, 2012 at 03:03:12PM +0800, Ni zhan Chen wrote: > >>>>On 10/26/2012 02:58 PM, Fengguang Wu wrote: > >>>>>> static void shrink_readahead_size_eio(struct file *filp, > >>>>>> struct file_ra_state *ra) > >>>>>> { > >>>>>>- ra->ra_pages /= 4; > >>>>>>+ spin_lock(&filp->f_lock); > >>>>>>+ filp->f_mode |= FMODE_RANDOM; > >>>>>>+ spin_unlock(&filp->f_lock); > >>>>>> > >>>>>>As the example in comment above this function, the read maybe still > >>>>>>sequential, and it will waste IO bandwith if modify to FMODE_RANDOM > >>>>>>directly. > >>>>>Yes immediately disabling readahead may hurt IO performance, the > >>>>>original '/ 4' may perform better when there are only 1-3 IO errors > >>>>>encountered. > >>>>Hi Fengguang, > >>>> > >>>>Why the number should be 1-3? > >>>The original behavior is '/= 4' on each error. > >>> > >>>After 1 errors, readahead size will be shrinked by 1/4 > >>>After 2 errors, readahead size will be shrinked by 1/16 > >>>After 3 errors, readahead size will be shrinked by 1/64 > >>>After 4 errors, readahead size will be effectively 0 (disabled) > >>But from function shrink_readahead_size_eio and its caller > >>filemap_fault I can't find the behavior you mentioned. How you > >>figure out it? > >It's this line in shrink_readahead_size_eio(): > > > > ra->ra_pages /= 4; > > Yeah, I mean why the 4th readahead size will be 0(disabled)? What's > the original value of ra->ra_pages? How can guarantee the 4th shrink > readahead size can be 0? Ah OK, I'm talking about the typical case. The default readahead size is 128k, which will become 0 after / 256. The reasonable good ra size for hard disks is 1MB=256pages, which also becomes 1page after 4 errors. Thanks, Fengguang -- 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