From mboxrd@z Thu Jan 1 00:00:00 1970 Subject: Re: [rfc][patch] remove racy sync_page? From: Arjan van de Ven In-Reply-To: <447D8C99.2080009@aitel.hist.no> References: <447AC011.8050708@yahoo.com.au> <20060529121556.349863b8.akpm@osdl.org> <447B8CE6.5000208@yahoo.com.au> <20060529183201.0e8173bc.akpm@osdl.org> <447BB3FD.1070707@yahoo.com.au> <447BD31E.7000503@yahoo.com.au> <447BD63D.2080900@yahoo.com.au> <447CE43A.6030700@yahoo.com.au> <447D8C99.2080009@aitel.hist.no> Content-Type: text/plain Date: Wed, 31 May 2006 14:36:25 +0200 Message-Id: <1149078985.3114.24.camel@laptopd505.fenrus.org> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Helge Hafting Cc: Linus Torvalds , Nick Piggin , linux-kernel@vger.kernel.org, linux-mm@kvack.org, mason@suse.com, andrea@suse.de, hugh@veritas.com, axboe@suse.de List-ID: On Wed, 2006-05-31 at 14:31 +0200, Helge Hafting wrote: > Linus Torvalds wrote: > > On Wed, 31 May 2006, Nick Piggin wrote: > > > >> The requests can only get merged if contiguous requests from the upper > >> layers come down, right? > >> > > > > It has nothing to do with merging. It has to do with IO patterns. > > > > Seeking. > > > > Seeking is damn expensive - much more so than command issue. People forget > > that sometimes. > > > > If you can sort the requests so that you don't have to seek back and > > forth, that's often a HUGE win. > > > > Yes, the requests will still be small, and yes, the IO might happen in 4kB > > chunks, but it happens a lot faster if you do it in a good elevator > > ordering and if you hit the track cache than if you seek back and forth. > > > This is correct, but doesn't really explain why plugging might be good. > > If requests go to disk immediately and the disk is able to keep > up with the seeks, then plugging doesn't help. This is a low-bandwith > case of course, but servicing each request immediately will > keep the latency lower. only for writes. afaik for reads and other synchronous beasts we already unplug immediately anyway. In addition unplugging happens after like 3 or 4 requests and after a few miliseconds, whatever comes first ... so you never wait really long (on a "what a seek costs" scale) anyway -- 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