From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <46AC3642.6C60A0EF@earthlink.net> Date: Sat, 28 Jul 2007 23:40:02 -0700 From: Erblichs MIME-Version: 1.0 Subject: Re: How can we make page replacement smarter (was: swap-prefetch) References: <200707272243.02336.a1426z@gawab.com> <200707280717.41250.a1426z@gawab.com> <46AAEFC4.8000006@redhat.com> <200707281411.57823.a1426z@gawab.com> <46AC1297.9030009@redhat.com> Content-Type: text/plain; charset=iso-8859-2 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Rik van Riel Cc: Al Boldi , Chris Snook , linux-kernel@vger.kernel.org, linux-mm@kvack.org List-ID: Inline.. Mitchell Erblich Rik van Riel wrote: > > Al Boldi wrote: > > Chris Snook wrote: > > >> At best, reads can be read-ahead and cached, which is why > >> sequential swap-in sucks less. On-demand reads are as expensive as I/O > >> can get. > > > > Which means that it should be at least as fast as swap-out, even faster > > because write to disk is usually slower than read on modern disks. But > > linux currently shows a distinct 2x slowdown for sequential swap-in wrt > > swap-out. > > That's because writes are faster than reads in moderate > quantities. Assuming that the write is not a partial write based on first doing a read.. Yes, a COW FS minimes this condition (ex: ZFS) However, since writes are mostly asynch in nature most writers shouldn't care when the write is actually commited, just that the data is stable at some point in the future.. Thus, who would care (as long as we are not waiting for the write to complete) if the write was slower. IMO, it would make sense to ALMOST always generate a certain amount of writable data before the write is completed to attempt for the write to be as "sequential" on the disk, so any later reads would have minimal seeks.. > > The disk caches writes, allowing the OS to write a whole > bunch of data into the disk cache and the disk can optimize > the IO a bit internally. > > The same optimization is not possible for reads. > > -- > Politics is the struggle between those who want to make their country > the best in the world, and those who believe it already is. Each group > calls the other unpatriotic. > - > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- 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