From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Fri, 26 Jul 2002 12:50:55 -0400 Subject: Re: [RFC] start_aggressive_readahead Content-Type: text/plain; charset=US-ASCII; format=flowed Mime-Version: 1.0 (Apple Message framework v482) From: Scott Kaplan In-Reply-To: <3D405428.7EC4B715@zip.com.au> Message-Id: Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Andrew Morton Cc: Rik van Riel , Christoph Hellwig , torvalds@transmeta.com, linux-mm@kvack.org List-ID: -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On Thursday, July 25, 2002, at 03:40 PM, Andrew Morton wrote: > What it boils down to is: which pages are we, in the immediate future, > more likely to use? Pages which are at the tail of the inactive list, > or pages which are in the file's readahead window? That is the right question to ask... > I'd say the latter, so readahead should just go and do reclaim. ...but the answer's not that simple, I'm afraid. You've got two groups of logical pages competing for physical page frames. Which is more valuable depends entirely on the reference behavior of workload. I'll point you to a recent paper of mine on exactly this problem (in two formats): http://www.cs.amherst.edu/~sfkaplan/papers/prepaging.pdf http://www.cs.amherst.edu/~sfkaplan/papers/prepaging.ps.bz2 The results presented are from uniprogrammed reference traces, but the principle still applies: For some reference patterns, caching of some number of readahead pages is a great idea. For other reference patterns, the pages at the tail end of the inactive list are *still* more valuable, and the readahead pages should be completely ignored. There's also a lot of space in the middle: Readahead pages should be cached, but only for a limited time, lest they displace too many pages on the tail end of the inactive list. What you really want is some kind of adaptivity that allows you to compare the rates at which these two pools of pages are referenced, and then decides how many readahead pages (if any) to cache. Scott -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.0.6 (Darwin) Comment: For info see http://www.gnupg.org iD8DBQE9QX3y8eFdWQtoOmgRAplfAKCLrmURjCkuf6snOfwrFQFmqXlYoACgnvCa IFEC/tDsVLY+isCC/qkxn5w= =8Jx5 -----END PGP SIGNATURE----- -- 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/