From owner-freebsd-hackers Sun Jan 26 17:43: 3 2003 Delivered-To: freebsd-hackers@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id 2850B37B401 for ; Sun, 26 Jan 2003 17:42:57 -0800 (PST) Received: from apollo.backplane.com (apollo.backplane.com [216.240.41.2]) by mx1.FreeBSD.org (Postfix) with ESMTP id 433C043EB2 for ; Sun, 26 Jan 2003 17:42:57 -0800 (PST) (envelope-from dillon@apollo.backplane.com) Received: from apollo.backplane.com (localhost [127.0.0.1]) by apollo.backplane.com (8.12.6/8.12.6) with ESMTP id h0R1gu0i070183; Sun, 26 Jan 2003 17:42:56 -0800 (PST) (envelope-from dillon@apollo.backplane.com) Received: (from dillon@localhost) by apollo.backplane.com (8.12.6/8.12.6/Submit) id h0R1guR3070182; Sun, 26 Jan 2003 17:42:56 -0800 (PST) Date: Sun, 26 Jan 2003 17:42:56 -0800 (PST) From: Matthew Dillon Message-Id: <200301270142.h0R1guR3070182@apollo.backplane.com> To: Terry Lambert Cc: Sean Hamilton , hackers@FreeBSD.ORG Subject: Re: Random disk cache expiry References: <000501c2c4dd$f43ed450$16e306cf@slugabed.org> <200301261931.h0QJVCp8052101@apollo.backplane.com> <3E348B51.6F4D6096@mindspring.com> Sender: owner-freebsd-hackers@FreeBSD.ORG Precedence: bulk List-ID: List-Archive: (Web Archive) List-Help: (List Instructions) List-Subscribe: List-Unsubscribe: X-Loop: FreeBSD.ORG :I really dislike the idea of random expiration; I don't understand :the point, unless you are trying to get better numbers on some :.. Well, the basic scenario is something like this: Lets say you have 512MB of ram and you are reading a 1GB file sequentially, over and over again. The cache scenario that gives you the greatest performance is basically to keep 512MB worth of the file in memory and throw away the rest as you read it. This is effectively what random disk cache expiration gives you (assuming you expire whole tracks worth of data, not just random pages). i.e. it's only useful when you are cycling through a data set that is larger then main memory over and over again. Most people deal with this issue simply by beefing up their disk subsystem. After all, you don't have to stripe very many disks (only 2 or 3) to get 100MB/sec+ in throughput from the disk subsystem. -Matt To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message