Date: Thu, 20 Dec 2001 07:35:54 +1100 From: Peter Jeremy <peter.jeremy@alcatel.com.au> To: "Justin T. Gibbs" <gibbs@scsiguy.com> Cc: freebsd-arch@FreeBSD.ORG Subject: Swap performance (was: Getting rid of /usr file system) Message-ID: <20011220073554.Q73243@gsmx07.alcatel.com.au> In-Reply-To: <200112161616.fBGGGKg49403@aslan.scsiguy.com>; from gibbs@scsiguy.com on Sun, Dec 16, 2001 at 09:16:20AM -0700 References: <200112120510.fBC5AEM33040@harmony.village.org> <200112161616.fBGGGKg49403@aslan.scsiguy.com>
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On Sun, Dec 16, 2001 at 09:16:20AM -0700, Justin T. Gibbs wrote: > I prefer to have my swap partition as close to the front >of the disk as possible to take advantage of the faster tracks. Is this particular piece of folklore still valid? It used to be true: When a process started, the text and data were read from the filesystem into memory. If necessary, the process was paged/swapped to/from swap. Pre-paging Unices in particular had comparatively high I/O load on swap since the entire process space needed to be read/written. (Hence the use of drums in better-equipped sites). For FreeBSD (and other modern Unices), processes are initially paged in from the filesystem. Under high memory load, pages will be paged to/from swap and from the filesystem. The removal of the need to swap entire processes and the removal of text (and other read-only data) from swap both reduce the I/O load of the swap device. It's no longer clear to me that dedicating the fastest piece of disk to swap is the best solution. The above is somewhat simplified - it ignores sticky executables (which transform filesystem I/O into swap I/O) and mmap(2) (which can result in paging to, as well as from, the filesystem) - but I believe it is generally representative. Peter To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
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