From owner-freebsd-hackers Sun Feb 14 22:50:07 1999 Return-Path: Received: (from majordom@localhost) by hub.freebsd.org (8.8.8/8.8.8) id WAA13369 for freebsd-hackers-outgoing; Sun, 14 Feb 1999 22:50:07 -0800 (PST) (envelope-from owner-freebsd-hackers@FreeBSD.ORG) Received: from apollo.backplane.com (apollo.backplane.com [209.157.86.2]) by hub.freebsd.org (8.8.8/8.8.8) with ESMTP id WAA13363 for ; Sun, 14 Feb 1999 22:50:05 -0800 (PST) (envelope-from dillon@apollo.backplane.com) Received: (from dillon@localhost) by apollo.backplane.com (8.9.3/8.9.1) id WAA12919; Sun, 14 Feb 1999 22:50:02 -0800 (PST) (envelope-from dillon) Date: Sun, 14 Feb 1999 22:50:02 -0800 (PST) From: Matthew Dillon Message-Id: <199902150650.WAA12919@apollo.backplane.com> To: Matthew Jacob Cc: Jaye Mathisen , hackers@FreeBSD.ORG Subject: Re: Processor affinity? References: Sender: owner-freebsd-hackers@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.ORG : : :Really? Hmm.. I would have thought for a machine that with local cache :but expensive global access (e.g., sun4d architecture) that affinity is a :win. Oh well, not my area of expertise. All modern SMP designs have per-cpu caches and nearly all have local L1 and L2 caches ( because the L2 caches are what tie into the SMP mechanisms of the backplane ). The expense of a main memory access verses a cache access is always pretty big. There are various kinds of SMP caching schemes, but most of the amount to the same generalities and only differ in their basic performance characteristic. For example, a number of modern SMP designs use an L2-read-shared and L2-write-allocate scheme and one L2 cache that misses is actually able to get the data from another L2 cache belonging to another cpu rather then have to go to main memory. The only issue with processor affinity, really, is the actual load on the main memory. Processor affinity only makes sense when the load on main memory is relatively high. Typically, SMP systems have a relatively low main memory load ( and high L1 and L2 cache memory load ), so until you have enough cpu's banging on the same main memory to saturate it, processor affinity is usually wash. It makes sense on a big 32+ cpu Solaris or SGI system, but not much sense on a 2 or 4 cpu system. -Matt Matthew Dillon To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message