Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site mips.UUCP Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!mhuxt!houxm!whuxl!whuxlm!akgua!gatech!seismo!hao!hplabs!oliveb!Glacier!mips!mash From: mash@mips.UUCP (John Mashey) Newsgroups: net.arch Subject: Re: 386 info Message-ID: <215@mips.UUCP> Date: Tue, 5-Nov-85 12:51:31 EST Article-I.D.: mips.215 Posted: Tue Nov 5 12:51:31 1985 Date-Received: Sun, 10-Nov-85 08:30:52 EST References: <965@mcnc.mcnc.UUCP> Distribution: net Organization: MIPS Computer Systems, Mountain View, CA Lines: 27 John N. White writes (on the 386): > Intel claims that either segments or paging can be used to provide > flexible memory management. Let's assume that the 386 is being used in a > high performance workstation. ... > enough to hold all active tasks. Assume there is one major task being run > and that we are most concerned about its performance. This task will use > over a Mbyte of data area accessed randomly. > First, paging will clearly work. But our main task will run slowly because > almost all of the data accesses to the >1MBbyte of data area will be > cache misses, forcing the slow operation described above. Note that this > is a built in problem with 386. The page size is 4k, the cache size is 32, > so any program that randomly accesses more than 128kbytes will slow down.... (remember that the cache referenced here is really the TLB, or cache of translations, not a data cache). 1) Fortunately for the 386 and similar designs, most real programs don't randomly access data. Instruction locality is quite good, and even data locality is not bad, unless you have programs walking arrays in more than page size steps. 2) Consider the 128Kbyte number. The equivalent number on a VAX-11/780, for user space, is 64*512 = 32Kbyte, and the VAX has to flush that on every context switch. It seems possible on the 386 to use segment id's as process/context identifiers to avoid the flush. -- -john mashey UUCP: {decvax,ucbvax,ihnp4}!decwrl!mips!mash DDD: 415-960-1200 USPS: MIPS Computer Systems, 1330 Charleston Rd, Mtn View, CA 94043