Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!zaphod.mps.ohio-state.edu!mips!winchester!mash From: mash@mips.COM (John Mashey) Newsgroups: comp.arch Subject: Re: IBM RS6000 Message-ID: <45069@mips.mips.COM> Date: 20 Jan 91 19:43:46 GMT References: <1991Jan10.214122.9506@news.arc.nasa.gov> <1991Jan20.033052.10919@athena.mit.edu> Sender: news@mips.COM Reply-To: mash@mips.COM (John Mashey) Organization: MIPS Computer Systems, Inc. Lines: 36 In article <1991Jan20.033052.10919@athena.mit.edu> jfc@athena.mit.edu (John F Carr) writes: > >>The RIOS MMU is an excersise in complexity. The inverted page table (IPT) >>with hardware reload and hardware lock bit support is too far gone. TLB >>reload is somewhat slow as a result. One might see performance problems >>with processes that thrash the TLB. > >I'm not convinced this is a problem. The RIOS MMU is very similar to ..... good info on RT PC refill. ..... can anybody post a correpsonding analysis for RS6000? > >Mach 2.5 is guilty of "all the world's a VAX" thinking. It requires >VAX style page tables; it emulates these on the RT by taking a page >fault each time the virtual address used to access the same physical >page changes. Performance may vary a lot depending on memory access >patterns and use of shared data. It isn't a fair test of MIPS vs. IBM >hardware to run an operating system that requires a certain MMU when >only one of the machines has it. This is a fair comment, although one must also point out that: a) Mach was trying to be runnable on a number of different machines with different kinds of MMUs; however, most machines look more like VAXen than RT PCs. b) Mach hardly requires a MIPS MMU c) Note that the RT PC MMU was designed for & in conjunction with one particular OS version, with little convern for whether other OS's would port well or not (and there's nothing worng with that, given IBM's viewpoint). On the other hand, the MIPS MMU was explicitly designed with a specific flexibility in mind, that lots of different people would be able to port different operating systems to it without having to do major redesigns. -- -john mashey DISCLAIMER: UUCP: mash@mips.com OR {ames,decwrl,prls,pyramid}!mips!mash DDD: 408-524-7015, 524-8253 or (main number) 408-720-1700 USPS: MIPS Computer Systems, 930 E. Arques, Sunnyvale, CA 94086