Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!thunder.mcrcim.mcgill.edu!snorkelwacker.mit.edu!apple!usc!sdd.hp.com!ucsd!ucbvax!mtxinu!taniwha!paul From: paul@taniwha.UUCP (Paul Campbell) Newsgroups: comp.arch Subject: Re: SunMMU history Message-ID: <756@taniwha.UUCP> Date: 21 Jan 91 20:01:52 GMT References: <1991Jan19.133914.23871@bellcore.bellcore.com> Reply-To: paul@taniwha.UUCP (Paul Campbell) Organization: Taniwha Systems Design, Oakland Lines: 42 In article pcg@cs.aber.ac.uk (Piercarlo Grandi) writes: >The 68451 was not that long coming, and could be used as a TLB. Yes, it >was slow. As to the issue of whether it is better to add a wait state or >to make the entire system thrash in the pager, it all depends on whether well actually usually 3-5 wait states :-). Most of the '451 based systems and most of the early systems that were based on clones of the SUN MMU were swapping systems - and of course swap based systems perform BETTER than VM systems - but marketting people don't want to hear that (no page fault overhead while running - but you can't run something bigger than real memory). >1) A lot of other manufacturers had the same problems. Nobody botched it >quite as badly as Sun. I have to disagree - around that time I found myself porting Unix to a whole host of '451 and SUN clones - lot's of people screwed up in their MMU design (some to the point where the kernel wasn't even protected from user mode) - I wont mention any names (to protect the guilty). >Incidentally, even a 4KB pagesize is still too large, even if it is >barely tolerable. 8KB was simply crazy, for a workstation/time sharing >server. Another issue that hasn't really been talked about here has to do with disk bandwidth, esp. on some of the slower transfer-rate disks (that can't do 1:1 interleaved transfers). With 8k pages you have to move 8k/page fault so latency from disk can be much higher. These days it's not such a big deal since most disks can do 1:1 but even a couple of years ago a system configured with 'big' ie 8k pages could perform much worse than a 4k or 2k system it also reduces the granularity of the in-core page cache but increases the efficiency of the TLB. These are hard trade-offs to make since their effects (TLB misses vs page-faults vs disk transfer rates) range across a whole range of scales and tend not to be measured together to give a single number you can measure them with Paul -- Paul Campbell UUCP: ..!mtxinu!taniwha!paul AppleLink: CAMPBELL.P Where do you find a "kinder gentler nation" when you need one these days?