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Path: sserve!newshost.anu.edu.au!munnari.oz.au!bunyip.cc.uq.oz.au!harbinger.cc.monash.edu.au!yeshua.marcam.com!news.kei.com!bloom-beacon.mit.edu!ai-lab!life.ai.mit.edu!mycroft From: mycroft@duality.gnu.ai.mit.edu (Charles Hannum) Newsgroups: comp.os.386bsd.questions Subject: Re: How different is VM twixt NetBSD and FreeBSD? Date: 06 Jan 1994 14:35:59 GMT Organization: MIT Artificial Intelligence Lab Lines: 31 Message-ID: <MYCROFT.94Jan6093600@duality.gnu.ai.mit.edu> References: <2fjgs0$5bt@news.service.uci.edu> <JKH.93Dec26154313@whisker.lotus.ie> <DERAADT.93Dec26153823@newt.fsa.ca> <2g5pu6$n96@u.cc.utah.edu> NNTP-Posting-Host: duality.ai.mit.edu In-reply-to: terry@cs.weber.edu's message of 2 Jan 1994 06:34:46 GMT In article <2g5pu6$n96@u.cc.utah.edu> terry@cs.weber.edu (A Wizard of Earth C) writes: NetBSD does suffer from the 4M problem, actually; [...] No, it does not, and as far as I know, it never has. What you are referring to is the standard problem of cache invalidation not working correctly. `The 4M problem' refers to bogosities in FreeBSD which caused it to not work (at least for a while) on *any* machine with 4M; and the same bug occured, but less frequently, on machines with more memory. As for the problem we do have, if your motherboard doesn't do snooping for the cache correctly, then it's broken. It could be kluged around by always invalidating the cache after a DMA operation, but I'm not sure I see the point in dealing with fundamentally broken hardware. Because of the allocation order of DMA buffers in the kernels, initial I/O buffers are allocated in locore in NetBSD; this means that for about 70% of the cases, the problem "goes away"; [...] Yes, this is well known, and I tell people who think their ISA boxes are working fine with >16MB the same thing. They just haven't lost *yet*. -- - Charles Hannum, mycroft@ai.mit.edu a straight dressing, mentally unchallenged, vertically unchallenged, ..., English-American of no color, no religion, and a strong mating preference for people of gender.