Date: Fri, 19 Nov 1999 20:01:32 +0000 (GMT) From: Jonathon McKitrick <jcm@dogma.freebsd-uk.eu.org> To: Lowell Gilbert <lowell@world.std.com> Cc: freebsd-questions@freebsd.org Subject: Re: Memory Info Message-ID: <Pine.BSF.4.02A.9911191956100.28523-100000@dogma.freebsd-uk.eu.org> In-Reply-To: <199911191951.OAA23688@world.std.com>
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On Fri, 19 Nov 1999, Lowell Gilbert wrote: >>If this is the case, does this mean FreeBSD keeps everything possible in >>memory and then swaps out to VM when it needs more? > >This is a badly-worded question -- that, in itself, indicates some of what >you're missing. VM *is* memory; the 'M' *stands* for the word "memory." OK, i meant swaps it to _disk_ here... >Specifically, VM is an abstraction of memory space that allows parts of >that space to be kept in different kinds of storage at any particular >That's only a rough approximation, though. Pages can get swapped to disk >well before they're needed for other purposes, and then there's no wait on >the disk write before they can be re-used. They don't have to be cleared >immediately, though, so if the original task accesses the page before it's >needed for anything else, that page is still in RAM (and the swap copy of >the page is invalidated). If pages are swapped to disk before they are needed, then there *IS* a wait on the disk write before the can be re-used, correct? >> How are unnecessary >>page faults and disk accesses avoided with this method? > >Quite well. A RAM page is never assigned to a new use unless that that >use *needed* RAM. [I realize I'm being a little disinguous here; the >problem is not when to reassign a page, but which one to reassign. Making >that guess is a rather complicated statistical problem, but amounts to >guessing which current page is least likely to be needed again soon.] FIFO, LRU, Second Chance, etc. And i don't quite understand that second sentences.. typo, maybe? >>I am a bit of a neophyte, but I am right in the middle of an 'Intro to >>Operating Systems' course, and we just finished memory mgmt and virtual >>memory. > >You might want to discuss some of this with your professor rather than a >bunch of people who don't know you and can't see you to pick up on body >language. I didn't realize body language could cause such a misinterpretation of email. ;-) -jm --------- He who laughs last... obviously didn't get the joke. To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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