Date: Mon, 12 Feb 1996 09:27:17 +0100 (MET) From: Luigi Rizzo <luigi@labinfo.iet.unipi.it> To: toor@dyson.iquest.net (John S. Dyson) Cc: dufault@hda.com, hackers@FreeBSD.ORG Subject: Zero Fill On Demand (was Re: Compressed RAM/SWAP) Message-ID: <199602120827.JAA00762@labinfo.iet.unipi.it> In-Reply-To: <199602101238.MAA03889@dyson.iquest.net> from "John S. Dyson" at Feb 10, 96 12:37:56 pm
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> > If there are lots of zero'd pages that aren't used that often, > > can't be eliminated, and would benefit from compression wouldn't > > it be better to look at a ZFOD (zero fill on demand) mechanism? > > Perhaps using that as a HINT as to which pages might be useful targets > for compression??? Maybe eliminating the wasteful attempted compression. One possible way to achieve ZFOD would be the following: + make sure that there is always a bzeroed page, say ZP + when creating a new page, just build a copy-on-write reference to ZP in the page table. this should be quite fast at creation (and implementation) time, and a bit more powerful than ZFOD, as the page is read-accessible from the beginning without any page fault. I believe it is going to be more expensive when the page is written to, because copying require twice the accesses of bzero-ing. John ? Luigi ==================================================================== Luigi Rizzo Dip. di Ingegneria dell'Informazione email: luigi@iet.unipi.it Universita' di Pisa tel: +39-50-568533 via Diotisalvi 2, 56126 PISA (Italy) fax: +39-50-568522 http://www.iet.unipi.it/~luigi/ ====================================================================
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