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From: boba@iscuva.ISCS.COM (Bob Alexander)
Newsgroups: comp.unix.wizards
Subject: Re: UNIX Facilities for Interpreters - (nf)
Message-ID: <544@iscuva.ISCS.COM>
Date: Wed, 17-Jun-87 15:01:55 EDT
Article-I.D.: iscuva.544
Posted: Wed Jun 17 15:01:55 1987
Date-Received: Sun, 21-Jun-87 10:11:26 EDT
References: <540@iscuva.UUCP> <8300006@iaoobelix.UUCP> <680@nu3b2.UUCP>
Reply-To: boba@iscuva.UUCP (Bob Alexander)
Organization: ISC Systems Corporation, Spokane, WA
Lines: 28

Robert, I think your reading a lot more complexity into this file
mapping business than there needs to be.  The key points are:

Although the call looks something like a read, no data is actually
transferred.  This file data is just mapped in to the caller's address
space.

The mechanism is completely independent from the file system buffer
cache.

The file's data is accessed just like a memory access.

Decisions about what parts of the file are read into real memory when
are made by exactly the same mechanisms as regular virtual memory.
Parts (i.e. pages) of the file are simply paged in as needed.  No
special tuning is required.  No consideration need be given to whether
the file is open or not (in fact, the mapped file can be accessed
after it is closed -- just like a "read" buffer can be accessed after
the file is closed).  If a part of the file is frequently accessed, it
will likely stay in real memory; if not it becomes a candidate to be
swapped out.

The nice thing about mapped files is that, for the most part, they are
just a repackaging of existing facilities.
-- 

Bob Alexander	   ISC Systems Corp.  Spokane, WA  (509)927-5445
		   UUCP: ihnp4!tektronix!reed!iscuva!boba