Path: utzoo!mnetor!uunet!lll-winken!lll-tis!ames!nrl-cmf!cmcl2!brl-adm!adm!rbj@icst-cmr.arpa
From: rbj@icst-cmr.arpa (Root Boy Jim)
Newsgroups: comp.unix.wizards
Subject: Shared Memory in BSD4.3 is lacking?
Message-ID: <12137@brl-adm.ARPA>
Date: 4 Mar 88 22:15:01 GMT
Sender: news@brl-adm.ARPA
Lines: 61


   In article <12033@brl-adm.ARPA> rbj@icst-cmr.arpa (Root Boy Jim) writes:
   >   From: Chris Torek <chris@trantor.umd.EDU>
   >   Nobody was quite sure how mmap `ought' to work, and it never got
   >   implemented....
   >
   >.... in BSD. So those vendors who needed it, SUN and Sequent, implemented
   >it *differently*. Encore probably has another variation.
		      ^^^^^^

   If you're currently running Umax4.2, you get a very simple shared
   memory semantic which basically says, "Share this piece of my address
   space with any processes that I fork off".  This shared memory does
   not survive across an exec, so basically any programs that try to make
   use of shared memory have to be monolithic.  Seems crude and ugly, I
   know, but you'd be surprised how many programs can be parallelized very
   nicely using just this semantic.

Great! Somebody actually implemented my idea! I hadn't thought about
sharing only *some* of the data space, my idea was to share it all.

   Trivia bowl time (related to another comp.unix.wizards topic).  Some have
   argued that a great way to access shared-memory regions is through the
   filesystem namespace.  Well, there is at least one operating system out
   there that does just that.  Which OS is that?

   [sound of Jeopardy theme music...]

   VMS.  They're called "global segments".  Of course, VMS has the
   interesting "feature" of not just treating the filesystem object as
   a handle for the shared memory region, but also using it to
   reference a FILE that the region is paged from/to.  Whether this is
   good or bad is a mixed bag.  Once the last person detaches from the
   global segment, voila` -- you have a regular file containing the
   data that was in the global segment (when the file is actually in
   use for paging it's inaccessible as a regular file).  This can be
   seen as either a benefit or a security risk.  One of the benefits
   is that once the last process detaches from the region it remains
   with its data intact to be attached by another process, consuming
   only filesystem space in the interim.  Hell, you could even survive
   system shutdowns that way! (clean shutdowns only; no telling what
   state the file will be in after a crash!)

[sound of Twilight Zone (Grateful Dead!) music...]

Your friends at Sequent do the same thing. However, what they are
really doing is marking certain pieces of the buffer cache resident
and non-reclaimable, then mapping them into the processes space.

      +----------------+  Jim Paradis                  linus--+
   +--+-------------+  |  Encore Computer Corp.       necntc--|
   |  | E N C O R E |  |  257 Cedar Hill St.           ihnp4--+-encore!paradis
   |  +-------------+--+  Marlboro MA 01752           decvax--|
   +----------------+     (617) 460-0500             talcott--+
   Well, what's the pleasure in THAT??!!

	(Root Boy) Jim Cottrell	<rbj@icst-cmr.arpa>
	National Bureau of Standards
	Flamer's Hotline: (301) 975-5688
All of a sudden, I want to THROW OVER my promising ACTING CAREER,
 grow a LONG BLACK BEARD and wear a BASEBALL HAT!!
 ...  Although I don't know WHY!!