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From: zwilling@edam.cs.wisc.edu (Mike Zwilling)
Newsgroups: comp.os.mach
Subject: External pagers and sharing
Message-ID: <10026@spool.cs.wisc.edu>
Date: 27 Mar 90 20:00:17 GMT
Sender: news@spool.cs.wisc.edu
Organization: U of Wisconsin CS Dept
Lines: 38

I have a question concerning physical memory sharing among multiple
clients of an external pager.  Consider the following scenario:

    Assume we have one the same machine an external pager with 2 
    clients.  Client 1 uses memory_object_1 and client 2 uses
    memory_object_2.  Both memory objects are maintained by the
    same external pager.  Further assume that page 1 in each object
    is identical (eg. in the same file) and that each client requires 
    only read access to page 1.  Now, it would be preferable for the 
    clients to share the same physical page containing page 1 (since
    they are not writing the page).  This could be "implemented" by 
    return a pointer to the same data in the 
    memory_object_data_provided calls.  

It is my understanding that when the external pager provides a pointer 
to the data, the data is mapped copy-on-write in the client thus giving 
the sharing I want.  But, according to the document, "The Duality of 
Memory and Communication in the Implementation of a Multiprocessor 
Operating System", each physical page has a *resident page* structure 
containing *the memory object* associated with the page.

So, my question is, will physical page be shared in the above scenario
or will a copy be done behind-the-scene?  If the page is shared, I'm
curious as to how that is implemented.

Note: I do realize that sharing would occur if each client mapped the
      same memory object.  But, I'm interested in the case where they
      don't.

Thanks for any help, and if the question isn't clear, just let me
know.

Mike
--
Mike Zwilling   University of Wisconsin -- Madison              
Computer Sciences Dept.
1210 W. Dayton St.
Madison, WI  53706    email: zwilling@cs.wisc.edu   ph# (608) 263-4076