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From: rab@mimsy.UUCP (Robert A Bruce)
Newsgroups: mod.os,comp.os.research
Subject: Shared Libraries
Message-ID: <2990@sdcsvax.UCSD.EDU>
Date: Wed, 15-Apr-87 02:06:35 EST
Article-I.D.: sdcsvax.2990
Posted: Wed Apr 15 02:06:35 1987
Date-Received: Sun, 19-Apr-87 04:46:05 EST
Sender: darrell@sdcsvax.UCSD.EDU
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
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Here at UM we are trying to implement shared libraries as part
of an operating system for a multiprocesser computer.  The computer
consists of 16 MC68010 bases cpu boards, each with a 68450 MMU.
We are trying to implement forking and load balancing between processors.
There is no shared memory between processor boards, so moving a process
to a different cpu requires copying the entire text space.  If we
use shared libraries the size of the text segment of each process whould 
be substantially reduced, so copying could be done much more efficiently.

I would like to start some discussion about the best way to support shared
libraries and how the operating system should support them.

Some ideas that I have heard are:

1) Locate the library at a know location in memory.  At link time bind
   all the addresses to the addresses of the library subroutines.
   This would involve the least overhead, but would also be the most
   inflexible.  Every time the library was changed, or moved to a new
   location in memory, all programs would need to be relinked.

2)  Same as 1) but bind all the addresses when a program is loaded.
    This would cause more overhead at load time, but minor library
    changes won't break all existing programs.

3)  Use a jump table.  The jump table is located at a known location in
    memory.  The location of each jump instruction is bound to the
    program at link time.  Each library call involves one extra jump
    instruction, but I don't think these jumps would cause a significant
    performance degradation.

4)  Use an `openlibrary' system call that returns a pointer to a table
    of pointers to library routines.  I have heard the amiga uses
    something like this.  This would would require an extra level of
    of indirection but would be even more flexible.