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From: allbery@ncoast.UUCP
Newsgroups: comp.unix.wizards
Subject: Re: Unix with dynamic loading (was Re: Big Programs Hurt Performance)
Message-ID: <4776@ncoast.UUCP>
Date: Tue, 29-Sep-87 18:14:30 EDT
Article-I.D.: ncoast.4776
Posted: Tue Sep 29 18:14:30 1987
Date-Received: Fri, 2-Oct-87 06:29:21 EDT
References: <1665@ncr-sd.SanDiego.NCR.COM) <8579@utzoo.UUCP)
Reply-To: allbery@ncoast.UUCP (Brandon Allbery)
Organization: Cleveland Public Access UN*X, Cleveland, Oh
Lines: 40

[Ncoast has neither comp.arch nor comp.os.misc, so I have restricted this
 article to comp.unix.wizards.  Since I'm talking UNIX specifics, that is
 probably appropriate anyway.  ++bsa]

As quoted from <2550@xanth.UUCP> by kent@xanth.UUCP (Kent Paul Dolan):
+---------------
| Well, ring my chimes!  If this is possible in Unix, it would sure help
| the commercial success of Unix if it became common practice.
+---------------

System V can do something like this; it can't patch addresses, but it can use
the jump-table approach.  What works for shared libraries works just as well
for loading part of the address space from a file, and using a loader ifile
to specify symbol addresses within the dynamically-loaded area.  Of course,
this requires that the read/write data space of a program have execute
permissions in the MMU (does it normally?  I've no idea) and pretty much
rules out sharing the code; without kernel support, you can't page it in from
the dynamic load file, either.  Alternatively, the dynamic library could be
read by a routine which builds a jump table from the library's namelist, and
"stubs" could be written which search the jump table for the actual routine
and jump to it.  (They could then store the jump address in a static variable
for faster access on subsequent calls.)  The library itself would be able to
access its static variables indirectly through the jump table, with little
loss in performance if the dynamic load routine insures that externally
visible static variables always have the same offset in the jump table as
their symbols have in the namelist.  (That was unclear, but I hope at least
someone gets the idea I'm trying to get across.)  This kind of dynamic loading
might be a performance loss, but it would be a major win for making the
library truly dynamic.

DId I make any sense, or did I come across as a lunatic, or have I got
something horribly wrong (I'm a programmer; when it comes to MMU-related stuff,
though, I'm out of my depth), or ???
-- 
	    Brandon S. Allbery, moderator of comp.sources.misc
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