Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!EDDIE.MIT.EDU!compass!worley
From: compass!worley@EDDIE.MIT.EDU (Dale Worley)
Newsgroups: gnu.emacs
Subject: Why Gnu Emacs doesn't release memory
Message-ID: <8910261557.AA02573@sn1987a.compass.com>
Date: 26 Oct 89 15:57:06 GMT
Sender: daemon@tut.cis.ohio-state.edu
Distribution: gnu
Organization: GNUs Not Usenet
Lines: 32

Gnu Emacs uses memory in a dynamically allocated manner.  Memory tends
to be allocated in small chunks (list cells, small strings) and in
large chunks (buffers, large strings).  (To prevent various problems,
small chunks are suballocated out of large chunks that are reserved
for this purpose.)  Memory becomes free in more-or-less random order,
so the memory space tends to be a mosaic of free and in-use areas.

Most Unix systems allow the user to change his memory allocation only
by enlarging or shrinking the data area, and then only by moving the
high-address end.  However, the high-address end of the data area is
likely to be a mosaic of free and in-use areas, and so it is very
unlikely that its end can be moved down by much at any time, even if a
large fraction of the data area is free.  Thus, there is no utility in
designing a mechanism to detect and free memory from the address
space.  (If the kernel provided a mechanism to free interior parts of
the data area address space, it might be worthwhile for Emacs to
deallocate large free chunks.)

The traditional solution to such problems is called 'compactification'
-- moving all in-use data areas into a compact mass at the low end of
the data area, and then deallocating the huge free area left at the
high end.  To do this successfully requires that all pointers to any
data area be altered to point to the new addresses, which (despite
some earlier speculations of mine) requires much careful design and is
very error-prone.

Dale Worley		Compass, Inc.			worley@compass.com
--
All through human history, tyrannies have tried to enforce obedience by
prohibiting disrespect for the symbols of their power.  The swastika is
only one example of many in recent history.
-- American Bar Association task force on flag burning