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From: mac@rhea.ardent.com (Mike McNamara)
Newsgroups: gnu.emacs
Subject: Re: Why Gnu Emacs doesn't release memory
Message-ID: <MAC.89Oct30112802@rhea.ardent.com>
Date: 30 Oct 89 19:28:02 GMT
References: <8910261557.AA02573@sn1987a.compass.com>
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Reply-To: mac@ardent.com (Michael McNamara)
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In-reply-to: compass!worley@EDDIE.MIT.EDU's message of 26 Oct 89 15:57:06 GMT


In article <8910261557.AA02573@sn1987a.compass.com> 
	compass!worley@EDDIE.MIT.EDU (Dale Worley) writes:

>  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.

	Could you just add one level of indirection to the memory
allocation scheme? 

        ie. define a new malloc to be
      char ** gc_malloc(unsigned int);
	instead of
      char * malloc(unsigned int);
	You also need a 
      gc_lock(char**); and 
      gc_unlock(char**),
        which allow modules to temporarily gain protection for a
        dereferenced pointer.  [Perhaps this is what you mean by
        careful design and error prone.]

	You slowly migrate the potentially large memory allocators to
	this scheme.

	I have programmed a number of PC's which used the above scheme
	to manage memory.

	Then when compactifing, you simply deal with those of your
	internal list of char*s which are not currently gc_lock'ed.

	At the expense of an additional dereference for every pointer
	usage (non trivial, yes, but 32MHz is pretty fast..), this allows
	simple, save memory compactification.

>  
>  Dale Worley		Compass, Inc.			worley@compass.com
>  --