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From: ahh@glyph.kingston.ny.us (Andy Heffernan)
Newsgroups: comp.sys.amiga.programmer
Subject: Re: Memory fragging
Message-ID: <1600@glyph.kingston.ny.us>
Date: 26 May 91 02:46:54 GMT
References: <1991May21.195251.16477@zorch.SF-Bay.ORG> <21795@cbmvax.commodore.com> <1991May23.114423.11039@medar.com> <21838@cbmvax.commodore.com>
Reply-To: ahh@glyph.UUCP (Andy Heffernan)
Organization: Moji Computing
Lines: 31

In article <21838@cbmvax.commodore.com> peter@cbmvax.commodore.com (Peter Cherna) writes:
>In article <1991May23.114423.11039@medar.com> jseymour@medar.com (James Seymour) writes:
>>But if you eventually F/free() the other two, the three chunks of memory
>>(in the above example) will be coalesced by the o/s, won't they?
>
>I'm not a fragging expert, but I've always assumed the system was
>happier if you tried to free things in reverse order.  (Intervening
>processes may make allocations.)  It's also a generally good programming
>practice.

Well, sorta.

For your typical coalescing memory allocator, given that the heap starts out 
unfragmented at time t1, and -all- allocations/frees are perfectly nested 
until time t2, there will be no fragmentation in the heap at any point 
between t1 and t2.  It's not really a heap anymore, it's a stack.
Given that nearly all allocations on the Amiga come from the common
system heap (AllocMem/FreeMem), however, maintaining perfect nesting 
of allocations is clearly impossible.  Performing allocations from a
per-process memory pool (a la sbrk()/malloc()) allows a process total 
control over its heap and helps to localize any eliminate fragmentation.

At any rate, if between t1 and t2 (same initial conditions as above), 
n random tasks madly allocate and free memory from the system heap
in any order, but all memory is guaranteed to have been freed by t2, 
there should be no fragmentation at time t2.  If there is, either the 
coalescer has a bug or not everything has actually been freed.

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  Andy Heffernan		文字		uunet!glyph!ahh