Re: Garbage Collection in Ada

[eachus@spectre.mitre.org (Robert I. Eachus)]

In article <JSA.96Oct25141553@alexandria> jsa@alexandria (Jon S Anthony) writes:

  > Exactly.  The "theory" just doesn't account for the observed facts.
  > Hence, anyone who is even remotely "scientifically motivated" would
  > have to just toss it on the incorrect theory junk pile.  While the
  > "science" in "computer science" has always struck me as dubious (at
  > best), one would hope that there would be at least _some_ adherence to
  > this basic principle.

   My observations are based on simulations which in some cases run
more than a week--of CPU time.  The characteristics of my experience
are: heap allocated objects containing a large amount of non-pointer
state, including random values and in some cases the state of a random
number generator.  This is the worst case environment for a
conservative collector.

    Hans-Juergen Boehm described a much different environment:

> Please try it for yourself.  Until April I was reading my mail on a
> conservatively garbage collected system that couldn't possibly have
> stayed up for weeks by this chain of reasoning.  (It did.)  Many other
> systems that are in routine use couldn't possibly work either.
   
     Heap memory values in such an environment will tend to contain
valid pointer values and substrings of mostly English ASCII text.  It
is possible that in Boehm's environment, no legal pointer corresponds
to any printable substring.

     So yes your milage WILL vary, but in the simulation business,
conservative collectors are an extremely poor choice.  (And what makes
them so bad is not that they fail often, but that when they fail three
days into a week long run, you are up the proverbial creek.)

--

					Robert I. Eachus

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