[GAP Forum] Memory management with GAP

[Keshav Rao Kini]

Well, I'll answer what I can...

2011/3/23 Mathieu Dutour <Mathieu.Dutour at ens.fr>
> ---Is there a way to know how much memory a variable occupies?
>   The commands GasmanStatistics() is helpful but gives you only
>   global information when what you want to know is more specific.
>   Would it be possible to know how much memory a given variable
>   uses? And the list of variables.

You can use MemoryUsage to get the memory usage of an object (with
some caveats - see the manual entry for details). NamesGVars will give
you a list of all the global variable names currently known to the GAP
interpreter.

> ---It is said that the garbage collection of GAP is "very conservative"
>   What does that really mean?

I assume this means that it tries to avoid garbage-collecting any
objects until it is absolutely sure that there are no references to
it, but I don't know the actual details.

> ---In garbage collection, the problem is with cyclical links,
>   a->b and b->a. Well I do not even know how to create such links,
>   let alone cyclical in GAP.
>   What command could lead to such cyclical links?

Well, one easy way is
gap> a := [];; b := [a];; a[1] := b;;
gap> a; b;
[ [ ~ ] ]
[ [ ~ ] ]
gap> a[1] = b; b[1] = a;
true
true

I guess this simple example is probably detectable to the garbage
collector, but there of course may be more complicated ones. By the
way, in the above, the notation "[ [ ~ ] ]" contains the symbol "~",
which means that object itself, i.e. the object is a list containing a
list containing the object itself. That's because a = b[1] = a[1][1].

-Keshav

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