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Avoiding memory loss...

by J.Perl.Man (Novice)
on Dec 27, 2012 at 21:36 UTC ( #1010583=perlquestion: print w/ replies, xml ) Need Help??
J.Perl.Man has asked for the wisdom of the Perl Monks concerning the following question:

My question has to do with potential memory leaks, and how to properly clean up what I'm doing... I'm creating an array off of a hash as follows:

if (! exists $cTable{$cID}{raw}) { $cTable{$cID}{raw}=[]; } push @{$Table{$cID}{raw}}, $currentVal;

and, periodically wanting to delete the hash.

delete $cTable{$cID};

My question is, do I first need to do something to remove the array or is that cleaned up automatically for me?

Comment on Avoiding memory loss...
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Re: Avoiding memory loss...
by LanX (Abbot) on Dec 27, 2012 at 21:53 UTC
    Hi

    first, you don't need to intialize an array, perl does the allocation automatically when you do the push.

    #if (! exists $cTable{$cID}{raw}) { # $cTable{$cID}{raw}=[]; #} push @{$Table{$cID}{raw}}, $currentVal;

    > and, periodically wanting to delete the hash.

    delete $cTable{$cID}
    > My question is, do I first need to do something to remove the array or is that cleaned up automatically for me?

    depends, garbage collections cleans up variables with an empty reference count.

    so as long as you don't have any $copies = $Table{$cID} somewhere else in the code the hash will be automatically destroyed when deleting the last reference.

    The same for all nested substructures/elements like the array $Table{$cID}{raw}, if there are no other references they will be destroyed, too.

    in other words: no reference left <=> no memory used

    for details see: perlref

    Cheers Rolf

      Excellent -- I hoped my read of the memory cleanup stuff was accurate. There are no copies, so I should be good to go I guess!

      Regarding the allocation, in this case, we're talking pointer to an array. Normally, I wouldn't create an empty array, but was pretty sure that I had to have the reference in place before the push, yes?

      Thanks!

        > Regarding the allocation, in this case, we're talking pointer to an array.

        yes, reference to an array (pointers are in C memory addresses but references are more complicated).

        But it doesn't matter @array and $array_ref=\@array are just two different ways to represent the same array.

        > Normally, I wouldn't create an empty array, but was pretty sure that I had to have the reference in place before the push, yes?

        As I told you: No!

        You are pushing to a variable starting with @. If the array doesn't exist already it will be allocated. If it exists but it's incompatible to push (like a hash) you,'ll get an error.

        DB<107> ref $a => "" DB<108> push @$a,1..3 => 3 DB<109> ref $a => "ARRAY" DB<110> $b={} => {} DB<111> push @$b,1..3 Not an ARRAY reference at (eval 44)[multi_perl5db.pl:644] line 2. DB<112> ref $b => "HASH"
        In newer versions of Perl you are even allowed to push directly to $array_refs.

        Cheers Rolf

Re: Avoiding memory loss...
by Anonymous Monk on Dec 27, 2012 at 22:33 UTC
    ... but they probably won't be destroyed immediately. Like most systems, Perl is "lazy." Unreferenced items are marked for deletion but nothing actually happens until the garbage-collector makes its midnight run, banging the cans together and waking everybody up.

      So perhaps I'm right to be concerned about cleanup... the program itself will be installed as a daemon, so anything I can do to prevent long term consumption, or to force garbage collection would be good.

      What method do you recommend to remove the array? My read of delete (man page) against an array leads me to believe that I'm still waiting on true garbage collection

        > So perhaps I'm right to be concerned about cleanup

        No don't worry, Anonymous probably confuses the garbage collection systems.

        Perl doesn't freeze!

        Just be careful to not produce circular references.

        Cheers Rolf

      but nothing actually happens until the garbage-collector makes its midnight run
      No, perl doesn't use a garbage collector. Items are freed immediately when their ref count goes to zero.

      Dave.

        > No, perl doesn't use a garbage collector.

        most probably a problem of fuzzy terminology¹, perldoc talks about "garbage collection":

         Perl uses a fast and simple, reference-based garbage collection system

        see perlobj#Two-Phased-Garbage-Collection

        But as opposed to mark-and-sweep Perl's runtime approach is very simple an efficient.

        OTOH it can't handle circular references.

        And even if mark-and-sweep was used, IIRC Python does it and is still very performant. ²

        Cheers Rolf

        UPDATES:

        ¹) from perlglossary

        garbage collection A misnamed feature--it should be called, "expecting your mo +ther to pick up after you". Strictly speaking, Perl doesn’t do thi +s, but it relies on a reference-counting mechanism to keep things +tidy. However, we rarely speak strictly and will often refer to t +he reference-counting scheme as a form of garbage collection. + (If it’s any comfort, when your interpreter exits, a "real" gar +bage collector runs to make sure everything is cleaned up if you +’ve been messy with circular references and such.)

        ²) more complicated: "While Python uses the traditional reference counting implementation, it also offers a cycle detector that works to detect reference cycles."

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