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Shrunk array takes more memory than original

by kroach (Pilgrim)
on Nov 10, 2015 at 12:03 UTC ( #1147349=perlquestion: print w/replies, xml ) Need Help??
kroach has asked for the wisdom of the Perl Monks concerning the following question:

I'm using the $# feature to shrink an array. After shrinking to half the size, the array seems to take a bit more memory. How is that possible? (perl 5.10 on 64-bit GNU/Linux)

use strict; use warnings; use feature qw(say); use Devel::Size qw(size); my @array = 1 .. 10_000; say size(\@array); $#array = 5_000; say size(\@array);
Output:
80064 80216

Is there a way to shrink the array without causing it to grow (and probably be reallocated)?



UPDATE: It seems splice doesn't cause the same effect.

my @array = 1 .. 10_000; say size(\@array); splice @array, 5_001; say size(\@array);

Output:

80064 80064

Furthermore, after running a few simple benchmarks, it turns out the splice method is about as fast as last index assignment, so it seems splice doesn't do unnecessary copying in void context.

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Re: Shrunk array takes more memory than original
by BrowserUk (Pope) on Nov 10, 2015 at 13:04 UTC

    Using total_size() rather than size():

    @a = 1 .. 10000;; print total_size \@a;; 320176 $#a = 5000;; print total_size \@a;; 200376

    As you can see, the total size occupied by the array has shrunk by 30%.

    When you initialise the array, you cause perl to allocate 10,000 * 8 (64-bit) pointers as a continuous piece of memory for the array, plus 10,000 * 24 bytes for the scalars = 320,000 bytes (plus a little extra for the array mechanics.

    When you assign to $#array, the size of the base array does not get reallocated (hence the size() doesn't change), but half of the scalars are reclaimed. So now you have 10,000 * 8 plus 5,000 * 24 = 200,000 (plus the little extra).


    With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
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Re: Shrunk array takes more memory than original
by choroba (Bishop) on Nov 10, 2015 at 12:26 UTC
    The only way how to shrink the memory is to allocate a new array (or undef, but then you can't keep any values after shrinking).
    my @shrunk = @array[0 .. 5_000]; # Array slice. say size(\@shrunk); # 40072

    See also How do I shrink an array in Perl? on StackOverflow.

    لսႽ ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

      I do not mean to shrink the memory, I would like it to stay the same instead of growing.

      I'm not adding anything new to the array and I don't get why it gets larger from having half of its elements removed.

        why it gets larger from having half of its elements removed.

        Because using $#array causes some magic to be attached to the array.

        You don't even have to assign to $#array for this to happen, only reference it:

        @b = 1 .. 10; print size \@b;; 256 print $#b; print size \@b;; 9 440

        And yes, 184 bytes does seem excessive. You'd think that the magic could be stored once and simply referenced for each array using an 4/8-byte pointer.


        With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
        Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
        "Science is about questioning the status quo. Questioning authority". I knew I was on the right track :)
        In the absence of evidence, opinion is indistinguishable from prejudice.

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