I've seen quite often that Perl was reusing reference addresses after garbage collection had them released before.

I was curious as to how often this address reusing happens:

#!/bin/bash

for i in {1..15}; do
perl -Mstrict -wle'
use strict;
use warnings;

my %addr;
my $count = 0;
my $tries = 2**$ARGV[0];
for my $try (1..$tries) {
    my %h;
    for (reverse 1..1000) {
        $h{$_} = { value => $_};
        $count++;
    }

    $addr{$_}++ for values %h;
}

my $reused = scalar grep $addr{$_}>1, keys %addr;
my $addrs  = scalar keys %addr;
print "Address of reference reused $reused times, all addresses $addrs
+, in $count tries";' $i
done
[download]

Address of reference reused 1 times, all addresses 1999, in 2000 tries
Address of reference reused 1001 times, all addresses 2999, in 4000 tr
+ies
Address of reference reused 2995 times, all addresses 3007, in 8000 tr
+ies
Address of reference reused 3011 times, all addresses 3023, in 16000 t
+ries
Address of reference reused 3043 times, all addresses 3055, in 32000 t
+ries
Address of reference reused 3107 times, all addresses 3119, in 64000 t
+ries
Address of reference reused 3235 times, all addresses 3247, in 128000 
+tries
Address of reference reused 3491 times, all addresses 3503, in 256000 
+tries
Address of reference reused 4003 times, all addresses 4015, in 512000 
+tries
Address of reference reused 5027 times, all addresses 5039, in 1024000
+ tries
Address of reference reused 7075 times, all addresses 7087, in 2048000
+ tries
Address of reference reused 11171 times, all addresses 11183, in 40960
+00 tries
Address of reference reused 19363 times, all addresses 19375, in 81920
+00 tries
Address of reference reused 35747 times, all addresses 35759, in 16384
+000 tries
Address of reference reused 68515 times, all addresses 68527, in 32768
+000 tries
[download]

These numbers seem to be stable on my machine across multiple runs.

The bottom line is that address reusing happens all the time.

And will all be drawn from the same lot: Perl's memory pool.

When memory is freed, it can be released to perl's free memory pool rather than being released to the OS. Future allocations will dip into this pool first.

From Re: Long running tasks, perl and garbage collection by ikegami

I suppose Perl is keeping its memory footprint low by reusing them.

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}

 What's wrong with them?

efficiency?

Elaborate?

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}

memory and time: What I had in mind was the solution of an array in tandem with a hashtable. The array receives items as they are created, and so it keeps the order of insertion. And the hashtable stores data with key to facilitate search-by-key. However deleting by key is not efficient as it needs to search the array too. Introducing more values to the hash (e.g. the corresponding array index) increases memory. Inserting also needs updating 2 data structures.

10 min edit: also deleting from middle of array is inefficient.