> it simply mentions reallocation and not leakage

yes, but the question could be if memory is released again.

IIRC, Perl's arrays are "improved" C-Arrays to have O(1) lookup.¹

The slots of the C-array are equally sized pointers to scalars holding the various data types

Furthermore will they always allocate a power of 2 of RAM, and only double if needed.

The empty slots are available as fill-in margins at the beginning and end of the C-array, The Perl is keeping a counters to see where the actual values start and end.

Like this unshift and push are O(1) if there is margin left. Otherwise reallocating more space only happens for doubling it. I.e. complexity is negligible since it only happens in O(log) occasions.

This bargaining of time-complexity for space-complexity allows nearly as efficient appending operations like linked lists have.

Now in theory it could happen that you have an array of a handful elements and after 1 million combined push and shift operations it allocates megabytes of space while not logically growing.

I doubt this happens and otherwise should be easy to demonstrate by the OP with a loop and using Devel::Size before and after.

edit

1) see also https://blob.perl.org/tpc/1998/Perl_Language_and_Modules/Perl%20Illustrated/#AV

update

It could also be an issue if the OS doesn't "take back" released memory.

Now in theory it could happen that you have an array of a handful elements and after 1 million combined push and shift operations it allocates megabytes of space while not logically growing. ... I doubt this happens

That's my position also. And I was bored, so here's the proof.

#!/usr/bin/env perl
#=====================================================================
+==========
#
#         FILE: fifo-mem.pl
#
#        USAGE: ./fifo-mem.pl  
#
#  DESCRIPTION: Test memory consumption of a push/shift FIFO
#               See https://www.perlmonks.org/?node_id=11157497
#
#=====================================================================
+==========

use strict;
use warnings;
use Memory::Usage;

my @fifo;

my $max = 100_000_000;

my $mu = Memory::Usage->new;

$mu->record ('start');

for my $count (1 .. $max) {
    push @fifo, 'dog';
    shift @fifo;
    $mu->record ("After $count iterations") unless $count % 10_000_000
+;
}

$mu->record ('finish');
$mu->dump;
[download]

And since not everyone can run this, here's the output:

$   time    vsz (  diff)    rss (  diff) shared (  diff)   code (  dif
+f)   data (  diff)
     0  10780 ( 10780)   6544 (  6544)   5448 (  5448)      4 (     4)
+   1356 (  1356) start
     3  10780 (     0)   6544 (     0)   5448 (     0)      4 (     0)
+   1356 (     0) After 10000000 iterations
     5  10780 (     0)   6544 (     0)   5448 (     0)      4 (     0)
+   1356 (     0) After 20000000 iterations
     8  10780 (     0)   6544 (     0)   5448 (     0)      4 (     0)
+   1356 (     0) After 30000000 iterations
    10  10780 (     0)   6544 (     0)   5448 (     0)      4 (     0)
+   1356 (     0) After 40000000 iterations
    12  10780 (     0)   6544 (     0)   5448 (     0)      4 (     0)
+   1356 (     0) After 50000000 iterations
    15  10780 (     0)   6544 (     0)   5448 (     0)      4 (     0)
+   1356 (     0) After 60000000 iterations
    17  10780 (     0)   6544 (     0)   5448 (     0)      4 (     0)
+   1356 (     0) After 70000000 iterations
    20  10780 (     0)   6544 (     0)   5448 (     0)      4 (     0)
+   1356 (     0) After 80000000 iterations
    22  10780 (     0)   6544 (     0)   5448 (     0)      4 (     0)
+   1356 (     0) After 90000000 iterations
    24  10780 (     0)   6544 (     0)   5448 (     0)      4 (     0)
+   1356 (     0) After 100000000 iterations
    24  10780 (     0)   6544 (     0)   5448 (     0)      4 (     0)
+   1356 (     0) finish
$
[download]

As expected, no leak whatsoever.

It could also be an issue if the OS doesn't "take back" released memory.

Of course, but so could any program doing anything. That doesn't constitute a leak.

---

🦛

> Of course, but so could any program doing anything. That doesn't constitute a leak.

maybe I misread "reallocating", if that means that at some point the whole array is moved (copied) to a formerly released memory location it's fine.

If Perl was only allocating forward after x push and releasing backwards after x shift and the released memory wasn't reused otherwise, then memory consumption for the process would grow.

This I'd call a leak.

It would be interesting to know how and when this reallocation happens and if it's automatically done on C-level.

update

I now seem to remember (in the context of paging to the disk) that memory management operates on physical blocks or pages which are dispersed over RAM, and translates a virtual address to a physical one. Hence a released block would be naturally reused, without much intervention. :)

Cheers Rolf
_{(addicted to the Perl Programming Language :)
see Wikisyntax for the Monastery}