Update

I got this excerpt from the book Pro Perl while searching with Google:

"This includes nested data structures and objects, making shared memory ties potentially very powerful. However, each reference becomes a new shared memory object, so a complex structure can quickly exceed the system limit on shared memory segments."

Here is the URL for it: http://books.google.com.br/books?id=1bbjLxkBLaMC&pg=PA888&lpg=PA888&dq=perl+ipc+shared+memory&source=bl&ots=GfhUOuXJND&sig=t1DqzIFeFCneihcnuK43r76mk4Y&hl=pt-BR&ei=9ZnBTM7-LMX_lgeg5fiyCg&sa=X&oi=book_result&ct=result&resnum=5&ved=0CCgQ6AEwBDgU#v=onepage&q=perl%20ipc%20shared%20memory&f=false. I think this explains enough.

Greetings monks,

I'm doing some testing with IPC shared memory by using IPC::Shareable with the code below (borrowed from a O'Reilly's book):

#!/usr/bin/perl 
# sharetest - test shared variables across forks
use strict;
use warnings;
use IPC::Shareable;

my %buffer;
my $handle = tie %buffer, 'IPC::Shareable', undef, { destroy => 1 };

$buffer{test}   = "inoangrnglkdtghnlkhnl";
$buffer{test1}   = "inoangrnglkdtghnlkhnl";
$buffer{test2}   = "inoangrnglkdtghnlkhnl";
$buffer{test3}   = "inoangrnglkdtghnlkhnl";
$buffer{test4}   = "inoangrnglkdtghnlkhnl";

$SIG{INT} = sub { die "$$ dying\n" };

my @kids;

for ( 1 .. 10 ) {

    my $child;

    unless ( $child = fork ) {    # i'm the child
        die "cannot fork: $!" unless defined $child;
        squabble();
        exit;
    }
    push @kids, $child;           # in case we care about their pids
}

while (1) {
    print "Buffer is $buffer{test}\n";
    sleep 1;
}
die "Not reached";

sub squabble {
    my $i = 0;
    while (1) {
        next if $buffer{test} =~ /^$$\b/o;
        $handle->shlock();
        $i++;
        $buffer{test} = "$$ $i";
        $handle->shunlock();
    }
}
[download]

After running the code, I got the following output from ipcs -m command in a shell:

- Segmentos da memória compartilhada -
chave      shmid      proprietário perms      bytes      nattch     st
+atus      
0x00000000 0          root       777        102400     1              
+         
0x00000000 196609     jackal     600        393216     2          dest
+         
0x00000000 229378     jackal     600        393216     2          dest
+         
0x00000000 262147     jackal     600        393216     2          dest
+         
0x00000000 294916     jackal     600        393216     2          dest
+         
0x00000000 327685     jackal     600        393216     2          dest
+         
0x00000000 360454     jackal     600        393216     2          dest
+         
0x00000000 393223     jackal     600        393216     2          dest
+         
0x00000000 425992     jackal     600        393216     2          dest
+         
0x00000000 458761     jackal     600        393216     2          dest
+         
0x00000000 491530     jackal     600        393216     2          dest
+         
0x00000000 524299     jackal     600        393216     2          dest
+         
0x00000000 557068     jackal     600        393216     2          dest
+         
0x00000000 589837     jackal     600        393216     2          dest
+         
0x00000000 655374     jackal     600        393216     2          dest
+         
0x00000000 688143     jackal     600        393216     2          dest
+         
0x00000000 1114128    jackal     600        393216     2          dest
+         
0x00000000 1146897    jackal     600        393216     2          dest
+         
0x00000000 851986     jackal     600        393216     2          dest
+         
0x00000000 983059     jackal     600        393216     2          dest
+         
0x00000000 1015828    jackal     600        393216     2          dest
+         
0x00000000 1212437    jackal     666        65536      1
[download]

The line regarding the created shared segment by the program is the last one.I think this is quite expected.

BUT if I change the program to use a array reference instead of a scalar as a value for each key:

$buffer{test}  = "inoangrnglkdtghnlkhnl";
$buffer{test1} = [ 'one', 'two' ];
$buffer{test2} = [ 'one', 'two' ];
$buffer{test3} = [ 'one', 'two' ];
$buffer{test4} = [ 'one', 'two' ];
[download]

I got a different amount of segments created:

- Segmentos da memória compartilhada -
chave      shmid      proprietário perms      bytes      nattch     st
+atus      
0x00000000 0          root       777        102400     1              
+         
0x00000000 196609     jackal     600        393216     2          dest
+         
0x00000000 229378     jackal     600        393216     2          dest
+         
0x00000000 262147     jackal     600        393216     2          dest
+         
0x00000000 294916     jackal     600        393216     2          dest
+         
0x00000000 327685     jackal     600        393216     2          dest
+         
0x00000000 360454     jackal     600        393216     2          dest
+         
0x00000000 393223     jackal     600        393216     2          dest
+         
0x00000000 425992     jackal     600        393216     2          dest
+         
0x00000000 458761     jackal     600        393216     2          dest
+         
0x00000000 491530     jackal     600        393216     2          dest
+         
0x00000000 524299     jackal     600        393216     2          dest
+         
0x00000000 557068     jackal     600        393216     2          dest
+         
0x00000000 589837     jackal     600        393216     2          dest
+         
0x00000000 655374     jackal     600        393216     2          dest
+         
0x00000000 688143     jackal     600        393216     2          dest
+         
0x00000000 1114128    jackal     600        393216     2          dest
+         
0x00000000 1146897    jackal     600        393216     2          dest
+         
0x00000000 851986     jackal     600        393216     2          dest
+         
0x00000000 983059     jackal     600        393216     2          dest
+         
0x00000000 1015828    jackal     600        393216     2          dest
+         
0x00000000 1474581    jackal     666        65536      1              
+         
0x00000000 1277974    jackal     600        393216     2          dest
+         
0x00000000 1507351    jackal     666        65536      0              
+         
0x00000000 1540120    jackal     666        65536      0              
+         
0x00000000 1572889    jackal     666        65536      0              
+         
0x00000000 1605658    jackal     666        65536      0
[download]

All segments created by the perl script have the size equal to 65536. Since the hash is being serialized with Storable, I would expect to see just one segment, but the script created four more.

Is this a behavior of IPC::Shareable or something related to the how IPC shared memory works?

Thank you in advance,