Alois Mahdal Alois Mahdal - 5 months ago 33
Perl Question

Deliberately eating certain amount of memory using perl script

I need to simulate a memory-hungry process. For example, On a machine with 4.0 GiB, I need a process that would eat 3.2 GiB (give or take few MiB).

I assumed it should be as easy as:

my $mbytes = 3276;
my $huge_string = 'X' x ($mbytes * 1024 * 1024);

But I end up with process eating twice as much memory as I need it to.

  • this is same on two Windows 7 amd64 machines: one with 64-bit, the other
    with 32-bit build of Strawberry Perl

  • I'm using Sysinternals Process Explorer and watching "Private Bytes"

Of course, I could just
$mbytes /= 2
(for now, I'll probably will do that), but:

  • Is there a better way?

  • Can anyone explain why the amount is twice as length of the string?


Code adapted from, all credit goes to Perlmonk BrowserUk.

my $huge_string = 'X';
$huge_string x= $mbytes * 1024 * 1024;

why the amount is twice as length of the string?

Think about the order of evaluation. The right-hand expression allocates memory for your x expression, and again so does the assignment operation into your new scalar. As usual for Perl, even though the right-hand expression is not referenced anymore, the memory is not freed right away.

Operating on an existing scalar avoids the second allocation, as shown above.