in reply to Memory usage double expected

RHS is compile-time constant and so no COW for it? Try $x = 'a' x (2**${\32});

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Re^2: Memory usage double expected (run-time)
by LanX (Saint) on Oct 27, 2022 at 09:00 UTC
    > Try $x = 'a' x (2**${\32});

    yes, forcing the creation of the RHS into runtime works.

    edit

    Dave_The_M will know better, why the space of an inline constant wasn't released.

    I suppose it's just an overlooked optimization.

    Cheers Rolf
    (addicted to the 𐍀𐌴𐍂𐌻 Programming Language :)
    Wikisyntax for the Monastery

[reply]
[d/l]

      Tim Bunce's excellent Profiling Memory Usage talk gives some insight into what's going on at the 09:20 minute mark: 

      sub foo {
   my $var = "X" x 10_000_000;
}
foo();

      [download]

      Tim explains that:

      1. The buffer for the $var lexical is preserved (for next time you call the subroutine).
      2. The compiler says hey "X" is a constant and 10_000_000 is a constant, so I'm gonna build you a 10 MB constant! And I'm going to keep it here to one side so when you call this subroutine again I can just copy it in for you! (aka Constant folding).

      With recursion, things get much worse (sorry, I couldn't bring myself to watch that part :).

      Though Tim's Devel::SizeMe module might be useful, AFAIK it is up for adoption and not being actively developed ATM.

      I also keep a list of Memory Tools References (from this list, Mini-Tutorial: Perl's Memory Management by ikegami is definitely worth reading).

[reply]
[d/l]
[select]
        OK this makes sense.

        Perl can't know beforehand, how often this line is executed.

        Even if it cared to check that it's not inside a loop or sub.

        Cheers Rolf
        (addicted to the 𐍀𐌴𐍂𐌻 Programming Language :)
        Wikisyntax for the Monastery

[reply]
      My initial thoughts:

      In general an assign should COW, but the rules are complex. It's not happening in this case. It's probably a bug.

      Dave.

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