FAQ 3.16 How can I make my Perl program take less memory?

Discussion in 'Perl Misc' started by PerlFAQ Server, Feb 4, 2011.

  1. This is an excerpt from the latest version perlfaq3.pod, which
    comes with the standard Perl distribution. These postings aim to
    reduce the number of repeated questions as well as allow the community
    to review and update the answers. The latest version of the complete
    perlfaq is at http://faq.perl.org .


    3.16: How can I make my Perl program take less memory?

    When it comes to time-space tradeoffs, Perl nearly always prefers to
    throw memory at a problem. Scalars in Perl use more memory than strings
    in C, arrays take more than that, and hashes use even more. While
    there's still a lot to be done, recent releases have been addressing
    these issues. For example, as of 5.004, duplicate hash keys are shared
    amongst all hashes using them, so require no reallocation.

    In some cases, using substr() or vec() to simulate arrays can be highly
    beneficial. For example, an array of a thousand booleans will take at
    least 20,000 bytes of space, but it can be turned into one 125-byte bit
    vector--a considerable memory savings. The standard Tie::SubstrHash
    module can also help for certain types of data structure. If you're
    working with specialist data structures (matrices, for instance) modules
    that implement these in C may use less memory than equivalent Perl

    Another thing to try is learning whether your Perl was compiled with the
    system malloc or with Perl's builtin malloc. Whichever one it is, try
    using the other one and see whether this makes a difference. Information
    about malloc is in the INSTALL file in the source distribution. You can
    find out whether you are using perl's malloc by typing "perl

    Of course, the best way to save memory is to not do anything to waste it
    in the first place. Good programming practices can go a long way toward

    * Don't slurp!

    Don't read an entire file into memory if you can process it line by
    line. Or more concretely, use a loop like this:

    # Good Idea
    while (<FILE>) {
    # ...

    instead of this:

    # Bad Idea
    @data = <FILE>;
    foreach (@data) {
    # ...

    When the files you're processing are small, it doesn't much matter
    which way you do it, but it makes a huge difference when they start
    getting larger.

    * Use map and grep selectively

    Remember that both map and grep expect a LIST argument, so doing

    @wanted = grep {/pattern/} <FILE>;

    will cause the entire file to be slurped. For large files, it's
    better to loop:

    while (<FILE>) {
    push(@wanted, $_) if /pattern/;

    * Avoid unnecessary quotes and stringification

    Don't quote large strings unless absolutely necessary:

    my $copy = "$large_string";

    makes 2 copies of $large_string (one for $copy and another for the
    quotes), whereas

    my $copy = $large_string;

    only makes one copy.

    Ditto for stringifying large arrays:

    local $, = "\n";
    print @big_array;

    is much more memory-efficient than either

    print join "\n", @big_array;


    local $" = "\n";
    print "@big_array";

    * Pass by reference

    Pass arrays and hashes by reference, not by value. For one thing,
    it's the only way to pass multiple lists or hashes (or both) in a
    single call/return. It also avoids creating a copy of all the
    contents. This requires some judgement, however, because any changes
    will be propagated back to the original data. If you really want to
    mangle (er, modify) a copy, you'll have to sacrifice the memory
    needed to make one.

    * Tie large variables to disk.

    For "big" data stores (i.e. ones that exceed available memory)
    consider using one of the DB modules to store it on disk instead of
    in RAM. This will incur a penalty in access time, but that's
    probably better than causing your hard disk to thrash due to massive


    The perlfaq-workers, a group of volunteers, maintain the perlfaq. They
    are not necessarily experts in every domain where Perl might show up,
    so please include as much information as possible and relevant in any
    corrections. The perlfaq-workers also don't have access to every
    operating system or platform, so please include relevant details for
    corrections to examples that do not work on particular platforms.
    Working code is greatly appreciated.

    If you'd like to help maintain the perlfaq, see the details in
    PerlFAQ Server, Feb 4, 2011
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