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[ #210 ] Perl - Using an expensive module Permalink

Perl Added less than a year ago

(Obviously expensive in this context refers to time or resources.)

From perldoc -f use, we know that use Module; is the same as:

BEGIN { require Module; Module->import( LIST ); }

Code in BEGIN blocks is executed at compile time, not run time. ('Compilation' in perl refers to the compilation to internal bytecode format.)

Consider an expensive module that your script uses. Imagine that the functionality that this module provides is only used in one of your subroutines, and that this subroutine is not necessary run in every invocation of your script.

If you used the expensive module, it would be included every time your script is run. If, instead, you simply required this module within the subroutine that actually needs it, you could avoid the cost of including the module when you don't need it!

Consider this expensive-to-use module, Expensive.pm:

package Expensive;

sleep 5;

1;

Now consider use.pl:

#!/usr/bin/env perl

use strict;
use warnings;

use Expensive;

sub rarely_called {
    # uses Expensive.pm's functionality here
    return;
}

rarely_called if defined $ARGV[0];

versus require.pl:

#!/usr/bin/env perl

use strict;
use warnings;

sub rarely_called {
    require Expensive;
    Expensive->import();
    # uses Expensive.pm's functionality here
    return;
}

rarely_called if defined $ARGV[0];

Let's time them:

$ time ./use.pl 

real    0m5.011s
user    0m0.008s
sys     0m0.008s

$ time ./use.pl xxx # see how both invocations of use.pl take the same amount of time

real    0m5.011s
user    0m0.008s
sys     0m0.004s

$ time ./require.pl # but when require doesn't call the sub it is fast!

real    0m0.010s
user    0m0.004s
sys     0m0.004s

$ time ./require.pl xxx

real    0m5.011s
user    0m0.012s
sys     0m0.000s

Oh, and in case you are wondering, no you can't just put the use statement in the sub... all uses are processed at compile time, so it would still be expensive even if that sub was never called.

Colophon

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