"I can make your tests take half the time", was the claim that Garland Snowboughs had made.

Twice as fast? Rustic Mullingfig had hardly started building out the implementation of his Christmas Tree class. It wasn't possible that he'd made the code go slow yet - there really wasn't much of it.

"Go on then mate, I'll bet you a mince pie that you can't manage it."

The bet was on.

The Tests in Question

There really wasn't much to the code yet. So far Rustic had only implemented the planted attribute (that defaulted to the time the object was created) and some stringification.

package XmasTree;
use Moose;
use experimental 'signatures';

use DateTime;
has 'planted' => (
    is => 'ro',
    default => sub { DateTime->now }
);

use overload '""' => 'as_string';
sub as_string ($self) {
    return "A Xmas Tree, planted on @{[ $self->planted ]}";
}

1;

And he'd written two basic tests - one for the stringification:

#!/usr/bin/perl

use Test2::V0;
use XmasTree;

# look, a year 2020 bug!
like( XmasTree->new, qr/An Xmas Tree, planted on 2019/ );

done_testing;

And one for the planted attribute itself:

#!/usr/bin/perl

use Test2::V0;
use XmasTree;

is( XmasTree->new, object {
    call planted => object {
        prop blessed => 'DateTime';

        # look, a year 2020 bug!
        call year => 2019;
    };
});
done_testing;

These ran pretty darn quickly:

    shell$ yath -Ilib t

    ** Defaulting to the 'test' command **

    ( PASSED )  job  1    t/planted.t
    ( PASSED )  job  2    t/stringification.t

    ================================================================================

    Run ID: A60F80C6-251F-11EA-ADBC-83BD753448D3

    All tests were successful!


    1.17659s on wallclock (0.19 usr 0.02 sys + 0.93 cusr 0.11 csys = 1.25 CPU)

How is Garland going to speed those tests up?

Faster

The first thing Garland changed was to add the -P option to yath.

yath is the Test2 test harness (yath stands for Yet Another Test Harness.) Each test it runs forks a new process, executes the test script, and gathers the STDERR and STDOUT of the child process in the parent test harness process to work out the results.

The -P option tells the harness to load the mentioned module before it forks. So by specifying -PDateTime we only have to load DateTime once in the parent process, not twice, once in each child process as it loads

    shell$ yath -PDateTime -PMoose  -Ilib t

    ** Defaulting to the 'test' command **

    ( PASSED )  job  1    t/planted.t
    ( PASSED )  job  2    t/stringification.t

    ================================================================================

    Run ID: D10B83C4-251F-11EA-BDAA-9B6707E666E2

    All tests were successful!


    0.90175s on wallclock (0.19 usr 0.02 sys + 0.68 cusr 0.10 csys = 0.99 CPU)

Whoo! That only took 77% of the time it previously did...but Garland had promised more! Can we make it faster still?

Faster Still

The key realization is that not only that DateTime and other CPAN modules we use do not change between the different test scripts we execute, they don't change between different runs of the same test script. Or, to put it another way...we want to load them into memory once and then every time after (until we eventually upgrade our Perl modules) we won't have to run them again.

To do this yath has a daemon mode. You can start it up by using the start command:

    shell$ yath start -PDateTime -PMoose
    Waiting for runner...

    Persistent runner started!
    Runner PID: 3199
    Runner dir: /tmp/yath-test-3198-JClS5vpt
    Runner logs:
    standard output: /tmp/yath-test-3198-JClS5vpt/output.log
    standard  error: /tmp/yath-test-3198-JClS5vpt/error.log

    Use `yath watch` to monitor the persistent runner

    0.58685s on wallclock (0.19 usr 0.02 sys + 0.00 cusr 0.00 csys = 0.21 CPU)

Now whenever we run yath from this directory it'll contact this daemon and have it run the tests instead - and its already loaded the modules!

    shell$ yath t

    ** Persistent runner detected, defaulting to the 'run' command **

    ( PASSED )  job 3485-1    t/planted.t
    ( PASSED )  job 3485-2    t/stringification.t

    ================================================================================

    Run ID: D682502A-2520-11EA-8563-D1A3990446BB

    All tests were successful!


    0.35264s on wallclock (0.18 usr 0.03 sys + 0.00 cusr 0.00 csys = 0.21 CPU)

Okay, that took less than 30% of the original time it took - over three times as fast!

Of course, this speed up will become more significant the more stable unchanging CPAN modules we pull in as our codebase becomes bigger.

We can even make this easier by having a preload module that we use whenever we get ready to start testing.

package reloadStandard;

use 5.024;
use warnings;

use Moose ();
use DateTime ();

1;

And then

    shell$ yath start -PreloadStandard
    ...

But more importantly, what this means is that Garland is owed some mince pies!