Mojo::UserAgent non-blocking vs blocking performance

I have the following code:

my $ua = Mojo::UserAgent->new ();

my @ids = qw(id1 id2 id3);

foreach (@ids) {    
    my $input = $_;
    my $res = $ua->get('http://my_site/rest/id/'.$input.'.json' => sub {
        my ($ua, $res) = @_;
        print "$input =>" . $res->result->json('/net/id/desc'), "\n";
    });    
}

Mojo::IOLoop->start unless Mojo::IOLoop->is_running;

Why when I run the above code (non-blocking) does it take about 6 seconds while when running the code as blocking, i.e. inside the loop something like:

 my $res = $ua->get('http://my_site/rest/id/'.$input.'.json');
 print "$input =>" . $res->result->json('/net/id/desc'), "\n";

without the latest line it takes about 1 second?

Why is the blocking code faster than the non-blocking code?

Solution

The first thing to check when things happened. I couldn't get the same delay. Remember to try each way several times to spot outliers where there's a network hiccup. Note that the second argument to the non-blocking sub is a transaction object, normally written as $tx, where the response object is normally written res:

use Mojo::Util qw(steady_time);

say "Begin: " . steady_time();
END { say "End: " . steady_time() }

my $ua = Mojo::UserAgent->new ();

my @ids = qw(id1 id2 id3);

foreach (@ids) {
    my $input = $_;
    my $res = $ua->get(
        $url =>
        sub {
            my ($ua, $tx) = @_;
            print "Fetched\n";
            }
        );
    }

One possibility is that keep-alive is holding an open connection. What happens if you turn that off?

    my $res = $ua->get(
        $url =>
        { Connection => 'close' }
        sub {
            my ($ua, $tx) = @_;
            print "Fetched\n";
            }
        );

Here's a version that uses promises, which you'll want to get used to as more Mojo stuff moves to it:

use feature qw(signatures);
no warnings qw(experimental::signatures);

use Mojo::Promise;
use Mojo::Util qw(steady_time);
use Mojo::UserAgent;
my $ua = Mojo::UserAgent->new;

say "Begin: " . steady_time();
END { say "End: " . steady_time() }

my @ids = qw(id1 id2 id3);

my @gets = map {
    $ua->get_p( 'http://www.perl.com' )->then(
        sub ( $tx ) { say "Fetched: " . steady_time() },
        sub { print "Error: @_" }
        );
    } @ids;

Mojo::Promise->all( @gets )->wait;