arunbhargav has asked for the wisdom of the Perl Monks concerning the following question: 

use threads;
my $thr = threads->create ( sub { for (5..10) { print $_,"\n";
                                   sleep(10);}});
$thr->join();
my $thr2 = threads->create ( sub { for (10..19) { print $_,"\n";}});
$thr2->join();

[download]
I see that the above code produces the following o/p which has nuthing to do with parallel run, what am i missing here? 
5
6
7
8
9
10
10
11
12
13
14
15
16
17
18
19

[download]
i was anticipating garbled output , not a sequential one

Replies are listed 'Best First'.
Re: Thread parallel execution (join blocks)
by Anonymous Monk on May 08, 2013 at 02:17 UTC

    Did you read about what join does? It blocks. Try 

    use threads;
my $thr = threads->create ( sub { for (5..10) { print $_,"\n";
                                   sleep(1);}});
my $thr2 = threads->create ( sub { for (10..19) { print $_,"\n";}});
$thr2->join();
$thr->join();

    [download]
[reply]
[d/l]
      #!/usr/boin/perl --
use strict;
use warnings;
use threads;

for my $ix ( 0 .. 20 ) {
    threads->create(
        sub {
            sleep rand $ix;
            print join ' ', $$, ix => $ix, tid => threads->tid(), "\n"
+;
        }
    );
}

exit if @ARGV;

while( threads->list ) {
    for my $joinable ( threads->list( threads::joinable ) ) {
        $joinable->join;
    }
}
__END__
$ perl fudge exit
1044 ix 0 tid 1
1044 ix 1 tid 2
1044 ix 2 tid 3
1044 ix 4 tid 5
1044 ix 17 tid 18
Perl exited with active threads:
        16 running and unjoined
        5 finished and unjoined
        0 running and detached

$ perl fudge
1948 ix 0 tid 1
1948 ix 1 tid 2
1948 ix 2 tid 3
1948 ix 3 tid 4
1948 ix 4 tid 5
1948 ix 7 tid 8
1948 ix 17 tid 18
1948 ix 9 tid 10
1948 ix 12 tid 13
1948 ix 13 tid 14
1948 ix 18 tid 19
1948 ix 6 tid 7
1948 ix 5 tid 6
1948 ix 10 tid 11
1948 ix 8 tid 9
1948 ix 14 tid 15
1948 ix 19 tid 20
1948 ix 11 tid 12
1948 ix 15 tid 16
1948 ix 16 tid 17
1948 ix 20 tid 21

      [download]
[reply]
[d/l]
      Thanks, My interpretation of join was it would only attach this thread to the main thread, not block until it finishes executing. Well, this clears up a lot.
[reply]
        My interpretation of join was it would only attach this thread to the main thread, not block until it finishes executing.

        I'm not getting on your case here, but this is a common enough misconception that it is worth drawing attention to.

        How could it "attach this thread to the main thread" without "block until it finishes executing"?

        What would "attachment" without "waiting" even mean?

        "join" is an awful method name. The primary purpose of join is to retrieve the results of the subroutine that was run in the thread.

        If the thread has finished before the call to join is made, then it can return immediately. However, if the thread hasn't finished, then join has no option but to wait until it does.

        With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
        Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
        "Science is about questioning the status quo. Questioning authority".
        In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]

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