Multi-threading in 5.8.0

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

Hi all.

In the following code:


#!/usr/bin/perl -w

use strict;
use threads qw( yield );
   
    my $thread1 = threads->new(\&sub1);
    my $thread2 = threads->new(\&sub2);

    my @data = $thread1->join;
    print "'Thread1' returned @data";

    my @data2 = $thread2->join;
    print "'Thread2' returned @data2";

    sub sub1 {
       yield;
       sleep 1;
       print "I'm a perl programmer!\n";
       return "Threading in perl is fun!\n";
    }

    sub sub2 {
      print "I like computing!\n";
      return "Computing is cool!\n";
   }
[download]

It seems I am unable to obtain different output ( thread1 always executes first even if I include the yield function inside of it ) unless I include a quantity of sleep time in sub1. Isn't that the purpose of 'yield' ( to give up it's CPU time to another thread )? This is my first exposure to perl threads so I apologize if this posting is overly simplistic.

Sample output without using sleep ( win98 - ActiveState 5.8.0 build ):

C:\Perl\bin\hw>perl pg5.pl
I'm a perl programmer!
I like computing!
'Thread1' returned Threading in perl is fun!
'Thread2' returned Computing is cool!

C:\Perl\bin\hw>perl pg5.pl
I'm a perl programmer!
I like computing!
'Thread1' returned Threading in perl is fun!
'Thread2' returned Computing is cool!

C:\Perl\bin\hw>perl pg5.pl
I'm a perl programmer!
I like computing!
'Thread1' returned Threading in perl is fun!
'Thread2' returned Computing is cool!

C:\Perl\bin\hw>perl pg5.pl
I'm a perl programmer!
I like computing!
'Thread1' returned Threading in perl is fun!
'Thread2' returned Computing is cool!

C:\Perl\bin\hw>
[download]

Sample output with sleep and yield:
C:\Perl\bin\hw>perl pg5.pl
I like computing! *pause for 1 second*
I'm a perl programmer!
'Thread1' returned Threading in perl is fun!
'Thread2' returned Computing is cool!

C:\Perl\bin\hw>perl pg5.pl
I like computing!
I'm a perl programmer!
'Thread1' returned Threading in perl is fun!
'Thread2' returned Computing is cool!

C:\Perl\bin\hw>perl pg5.pl
I like computing! 
I'm a perl programmer!
'Thread1' returned Threading in perl is fun!
'Thread2' returned Computing is cool!

C:\Perl\bin\hw>
[download]

From what I understand, perl 6 is going to include an improved thread model. Very interesting indeed.

Thanks,
-Katie.

Comment on Multi-threading in 5.8.0 Select or Download Code

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Re: Multi-threading in 5.8.0
by pfaut (Priest) on Apr 20, 2003 at 00:01 UTC

yield will give up the CPU and cause the scheduler to pick a new current thread which might very well be the same thread that just called yield. You can't make assumptions about how threads will be scheduled. If you need to control which thread will execute when, you need to use locking mechanisms.

90% of every Perl application is already written. ⇒

dragonchild

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Re: Multi-threading in 5.8.0
by JamesNC (Chaplain) on Apr 20, 2003 at 03:10 UTC

post

use threads;
use threads::shared;
use strict;
my $wait = 500; my @up: shared; my $th;
my @childs; my $child;
my @down: shared;

my @list = qw ( www.apple.com www.perl.org 
www.ibm.com www.coke.com www.oracle.com www.hp.com );


foreach(@list){
    push @childs, threads->create("ping","$_");
}
foreach $child (@childs){
    $child->join();
}
print @up;
print @down;

sub ping {
 my $ip = shift;
 my @rv = `ping -n 1 -w $wait $ip`;
 foreach(@rv){ push @up, "$ip up\n" if /\(0% loss\)/;
    push @down, "$ip down\n" if /\(100% loss\)/; }
}
[download]

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Re: Multi-threading in 5.8.0
by halley (Prior) on Apr 20, 2003 at 13:14 UTC

When it comes to testing a multi-threaded app, there is no substitute for getting a few hours on a multiple-processor system. All your tests which you ran on the single-processor machine are valid, but only a part of the testing you'll need to do. You'll soon discover the difference between empirical evidence and proven robustness.

What's worse is that each new threading model can also affect borderline code. What worked on POSIX will have a race condition on green threads, or vice versa. Knowing your OS's threading model won't help very much from an application level: you just have to get it right, not merely apparently okay. It'll come from experience.

--
[ e d @ h a l l e y . c c ]

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Re: Multi-threading in 5.8.0
by petesmiley (Friar) on Apr 21, 2003 at 15:58 UTC

If you are depending on the order in which your threads execute then you should carefully reconsider your design.

smiles

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