Question

Perl’s documentation says:
Since Perl 5.8, thread programming has been available using a model called interpreter threads which provides a new Perl interpreter for each thread

Using ps -Lm <pid> with the program below I can see that threads run in parallel, i.e., they are being run at the same time in different cores. But even when there are 4 threads (3 and the main) ps aux shows only one Perl process.

1. Does this mean that there is a whole Perl interpreter on each thread?
2. Are Perl threads mapped to system threads?
3. If 2 is true, how is possible to have multiple Perl interpreters within a single process?

use threads;

$thr = threads->new(\&sub1);
$thr2 = threads->new(\&sub1);
$thr3 = threads->new(\&sub1);

sub sub1 { 
      $i = 0;
      while(true){
        $i = int(rand(10)) + $i;
      }
}


$thr->join;

Answer 1

“Perl interpreter” refers to the environment in which the Perl code executes. From a user’s perspective, that’s mostly the symbol table and the globals therein, but it also includes a slew of internal variables (e.g. those used during parsing, the current op, etc).

1. Yes, there’s a Perl interpreter for each thread.

2. Yes, Perl threads are system threads.

3. Think of “Perl interpreter” as a class of which you can make any number of instances.* Perl refers to this as Multiplicity. See perlembed for how to embed a Perl interpreter in your application.

---

* — Requires the use of -Dusemulitplicity when building Perl, which is implied by -Dusethreads, which is how thread support is added to Perl. Otherwise, a whole bunch of globals are used instead of a “class”.