Congratulations on the PDL parallelisation effort.

Thank you for sharing the article. I'm trying to run the demonstration, beginning on page 6, and encountered a C build failure. See issue on GitHub. Thanks to @mohawk2, this is working.

Update: Remove the size args to pgswin. Thanks, etj.
Update: Display the compute time.

use strict;
use warnings;

use PDL;
use PDL::Graphics::Simple;
use Time::HiRes 'time';

no PDL::NiceSlice; # prevents interference
use Inline Pdlpp => <<'EOPP';
pp_def('pp_mandel',
    # Pars (signature) specs threadable arguments.
    Pars => 'c(n=2); [o]o()',
    # OtherPars specs scalar arguments.
    OtherPars => 'int max_it',
    Code => <<'EOC',
    // All this code gets wrapped automagically in a thread loop.
    // It starts a fresh C block, so you can declare stuff up top.
    // The $GENERIC() macro is the overall expression type.
    int i;                                     // Iterator
    $GENERIC() rp0 = $c(n=>0), ip0 = $c(n=>1); // Copy the initial val
+ue to rp0/ip0
    $GENERIC() rp = rp0, ip = ip0;             // Copy again for the i
+nitial iteration
    $GENERIC() rp2 = rp*rp, ip2 = ip*ip;       // Find RP^2 and IP^2 f
+or magnitude and z^2
    // The OtherPars are in the $COMP macro.
    for(i=$COMP(max_it); rp2+ip2 < 4 && i; i--) {
        ip *= 2 * rp; rp = rp2 - ip2; // Calculate M_i(z)^2
        rp += rp0; ip += ip0;         // Add z
        rp2 = rp*rp; ip2 = ip*ip;     // Calculate rp^2 and ip^2 for n
+ext time
    }
    $o()= i; // Assign the iterator to the output value
EOC
);
EOPP

my $cen = pdl(-0.74897,0.05708);
my $coords = $cen + (ndcoords(501,501)/250 - 1) * 0.001;
my $w = pgswin();

my $start = time();
$w->image( $coords->using(0,1), $coords->pp_mandel(2500), {title=>"Man
+delbrot"} );
print "Compute time: ", time() - $start, $/;

print "Press the enter key to exit\n";
<STDIN>;
[download]

Another thing that changed in 2021 (as well as the parallelisation improvements) is that PDL now has native complex numbers, which means the above code could be simplified to using C99 complex numbers rather than manually doing complex-number / magnitude calculations EDIT TO REMOVE size args to pgswin that caused gnuplot to try to make a gigantic window and run out of memory:

use strict;
use warnings;

use PDL;
use PDL::Graphics::Simple;

use Inline Pdlpp => <<'EOPP';
pp_def('pp_mandel',
    # Pars (signature) specs threadable arguments.
    Pars => 'coord(n=2); [o]o(); complex [t]c()',
    # OtherPars specs scalar arguments.
    OtherPars => 'int max_it',
    Code => <<'EOC',
    /* All this code gets wrapped automagically in a thread loop.  */
    /* The $GENERIC(c) macro is the type of ndarray "c".        */
    // using the Wikipedia formula: z_{n+1} = {z_n}^2 + c
    int i;                        // iterator.
    $c() = $coord(n=>0) + I*$coord(n=>1); // Copy the initial value
    $GENERIC(c) z = $c(); // Copy for the initial iteration
    // the OtherPars are in the $COMP macro.
    for(i=$COMP(max_it); cabs(z) < 2 && i; i--)
        z = z*z + $c();
    $o()= i; // Assign the iterator to the output value
EOC
);
EOPP

my $cen = pdl(-0.74897,0.05708);
my $coords = $cen + (ndcoords(501,501)/250 - 1) * 0.001;
my $w = pgswin();

$w->image( $coords->using(0,1), $coords->pp_mandel(2500),
  {title=>"Mandelbrot"} );

<STDIN>
[download]

EDIT apparently this version takes twice as long to run as the one that doesn't use C99 complex numbers. I assume this is because mine uses cabs, while the other one uses some cleverness to reduce the amount of multiplications etc.

Thanks for raising the issue, and for fixing it :-D