From etj: Could you take a look at the srand code and see if anything is obviously wrong? Also, are you able to see if the duplicates are in groups i.e. sequences?

From Rob: We need to be looking at how those values were derived.

I'm hoping for the PDL development team to chime in. The testing was out of curiosity. Running non-threads here, PDL generates approximately one billion unique. For threads (uncomment use threads at the top of the script), greater than 92% unique. Better than Math::Random::random_normal/random_uniform.

Calling PDL::srandom has no effect for predictability, before spawning workers. I commented out the line in my last post. It requires calling CORE::srand instead.

Care is needed whether to call a PDL function or class method.

PDL::srandom(42);    # not PDL->srandom(42)
$r = PDL->random();  # not PDL::random();
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I completed validation on the MCE side. Spawning child processes (non-threads), nearly one billion unique > 99.99995% for PDL->random, Math::Prime::Util::drand(), and Math::Random::MT::Auto::rand(). 80% ~ 82% unique for Math::Random::random_uniform() and Math::Random::random_normal().

Running threads, Math::Random::random_normal and random_uniform take beyond 2 minutes to output one billion lines and score less than 20% unique.

The relevant code in MCE 1.891 can be found here lines 644-662 and here lines 2036-2071. MCE::Child and MCE::Hobo have similar code.

See also, Random Numbers Overview by danaj.

Update:

It turns out that I can seed the generator inside threads for CORE, Math::Prime::Util, and Math::Random::MT::Auto and have predictable results, matching non-threads. I released MCE v1.892/v1.893, removing the check whether spinning threads. v1.893 preserves calling CORE::srand(N) for older Perl, non-threads.

# Sets the seed of the base generator uniquely between workers.
# The new seed is computed using the current seed and ID value.
# One may set the seed at the application level for predictable
# results (non-thread workers only). Ditto for Math::Prime::Util,
# Math::Random, Math::Random::MT::Auto, and PDL.
#
# MCE 1.892, 2024-06-08
#     Removed check if spawning threads i.e. use_threads.
#     Predictable output matches non-threads for CORE,
#     Math::Prime::Util, and Math::Random::MT::Auto.
#     https://perlmonks.org/?node_id=11159834

{
   my $_wid  = $_args[1];
   my $_seed = abs($self->{_seed} - ($_wid * 100000)) % 2147483560;

   CORE::srand($_seed) if (!$self->{use_threads} || $] ge '5.020000');
+ # drand48
   Math::Prime::Util::srand($_seed) if $INC{'Math/Prime/Util.pm'};

   if (!$self->{use_threads}) {
      PDL::srand($_seed) if $INC{'PDL.pm'} && PDL->can('srand'); # PDL
+ 2.062 ~ 2.089
      PDL::srandom($_seed) if $INC{'PDL.pm'} && PDL->can('srandom'); #
+ PDL 2.089_01+
   }
}

if (!$self->{use_threads} && $INC{'Math/Random.pm'}) {
   my ($_wid, $_cur_seed) = ($_args[1], Math::Random::random_get_seed(
+));

   my $_new_seed = ($_cur_seed < 1073741781)
      ? $_cur_seed + (($_wid * 100000) % 1073741780)
      : $_cur_seed - (($_wid * 100000) % 1073741780);

   Math::Random::random_set_seed($_new_seed, $_new_seed);
}

if ($INC{'Math/Random/MT/Auto.pm'}) {
   my ($_wid, $_cur_seed) = (
      $_args[1], Math::Random::MT::Auto::get_seed()->[0]
   );
   my $_new_seed = ($_cur_seed < 1073741781)
      ? $_cur_seed + (($_wid * 100000) % 1073741780)
      : $_cur_seed - (($_wid * 100000) % 1073741780);

   Math::Random::MT::Auto::set_seed($_new_seed);
}
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I'm hoping for the PDL development team to chime in.

I honestly wonder who you think that is.