Hi choroba and 1nickt,

I wonder about choroba's example involving several array fetches inside the loop. The following attempts to find out.

Diff output

$ diff choroba.pl 1nickt.pl
8,9c8,11
<     push @seq, ( $seq[-1] / 2, 3 * $seq[-1] + 1 )[ $seq[-1] % 2 ]
<         while $seq[-1] != 1;
---
>     while ( $n != 1 ) {
>         $n = $n % 2 ? 3 * $n + 1 : $n / 2;
>         push @seq, $n;
>     }
[download]

Demo choroba.pl

use warnings; use strict;
use feature 'say';
use MCE::Flow;

sub collatz {
    my ($n) = @_;
    my @seq = $n;
    push @seq, ( $seq[-1] / 2, 3 * $seq[-1] + 1 )[ $seq[-1] % 2 ]
        while $seq[-1] != 1;
    return @seq;
}

my @sizes;

mce_flow_s {
    max_workers => MCE::Util::get_ncpu(),
    bounds_only => 1,
    gather      => \@sizes,
}, sub {
    my ($start, $end) = @{ $_[1] };
    my @chunk_sizes;
    push @chunk_sizes, [ $_, scalar collatz($_) ] for $start .. $end;
    MCE->gather( @chunk_sizes );
}, 1, 1e6;

MCE::Flow->finish;

say "@$_" for reverse +(sort { $b->[1] <=> $a->[1] } @sizes)[0..19];
[download]

Demo 1nickt.pl

use warnings; use strict;
use feature 'say';
use MCE::Flow;

sub collatz {
    my ($n) = @_;
    my @seq = $n;
    while ( $n != 1 ) {
        $n = $n % 2 ? 3 * $n + 1 : $n / 2;
        push @seq, $n;
    }
    return @seq;
}

my @sizes;

mce_flow_s {
    max_workers => MCE::Util::get_ncpu(),
    bounds_only => 1,
    gather      => \@sizes,
}, sub {
    my ($start, $end) = @{ $_[1] };
    my @chunk_sizes;
    push @chunk_sizes, [ $_, scalar collatz($_) ] for $start .. $end;
    MCE->gather( @chunk_sizes );
}, 1, 1e6;

MCE::Flow->finish;

say "@$_" for reverse +(sort { $b->[1] <=> $a->[1] } @sizes)[0..19];
[download]

Run time on an AMD 3rd Gen Ryzen Threadripper 3970x box - SMT disabled

max_workers =>  1
  choroba  21.432 seconds
  1nickt   18.644 seconds

max_workers =>  2
  choroba  10.808 seconds
  1nickt    9.348 seconds

max_workers =>  4
  choroba   5.836 seconds
  1nickt    4.992 seconds

max_workers =>  8
  choroba   3.163 seconds
  1nickt    2.731 seconds

max_workers => 16
  choroba   1.835 seconds
  1nickt    1.623 seconds

max_workers => 32
  choroba   1.218 seconds
  1nickt    1.105 seconds
[download]

Interestingly, the more cores the lesser the difference on this hardware.

Regards, Mario