Compiling perl 5.10.1 without USE_IMP_SYS and with USE_PERL_MALLOC makes a huge difference. The following script grows two strings, each in a separate thread, to 1/2GB in 1000-byte increments in a little under 2 1/2 seconds:

#! perl -slw
use strict;
use Time::HiRes qw[ time ];
use threads;

<>;
my $start = time;

async {
    my $s = chr(0) x 1000;
    for( 1 .. 5e5 ) {
        $s .= chr( 0 ) x 1000;
    }
}->detach;

my $s = chr(0) x 1000;
for( 1 .. 5e5 ) {
    $s .= chr( 0 ) x 1000;
}

printf "Re-allocated 2 x 500MB (in 1000 byte incements) on two threads
+ in %.3f\n",
    time() - $start;

<>;

__END__
C:\perl\5.10.1\bin>.\perl.exe \test\mem.pl

Re-allocated 2 x 500MB (in 1000 byte incements) on two threads in 2.41
+6
[download]

Now the question is: why is USE_IMP_SYS required for fork emulation? And can that be corrected?

Cluebats welcomed. Along with any thoughts on more thorough testing of Perl_malloc and threading.