in reply to Is rand() really that slow or am I just using it wrong?

Most of your time is being spent building an 8-char string from 8 random bytes, then copying that to a 16-byte string and adding 8 more random bytes; then copying that to a 32-byte string and adding 16 more random bytes; then copying that to a 64-byte string ...

Try this version: 

#! perl -slw
use strict;

my @chars = ('a'..'z', 'A'..'Z',0..9, qw[- _ ! & ? = ] );
sub genFile {
    my( $fh, $mb ) = @_;
    my $buf = chr(0) x 1024**2;
    for( 1 .. $mb ) {
        substr $buf, $_, 1, $chars[ rand @chars ] for 0 .. 1024**2-1;
        print $fh $buf;
    }
}

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Re^2: Is rand() really that slow or am I just using it wrong?
by adiuva (Sexton) on Aug 08, 2013 at 18:56 UTC
    Thanks for the fast reply! But even though your code looks a lot nicer than mine it's not faster. :(
    If you generate a 10MB file you can still see 1MB chunks being written to the harddisk, while using the sub superfast() the file is there instantly.
[reply]
      it's not faster

      It *is* faster than your method.

      Not as fast as writing 10MB of a single character for sure, but then you are calling a function, rand 10 million times; it has to be slower.

      The only question is; how much faster do you need it to be?

      With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
      Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
      "Science is about questioning the status quo. Questioning authority".
      In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]

        OK, I'm confused. I wanted to see how much faster BrowserUk's version was. But Benchmark is showing that it is slower. 

        #!/usr/bin/perl

use warnings;
use strict;
use Benchmark qw( cmpthese );

my @chars = ('a'..'z', 'A'..'Z',0..9, qw[- _ ! & ? = ] );

open my $fh, ">", "/dev/null";

cmpthese( -20, {
            Original => sub { generateRandomFile($fh, 5) },
            BrowserUk => sub {genFile($fh, 5) },
        });


sub generateRandomFile
{
    my( $gen_file_h, $final_size ) = @_;

    for (my $mbytes = 0; $mbytes < $final_size; $mbytes ++) 
        {
            my $string="";
            my @chars = split(" ","a b c d e f g h i j k l m n o p q r
+ s t u v w x y z A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 
+- _ ! & ? = 0 1 2 3 4 5 6 7 8 9");
            for (1..1048576)
                {
                    my $rand = int(rand(68));
                    $string .= $chars[$rand];
                }
            print $gen_file_h $string;
        }
}
sub genFile {
    my( $fh, $mb ) = @_;
    my $buf = chr(0) x 1024**2;
    for my $chi ( 1 .. $mb ) {
        substr $buf, $_, 1, $chars[ rand @chars ] for 0 .. 1024**2-1;
        print $fh $buf;
    }
}
__END__

          s/iter BrowserUk  Original
BrowserUk   1.34        --      -15%
Original    1.13       18%        --

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