#!/usr/bin/perl;

use strict;
use warnings;
use autodie;
use Benchmark qw(timethis);
use List::Util qw(shuffle);

my $block_size = 8 * 1024;     #   8 KB
my $size = 128 * 1024 * 1024;  # 128 MB
my $blocks = $size / $block_size;

my $fn = '/tmp/seekme.pl';

warn "file size: $size, block size: $block_size, blocks: $blocks\n";

warn "writing file...\n";
my $kb = '1' x $block_size;
open my $fh, ">", $fn;
print $fh $kb for (1..$blocks);
close $fh;
warn "file written\n";

sub clean_cache {
    warn "cleaning disk cache\n";
    system "sync ; sudo echo 3 > /proc/sys/vm/drop_caches";
    warn "disk cache clean\n"
}


my @order = shuffle 0..($blocks -1);
my $buf;

clean_cache();

warn "timing preload and random seek\n";
timethis 1 => sub {
    open my $fh, '<', $fn;
    for (@order) {
        read $fh, $buf, $block_size;
    }

    for (@order) {
        seek($fh, $_ * $block_size, 0);
        read $fh, $buf, $block_size;
    }
};

warn "timing random seek with preloaded disk cache\n";
timethis 1 => sub {
    open my $fh, '<', $fn;
    for (@order) {
        seek($fh, $_ * $block_size, 0);
        read $fh, $buf, $block_size;
    }
};

clean_cache();

warn "timing random seek with empty disk cache\n";
timethis 1 => sub {
    open my $fh, '<', $fn;
    for (@order) {
        seek($fh, $_ * $block_size, 0);
        read $fh, $buf, $block_size;
    }
};

##</code><code>##

file size: 134217728, block size: 8192, blocks: 16384
writing file...
file written
cleaning disk cache
disk cache clean
timing preload and random seek
timethis 1:  6 wallclock secs ( 1.34 usr +  0.36 sys =  1.70 CPU) @  0.59/s (n=1)
timing random seek with preloaded disk cache
timethis 1:  1 wallclock secs ( 0.88 usr +  0.06 sys =  0.94 CPU) @  1.06/s (n=1)
cleaning disk cache
disk cache clean
timing random seek with empty disk cache
timethis 1: 93 wallclock secs ( 1.29 usr +  1.15 sys =  2.44 CPU) @  0.41/s (n=1)