#!/usr/bin/perl

use strict;

my $usegb = 1000*shift;
my $timeout = shift || 10;

my $giga = 2**30;
my $forks = 1;

my $free;

my $memtotal = $usegb || int(
	qx{ awk '/^(MemTotal):/ { sum += \$2} END { print sum }' /proc/meminfo }
    / 1024);

my $total = 1;

# 1 MB random data so it is not compressed
my $onemb = pack("L*", map { rand(2**32) } 1..(2**18));
my ($thisround, $sofar,$timediff, %buf, $pid,$shift);
do {
    my $start = time;
    $free =
	int (
	    qx{ awk '/^((Swap)?Cached|MemFree|Buffers):/ { sum += \$2} END { print sum }' /proc/meminfo }
	    / 1024)
	or
	int (
	    qx{ awk '/^MemAvailable:/ { sum += \$2 } END { print sum }' /proc/meminfo }
	    / 1024);
    print "Free $free ";
    if($free <= 1) {
	print "\nFree < 1\n";
	exit(1);
    }
    # Use at most 1 GB per round
    $thisround = min(1000,int($free/100));
    $total += $thisround;
    for(1..$thisround) {
	# Shift every block 1 byte, so no blocks have the same content
	$buf{$forks}{$total}{$_} = "x"x(++$shift) . $onemb;
	if($shift > 4095) {
	    # If shifted a full page: Recompute 1mb random
	    $shift -= 4096;
	    $onemb = pack("L*", map { rand(2**32) } 1..(2**18));
	}
    }
    $timediff = time - $start;

    print "Chunk size: $thisround Time for swapping: $timediff seconds. Total memory used: $total\n";
    if($total * 1048576 > $forks * $giga) {
	if($pid = fork()) {
	    print "child spawn ",$forks,"\n";
	    wait;
	    print "child exit ",$forks,"\n";
	} else {
	    $buf{$forks} = 1;
	    $forks++;
	}
    }
} until ($pid or $timediff > $timeout or $total > $memtotal);
print "exit ",$forks,"\n";

sub min {
    # Returns:
    #   Minimum value of array
    my $min;
    for (@_) {
        # Skip undefs
        defined $_ or next;
        defined $min or do { $min = $_; next; }; # Set $_ to the first non-undef
        $min = ($min < $_) ? $min : $_;
    }
    return $min;
}