public function percentUsage()
    {
        $total = $this->systemTotal();
        $free  = $this->systemFree();

        $usage = [];

        foreach( $total as $slotId => $amount ) {
            $usage[ $slotId ] = intval( ceil( ( ( $amount - $free[ $slotId ] ) * 100 ) / $amount ) );
        }

        return $usage;
    }