Problem updating content with ajax?

Question

I’m trying to keep my page always updated with ajax , but as I’m beginner I created some functions that worked , but requires a lot of server process.

LIKE I DID?

1º I created a function that calls a page off (busca_p.php) who has an appointment at the bank and a while

//Função que chama a pagina de consulta 

<script type="application/javascript">
function busca (){
        var numero1 = $("#n1").val();
        var numero2 = $("#n2").val();
        $.post("busca_p.php", {n1:numero1, n2:numero2}, function(retorno){
            $(".msg_porto").html(retorno);
            });

        };

</script>

2nd I used the function setInterval to call the function every 1 second.

<script>
window.setInterval(busca,500);
</script>

// Pagina que consulta e carrega em uma tabela
<table width="95%" border="0" align="left" cellpadding="0" cellspacing="0">
 <tr><td>
 </td></tr>
 <?php
include('conexao.php');
$sql=mysql_query("select * FROM porto ORDER BY id ASC ");
while($dados=mysql_fetch_array($sql)){ 
     $filial = $dados['m'];

?>


  <tr>
    <th width="50%" scope="row">
    <br>

 <?php if($dados['m'] != ''){

     ?>   
<table width="85%" border="0" align="left" cellspacing="0" cellpadding="0">
<tr class="info">
<th scope="row" bgcolor="#DEEBEB">
<div class="fm">
<label> <h6><?php echo $dados['n'].' diz em '.$dados['d']; ?></h6> 
<span style="color: #FF0000"><?php echo wordwrap($dados['m'], 45, "<br />\n"); ?></span></label>
</div>
</th>
</tr>  
</table> 
<br>

<?php }?>

<br>
<?php if($dados['mm'] != ''){?>
<table width="85%" border="0" align="right" cellspacing="0" cellpadding="0">
<tr class="success">
<th scope="row" bgcolor="#C5C5C5"> 
<div class="mm">
<label><h6><?php echo $dados['nm'].' diz em '.$dados['dm'] ?></h6>
 <span style="color: #000000"><?php echo wordwrap($dados['mm'], 45, "<br />\n"); ?></span> </label>  
 </div>
</th>
</tr>
</table>
<br>

<?php }?> 

    </th>   
    </td>
    </tr>


    <?php }?>
    <tr><td>&nbsp;</td></tr>

3º the function calling the query page returns the result and loads in a div.

<div class="msg_porto">...</div>

The problem is that it keeps calling the page of query every 1 second and giving a while in the server and generating a high consumption of processing in the server.

how should I do ?