How many times does curl_multi_exec have to be called?

Question

I'm using curl_multi with multi to upload files to different servers. Each server has multiple files that need uploading, so I have a curl_multi request for each server. When I execute the curl_multi handles, I just execute all the curl_multi handles in the same loop, like so:

<?php
do {

 $continue_running=false;

 foreach($handles as $host => $handle) {

  if(!is_resource($handle[0])) {
   die("\nHandle is not a resource!\n");
  }

  if($running_{$host}) {
   if(curl_multi_exec($handles[$host][0], $running_{$host}) != CURLM_OK) {
    die("\ncurl_multi_exec failed!\n");
   }
   $continue_running=true;
  }

  if(!$running_{$host} && !$done_{$host}) {
   echo "$host finished in ".(microtime(1)-$start)." seconds\n";
   $done_{$host}=true;
  }
 }

} while ($continue_running);
?>

What I'm wondering is, how many times does curl_multi_exec actually have to be called in a curl request? Does it need to be called for each little bit of data transfered? Its using a lot of cpu and I'm thinking that its because its "busy looping" too much. So can I add sleep(5); at the end of each loop to make it use less cpu cycles, or will this slow down the requests majorly?

I would use curl_multi_select, but I cant because theres multiple curl_multi_execs being processed.