do while loop causing 100% CPU usage with curl_multi_exec

Try this: http://php.net/manual/function.curl-multi-select.php

Adding a call to http://php.net/sleep or http://php.net/usleep in every iteration should reduce the CPU usage by allowing other running processes to be scheduled by the operating system.

Unfortunately you didn't post whole code. I suppose you are doing something like

$mh = curl_multi_init();
for ($i = 0; $i < $desiredThreadsNumber; $i++) {
    $ch = curl_init();
    // set up $ch here
    curl_multi_add_handle($mh, $ch);
}

You should understand that you haven't run threads yet here. curl_multi_exec() runs all threads. But it can't run all $desiredThreadsNumber threads simultaneously. If you look on example on curl_multi_exec() php.net page, you will see that you must wait while curl_multi_exec() run all threads. In other words, you need next nested loop here:

$running = null;
do {
    do {
        $mrc = curl_multi_exec($mh, $running);
    } while ($mrc == CURLM_CALL_MULTI_PERFORM);
} while($running > 0);

At the end let me suggest you to read this article http://www.onlineaspect.com/2009/01/26/how-to-use-curl_multi-without-blocking/ and use code snippet from there, I used it in 2 or 3 projects.

You could add "sleep 1", which sleeps for one second, into the loop.

Try:

 $running = null;
    do {
        do {
            $mrc = curl_multi_exec($mh, $running);
        } while ($mrc == CURLM_CALL_MULTI_PERFORM && curl_multi_select($mh) === 0 );
    } while($running > 0  && $mrc == CURLM_OK );

curl_multi_select (http://php.net/manual/function.curl-multi-select.php) is indeed the way to go, but there are a couple caveats.

First, if curl_multi_exec returns CURLM_CALL_MULTI_PERFORM, it has more data to be processed immediately, so should be run again. Also, it is important to check that curl_multi_exec did not fail immediately; in that case curl_multi_select could block forever.

This should work:

do {
    while (CURLM_CALL_MULTI_PERFORM === curl_multi_exec($mh, $running)) {};
    if (!$running) break;
    while (curl_multi_select($mh) === 0) {};
} while (true);

If anyone sees a good way to avoid the while(true) without duplicating code, please point it out.