Question

I’m testing out an interesting “threading” concept in PHP. Basically, I’m working to have a parent process spawn children that do stuff and return the results back to the parent via STDOUT in sort of a callback manner.

The way my example does it, the inner loop is too tight and burns 99% CPU:

while(true){
        $ret = $thread->Listen();
        if( $ret == 'DOINGSTUFF' ){
                break;
        }
}

I’d like to tone this while loop down to looping every 5 seconds using, for example, a sleep(). This brings CPU utilization to a much more rational level; however I’m concerned it will then “miss” some of the children “threads” returning results in case one child finishes during the parent’s sleeping process.

Can a PHP expert tell me whether this concern is well founded and if so, whether there’s anything I can do to cope?

Answer 1

Your example uses proc_open, which isn’t exactly multi-threading. Anyway, when using proc_open, the pipes streams will be available as long as proc_close is not called, even if the process has actually terminated:

$proc = proc_open('which ls', $descriptorspec, $pipes);
sleep(5);
echo stream_get_contents($pipes[1]); // output '/bin/ls'

$proc = proc_open('which ls', $descriptorspec, $pipes);
sleep(5);
proc_close($proc);
echo stream_get_contents($pipes[1]); // E_WARNING: $pipes[1] not a valid stream

Also, it is not necessary to use sleep. Usually, you don’t want a script hanging for 5 seconds every pass doing nothing. Use usleep instead, which will also reduce the CPU usage but will still make the script responsive:

while (true) {
   // do stuff
   usleep(50);
}

The above will sleep for 50 micro-seconds, leaving CPU cycles to other running processes.

As for pure multi-threading, it is not possible in PHP. All you can do is fork a process. The difference with actual forking and the method you use (proc_open) is that parent process data (variables, etc.) are forked to the child process as well. Forking should not be used in web applications, but only command-line programs, because web servers do not support it very well.