PHP Exec() Performance

PHP exec() performance

When you log in to a Unix machine, either at the keyboard, or over ssh, you create a new instance of a shell. The shell is usually something like /bin/sh or /bin/bash. The shell allows you to execute commands.

When you use exec(), it also creates a new instance of a shell. That instance executes the commands you sent to it, and then exits.

When you create a new instance of a shell command, it has it's own environment variables. So if you do this:

exec('env MAGICK_THREAD_LIMIT=1');
exec('/usr/local/bin/convert 1.pdf -density 200 -quality 85% 1.jpg');

Then you create two shells, and the setting in the first shell never gets to the second shell. To get the environment variable into in the second shell, you need something like this:

exec('env MAGICK_THREAD_LIMIT=1; /usr/local/bin/convert 1.pdf -density 200 -quality 85% 1.jpg');

Now, if you think that the shell itself may be the problem, because it takes too long to make a shell, test it with something that you know takes almost no time:

$starttime = microtime(true);
exec('echo hi');
$endtime = microtime(true);
$time_taken = $endtime-$starttime;

At that point you know to try and find some way to make the shell instantiate faster.

Hope this helps!

Tracking the script execution time in PHP

On unixoid systems (and in php 7+ on Windows as well), you can use getrusage, like:

// Script start
$rustart = getrusage();

// Code ...

// Script end
function rutime($ru, $rus, $index) {
    return ($ru["ru_$index.tv_sec"]*1000 + intval($ru["ru_$index.tv_usec"]/1000))
     -  ($rus["ru_$index.tv_sec"]*1000 + intval($rus["ru_$index.tv_usec"]/1000));
}

$ru = getrusage();
echo "This process used " . rutime($ru, $rustart, "utime") .
    " ms for its computations\n";
echo "It spent " . rutime($ru, $rustart, "stime") .
    " ms in system calls\n";

Note that you don't need to calculate a difference if you are spawning a php instance for every test.

Feasibility of using PHP exec() to speed up backend

While you could farm work out to other programs using exec and friends, the thing you're trying to do is best accomplished using a message queue system. A well-designed message queue will let you write worker programs in any language you need. They're a great solution when you need to do something in another language, environment, or server.

There are lots of message queues, but I'm a big fan of Gearman. It was built by the same folks that brought us memcached. Take a peek at the PECL extension.

Increasing performance of exec() statement

$ php -r 'var_dump(dns_get_record("35.174.0.154.in-addr.arpa"));'

Returns the following in under a one fifth of a second, including invoking the PHP interpreter:

array(2) {
  [0]=>
  array(5) {
    ["host"]=>
    string(25) "35.174.0.154.in-addr.arpa"
    ["class"]=>
    string(2) "IN"
    ["ttl"]=>
    int(7192)
    ["type"]=>
    string(3) "PTR"
    ["target"]=>
    string(16) "kent.aserv.co.za"
  }
  [1]=>
  array(5) {
    ["host"]=>
    string(25) "35.174.0.154.in-addr.arpa"
    ["class"]=>
    string(2) "IN"
    ["ttl"]=>
    int(7192)
    ["type"]=>
    string(3) "PTR"
    ["target"]=>
    string(25) "motairgdiool.hosted.co.za"
  }
}

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