Why is debug_backtrace() using so much memory?

Question

When trying to trace some memory issues in PHP, I noticed that debug_backtrace(), which I call in my logging code, seemed to be using a lot of memory.

In most cases, the following code prints something like 0.02 MB. But in one case, it prints 171.85 MB!

$before = memory_get_usage();
$backtrace = debug_backtrace(false);
$after = memory_get_usage();
echo round(($after - $before)/1024/1024, 2)." MB";

My question is, does this mean that debug_backtrace is actually using that much memory? Or could something else be happening, like garbage collection, that messes up the return value from memory_get_usage?

Answer 1

Its the objects, most likely, that are causing the bloat. Try passing false to the function so you don't pull the objects and your traces will be much smaller.

EDIT: If passing false doesn't work then if you're running PHP 5.3.6+ you can use a bitmask to limit what the function returns. What it sounds like is that you have objects being passed as args that are huge.

http://php.net/manual/en/function.debug-backtrace.php Reference

Additionally if you are using PHP 5.4.0+ they added a second param that will allow you to limit the number of stack frames.

EDIT2: total <<HACK>> here, but works ... add a try/catch, throw an exception and catch it then convert to string or call exception getTraceAsString() to get the full stack. Example:

try {
    throw new Exception('ignore this string');
} catch(Exception $e) {
    /* @var $trace array */
    $trace = $e->getTrace();

    // OR

    /* @var $str string */
    $str = $e->getTraceAsString();
    $e = null;
}

In the above snipped you can use $trace and build your own output or just use the standard exception as string $str. Easier to get the stack frame output.