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

Okay, I think I figured it out. I printed out the backtrace, and the "args" array was huge. This is because I was passing around some enormous strings. I guess it’s making copies of them (instead of references) when returning the results.

For example:

function test($str) {
    test2($str);
}
function test2($str) {
    test3($str);
}
function test3($str) {
    echo "before: ".round(memory_get_usage()/1024/1024, 2)." MB\n";
    debug_backtrace(false);
    echo "after: ".round(memory_get_usage()/1024/1024, 2)." MB\n";
}
test(str_repeat('a', 10000000));

Try this with and without the debug_backtrace() call. With it, the memory usage is increased by about 28 MB. It’s only cleared when test3() returns.