How can I tune the PHP realpath cache?

Recent versions of PHP have a cache of filenames for knowing the real path of files, and require_once() and include_once() can take advantage of it.

There's a value you can set in your php.ini to set the size of the cache, but I have no idea how to tell what the size should be. The default value is 16k, but I see no way of telling how much of that cache we're using. The docs are vague:

Determines the size of the realpath cache to be used by PHP. This value should be increased on systems where PHP opens many files, to reflect the quantity of the file operations performed.

Yes, I can jack up the amount of cache allowed, and run tests with ab or some other testing, but I'd like something with a little more introspection than just timing from a distance.

You've probably already found this, but for those who come across this question, you can use realpath_cache_size() and realpath_cache_get() to figure out how much of the realpath cache is being used on your site and tune the settings accordingly.