There is a bug in APCIterator: whatever the chunk size, APCIterator gets ALL keys matching the specified pattern (it seems that $chunk_size parameter is ignored).
Also, the documentation states that the default chunk size is 100, but even that is ignored.
Tested APC version is 3.1.13.
Example (I have 180 keys for this pattern in total):
$objIterator = new APCIterator('user', '/^key_prefix_\.*/', APC_ITER_ALL, 20);
If I itterate $objIterator and then count() the results, I get 180.
Also, for:
$objIterator = new APCIterator('user', '/^key_prefix_\.*/');
Same count(), 180.
Even $objIterator->getTotalCount() returns 180.