Unlocking the APC Code

APC (Alternative PHP Cache) started its life as fairly simple opcode cache for PHP. But along its development, a data cache seemed like a useful, yet simple addition to the package. As the usage of this grew, it became apparent that it was fraught with locking issues.

To ensure proper usage of shared memory, locks are essential. But the acquisition of locks are often hierarchical and assumptions about critical sections in code paths can lead to reduction in the number of lock and unlock calls involved. Also thread/process data separation can ensure that no two processes/threads hold pointers to the same area, allowing the removal of locks around such code.

Even though locks are the critical bottleneck that has been addressed, the talk also touches upon the other performance tweaks, tried and tested best practices, and a general summary of the new features in APC.