The PtrQueue class provides a free list for recycling buffers that have been used and processed.  This free list is presently protected by a Mutex.  Because the free list may need to be accessed as part of an Access barrier, the mutex must have "access" rank.  This mutex can also be a significant source of contention.

This free list could use a lock-free stack. The ABA problem for pop operations can be addressed by using GlobalCounter critical sections.  A pop operation is performed within a critical section.  A push operation first write_synchronizes then adds the buffer to the list.  This puts any delays on the "GC" processing side of the free list, making the mutator pops to obtain buffers inexpensive.