2.2. Ticket Spinlocks

A key part of any system design is ensuring that one process does not alter memory used by another process. Uncontrolled data change in memory can result in data corruption and system crashes. To prevent this, the operating system allows a process to lock a piece of memory, perform an operation, then unlock or "free" the memory.

One common implementation of memory locking is through spin locks, which allow a process to keep checking to see if a lock is available and take the lock as soon as it becomes available. If there are multiple processes competing for the same lock, the first one to request the lock after it has been freed gets it. When all processes have the same access to memory, this approach is "fair" and works quite well.

Unfortunately, on a NUMA system, not all processes have equal access to the locks. Processes on the same NUMA node as the lock have an unfair advantage in obtaining the lock. Processes on remote NUMA nodes experience lock starvation and degraded performance.

To address this, Red Hat Enterprise Linux implemented ticket spinlocks. This feature adds a reservation queue mechanism to the lock, allowing all processes to take a lock in the order that they requested it. This eliminates timing problems and unfair advantages in lock requests.

While a ticket spinlock has slightly more overhead than an ordinary spinlock, it scales better and provides better performance on NUMA systems.