Light-weight Locks

Abstract: In this paper, we propose a new approach to building synchronization primitives, dubbed "lwlocks" (short for light-weight locks). The primitives are optimized for small memory footprint while maintaining efficient performance in low contention scenarios. A read-write lwlock occupies 4 bytes, a mutex occupies 4 bytes (2 if deadlock detection is not required), and a condition variable occupies 4 bytes. The corresponding primitives of the popular pthread library occupy 56 bytes, 40 bytes and 48 bytes respectively on the x86-64 platform. The API for lwlocks is similar to that of the pthread library but covering only the most common use cases. Lwlocks allow explicit control of queuing and scheduling decisions in contention situations and support "asynchronous" or "deferred blocking" acquisition of locks. Asynchronous locking helps in working around the constraints of lock-ordering which otherwise limits concurrency. The small footprint of lwlocks enables the construction of data structures with very fine-grained locking, which in turn is crucial for lowering contention and supporting highly concurrent access to a data structure. Currently, the Data Domain File System uses lwlocks for its in-memory inode cache as well as in a generic doubly-linked concurrent list which forms the building block for more sophisticated structures.