futexbased

Futexbased is an adjective used to describe synchronization primitives and designs that rely on futexes, or fast userspace mutexes, to coordinate multiple threads in user space with kernel support for blocking and waking when contention occurs. Futex-based techniques aim to perform most synchronization in user space, reserving kernel intervention for the contended path.

The core idea is a two-path approach: an uncontended fast path uses atomic operations in userspace to acquire or release a lock; when contention is detected, a futex syscall is used to block the thread efficiently. Other threads waiting on the same lock are awakened via futex wake calls. This minimizes system calls for typical operation while providing correct synchronization under load.

Futex-based synchronization is a central mechanism in Linux thread libraries, notably the GNU C Library's implementation of pthread mutexes and condition variables. In such implementations, the futex-based approach underpins the standard pthread primitives on Linux, enabling scalable multithreading for server and desktop workloads.

Advantages include reduced overhead for uncontended locks and scalable behavior for many cores. Limitations include the need for the Linux futex subsystem, potential complexity in implementation, and sensitivity to kernel scheduling, memory ordering, and priority inversion. Portability is limited to Linux or environments implementing the futex interface.

See also futex, Linux kernel, pthreads, glibc, synchronization primitives.