PR_FUTEX_HASH(2const) PR_FUTEX_HASH(2const)

PR_FUTEX_HASH - configure the private futex hash

Standard C library (libc, -lc)

#include <linux/prctl.h>  /* Definition of PR_* constants */
#include <sys/prctl.h>
int prctl(PR_FUTEX_HASH, unsigned long op, ...);

Configure the attributes for the underlying hash used by the futex(2) family of operations. The Linux kernel uses a hash to distribute the unrelated futex(2) requests to different data structures in order to reduce the lock contention. Unrelated requests are requests which are not related to one another because they use a different futex word. The data structure holds the in-kernel representation of the operation and keeps track of the current users which are enqueued and wait for a wake up. It also provides synchronisation of waiters against wakers. The size of the global hash is determined at boot time and is based on the number of CPUs in the system. Due to hash collision, two unrelated futex(2) requests can share the same hash bucket. This in turn can lead to delays of the futex(2) operation due to lock contention while accessing the data structure. These delays can be problematic on a real-time system since random processes can share in-kernel locks and it is not deterministic which process will be involved.

Linux 6.16 implements a process-wide private hash which is used by all futex(2) operations that specify the FUTEX_PRIVATE_FLAG option as part of the operation. Without any configuration the kernel will allocate 16 hash slots once the first thread has been created. If the process continues to create threads, the kernel will try to resize the private hash based on the number of threads and available CPUs in the system. The kernel will only increase the size and will make sure it does not exceed the size of the global hash.

The user can configure the size of the private hash which will also disable the automatic resize provided by the kernel.

The value in op is one of the options below.

On success, these calls return a nonnegative value. On error, -1 is returned, and errno is set to indicate the error.

Linux.

Linux 6.16.

prctl(2), futex(2), PR_FUTEX_HASH_GET_IMMUTABLE(2const), PR_FUTEX_HASH_GET_SLOTS(2const), PR_FUTEX_HASH_SET_SLOTS(2const)

2025-06-11 Linux man-pages 6.15