--- /dev/null
+.\" Copyright, the authors of the Linux man-pages project
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.TH PR_FUTEX_HASH 2const (date) "Linux man-pages (unreleased)"
+.SH NAME
+PR_FUTEX_HASH
+\-
+configure the private futex hash
+.SH LIBRARY
+Standard C library
+.RI ( libc ,\~ \-lc )
+.SH SYNOPSIS
+.nf
+.BR "#include <linux/prctl.h>" " /* Definition of " PR_* " constants */"
+.B #include <sys/prctl.h>
+.P
+.BI "int prctl(PR_FUTEX_HASH, unsigned long " op ", ...);"
+.fi
+.SH DESCRIPTION
+Configure the attributes for the underlying hash used by the
+.BR futex (2)
+family of operations.
+The Linux kernel uses a hash to distribute the unrelated
+.BR 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
+.BR futex (2)
+requests can share the same hash bucket.
+This in turn can lead to delays of the
+.BR 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.
+.P
+Linux 6.16 implements a process-wide private hash which is used by all
+.BR futex (2)
+operations that specify the
+.B 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.
+.P
+The user can configure the size of the private hash
+which will also
+disable the automatic resize provided by the kernel.
+.P
+The value in
+.I op
+is one of the options below.
+.TP
+.B PR_FUTEX_HASH_GET_IMMUTABLE
+.TQ
+.B PR_FUTEX_HASH_GET_SLOTS
+.TQ
+.B PR_FUTEX_HASH_SET_SLOTS
+.SH RETURN VALUE
+On success,
+these calls return a nonnegative value.
+On error, \-1 is returned, and
+.I errno
+is set to indicate the error.
+.SH STANDARDS
+Linux.
+.SH HISTORY
+Linux 6.16.
+.SH SEE ALSO
+.BR prctl (2),
+.BR futex (2),
+.BR PR_FUTEX_HASH_GET_IMMUTABLE (2const),
+.BR PR_FUTEX_HASH_GET_SLOTS (2const),
+.BR PR_FUTEX_HASH_SET_SLOTS (2const)