HP-UX Reference (11i v2 04/09) - 5 Miscellaneous Topics (vol 9)

p
pfdat_hash_locks(5) pfdat_hash_locks(5)
(Tunable Kernel Parameters)
NAME
pfdat_hash_locks - determines the size of the pfdat spinlock pool
VALUES
Default
128
Allowed values
64 to 262144
DESCRIPTION
Global kernel structures containing information on a running process or memory usage are frequently
accessed or modified by several threads concurrently. To prevent race conditions, these structures are
protected by spinlocks (kernel data used for synchronization) which allow only the spinlock structure
must wait.
Hashed spinlocks are used when each instance of such a data structure is to be protected, and there are
several instances. Using a single spinlock for all instances would cause too much contention, but using
one spinlock per structure wastes memory while the majority of the locks are unused at any given time.
By allocating a pool of hashed locks, a hash function picks one lock per group of structures, reducing con-
tention while conserving memory. This tunable sets the size of such a pool for the pfdat data structure
spinlocks.
Who Is Expected to Change This Tunable?
Only HP Field engineers should change the value of this tunable.
Restrictions on Changing
Changes to this tunable take effect at the next reboot.
When Should the Value of This Tunable Be Raised?
This tunable should be raised on systems with both low memory contention and high lock contention due
to a large number of pfdat structures. Typically, such a system would be a large memory system with a
large amount of processes or threads.
What Are the Side Effects of Raising the Value?
More memory is used by the kernel.
When Should the Value of This Tunable Be Lowered?
This tunable should only be lowered on systems with high contention for physical memory and few
processes and threads. The memory saved by making the table smaller will be freed for general use, and
could cause contention if many pfdat structures are present and requiring locks.
What Are the Side Effects of Lowering the Value?
Hash contention for pfdat locks is more likely, which increases the possibility of threads needing to wait
on the lock shared between two or more pfdats.
What Other Tunables Should Be Changed at the Same Time?
None.
WARNINGS
All HP-UX kernel tunable parameters are release specific. This parameter may be removed or have its
meaning changed in future releases of HP-UX.
Installation of optional kernel software, from HP or other vendors, may cause changes to tunable parame-
ter values. After installation, some tunable parameters may no longer be at the default or recommended
values. For information about the effects of installation on tunable values, consult the documentation for
the kernel software being installed. For information about optional kernel software that was factory
installed on your system, see HP-UX Release Notes at
http://docs.hp.com.
AUTHOR
pfdat_hash_locks was developed by HP.
Section 5−−262 Hewlett-Packard Company − 1 − HP-UX 11i Version 2: September 2004