Tunable Kernel Parameters
NAME [Toc] [Back]
pfdat_hash_locks - determines the size of the pfdat spinlock pool
VALUES [Toc] [Back]
Allowed values [Toc] [Back]
64 to 262144
DESCRIPTION [Toc] [Back]
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 contention while conserving memory.
This tunable sets the size of such a pool for the pfdat data structure
Who Is Expected to Change This Tunable?
Only HP Field engineers should change the value of this tunable.
Restrictions on Changing [Toc] [Back]
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
Hewlett-Packard Company - 1 - HP-UX 11i Version 2: Sep 2004
Tunable Kernel Parameters
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?
WARNINGS [Toc] [Back]
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 parameter 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
AUTHOR [Toc] [Back]
pfdat_hash_locks was developed by HP.
Hewlett-Packard Company - 2 - HP-UX 11i Version 2: Sep 2004 [ Back ]