sched_thread_affinity(5)

NAME

sched_thread_affinity — adjust scheduler thread affinity

VALUES

Failsafe

6

Default

6

Allowed values

Any value in the range of 1 to 10 is allowed.

A value of 1 indicates weak thread affinity, and the threads may run on different processors. A value of 10 indicates strong thread affinity, and the threads will likely remain on the same processor.

Recommended values

Use the default value in normal cases.

In special cases where it is imperative that threads run as quickly as possible, a value closer to 1 may be used. However, this may lead to an increase in cache misses.

Customers must evaluate the performance impact in their workload environment before changing the value of this tunable on production systems.

DESCRIPTION

The sched_thread_affinity tunable defines on which processor a thread may likely run. A thread with lower affinity may move from one processor to another during its lifetime, whereas a thread with higher affinity will likely experience minimal movement across processors.

A change in the sched_thread_affinity value may have direct impact on system throughput and response times. A very small value may result in threads being run much quicker at the possible expense of higher cache misses, and a very large value may result in threads being run less frequently if they have run (or are running) on a loaded processor, with a possible benefit of improved cache affinity.

I/O intensive applications are likely to benefit from a lower value for this tunable.

Who Is Expected to Change This Tunable?

Anyone.

Restrictions on Changing

Changes to this tunable take effect immediately.

When Should the Value of This Tunable Be Raised?

Any increase in the value of this tunable will cause most threads to remain on the processor where they have run during their lifetimes.

What Are the Side Effects of Raising the Value?

Raising the value of the sched_thread_affinity tunable may cause some threads to wait to run, even though there may be less loaded or idle processors in the system.

When Should the Value of This Tunable Be Lowered?

The sched_thread_affinity tunable value should be lowered if the workload expects to run as quickly as possible, regardless of whether it is moved from processor to processor at the cost of possible increases in cache misses.

What Are the Side Effects of Lowering the Value?

Lowering sched_thread_affinity may result in an increase in cache misses as threads begin to run on different processors during their lifetimes.

What Other Tunable Values 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 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 http://docs.hp.com.

AUTHOR

sched_thread_affinity was developed by HP.