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Tunable Kernel Parameters
HP-UX 11i Version 3: February 2007

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lcpu_attr — dynamically enable or disable LCPU attribute of the default processor set


Fail Safe

0 (off)


0 (off)

the LCPU attribute of the default processor set is disabled

Allowed values

0 (off) or 1 (on)


This tunable dynamically enables (1) or disables (0) the logical processor (LCPU) attribute in the default processor set. On systems supporting hyper-threading technology, each hyper-thread is represented as an LCPU.

When the LCPU attribute is enabled, the processor cores in the default processor have hyper-threading enabled. If the LCPU attribute is disabled, the physical processors in the default processor set behave as single threaded processor cores.

See pset_setattr(2) for information about managing the LCPU attribute in processor sets other than the default processor set.

Who Is Expected to Change This Tunable?

System administrators that wish to change hyper-threading ability in the default processor set.

Restrictions on Changing

The tunable has no effect on platforms that do not have hyper-threading capability or systems with hyper-threading disabled at the firmware level. On systems that support hyper-threading, the firmware feature must be enabled to dynamically enable/disable the LCPU attribute in the default processor set.

When Should the Tunable Be Turned On?

The tunable should be turned on to enable the LCPU attribute in the default processor set to take advantage of hyper-threading. If the applications running on the system see performance benefits, the LCPU attribute should remain enabled.

What Are the Side Effects of Turning the Tunable On?

Some workloads may experience performance degradation when the LCPU attribute is enabled.

When Should the Tunable Be Turned Off?

Some applications may experience performance degradation or may not see any performance benefits from using hyper-threading. In this case, this tunable should be turned off.

What Are the Side Effects of Turning the Tunable Off?

With LCPU turned off, applications may not take advantage of hyper-threading.

What Other Tunables Should Be Changed at the Same Time?



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.


lcpu_attr was developed by HP.

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