CPU Configuration Guidelines for vPars

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1 Audience
This white paper is intended for individuals who are interested in using HP-UX virtual partitions (vPars)
to consolidate multiple systems onto a single system or hard partition. This document explains how to
manage CPU assignments on vPars to achieve predictable performance under varying workloads.
This paper assumes that the reader has a good knowledge of the HP-UX operating system and is
familiar with the concepts of vPars, processor sets, and I/O interrupts processing.
2 HP-UX Virtual Partitions Overview
vPars allows the system administrator to create independent HP-UX operating environments from
hardware resources in a system or hard partition. Each of the HP-UX operating environments can be
tuned specifically for the applications that will be hosted.
A vPar is composed of one or more CPU’s, memory, and I/O devices for mass storage and
networking. The minimum amount of memory necessary to boot a vPar is based on the minimum
required by the operating system version (HP-UX 11i v1 requires 128 MB minimum but 256 MB is
recommended and HP-UX 11i v2 requires a minimum of 1 GB).
All the resources of each vPar are specified in a partition plan (vPars database) that resides on the
boot disk of each vPar. The vPar commands vparcreate and vparmodify are used to create, add,
delete, and modify resources in the partition plan. Each vPar should be configured with enough
memory and I/O to sustain the peak workload that will be placed on it. The CPU’s on the other hand
can be configured to handle steady state and peak workload conditions by using the CPU migration
facilities of vPars.
2.1 CPU Assignment on vPars Releases Prior to A.04.01
vPars releases falling into this category have CPU’s that can be configured as bound or unbound
(floating) CPU’s. The basic differences between a bound CPU and unbound CPU are that an unbound
CPU does not participate in I/O interrupt processing. Unbound CPU’s on the other hand are more
flexible because they can be dynamically migrated into a running vPar when additional processing
capacity is needed and migrated out when the need subsides.
The following are definitions of configuration parameters for vPar CPU’s.
• min – user configurable setting for the minimum number of CPU’s that are assigned to a vPar.
The “min” values equates to the number of Bound CPU’s for a vPar.
• max – user configurable setting for the maximum number of CPU’s that can be assigned to a
vPar. The “max” value includes both Bound and Unbound CPU’s.
• Num – user configurable setting for the current number of CPU’s that are assigned to a vPar.
The “num” value includes both Bound and Unbound CPU’s. The “num” value minus the “min”
value gives you the number of Unbound CPU’s for a vPar.
2.2 CPU Assignment on vPars Releases Starting with A.04.01
In A.04.01 and subsequent releases, all CPU’s are capable of participating in I/O interrupt
processing and all CPU’s other than the boot processor are capable of migrating while the vPar is
running. The HP-UX kernel assigns I/O interrupts to the CPU’s made visible to it by the vPars monitor
during boot of the operating system. The number of CPU’s that will be assigned interrupts is based on
the value num (num is the current number of CPU’s assigned to the vPar and ranges from min to max).
The value associated with num can be obtained by issuing a vparstatus command by matching the