At Time C:

•CPU resource use for Group1 and Group2 is limited to the assigned shares. After Time C:

•PRM holds each group to its assigned available CPU resource percentage until total available CPU resource demand is less than 100%. This gives Group2 a priority for CPU resources over Group1. In contrast, in the standard HP-UX scheduler, CPU time is allocated based upon the assumption that all processes are of equal importance. Assuming there is one process associated with each PRM group, the standard HP-UX scheduler would allocate each process 50% of the available CPU resources after Time C.

CPU allocation and number of shares assigned

When managing FSS PRM groups, PRM favors processes in groups with a larger number of CPU shares over processes in groups with fewer CPU shares. Processes in FSS PRM groups with a larger number of CPU shares are scheduled to run more often and are given more opportunities to consume CPU time than processes in other FSS PRM groups. This preference implies that the process in an FSS PRM group with a larger number of shares may have better response times with PRM than with the standard HP-UX scheduler.

An FSS PRM group can use more than its configured CPU allocation when the system is at nonpeak load—unless CPUCAPON mode is enabled or a per-group cap equal to its allocation has been assigned. (For more information on capping options, see the next section, “Capping CPU resource use” (page 24).)

Capping CPU resource usePRM gives you two options for capping CPU resource use by FSS PRM groups:•On a per-group basis

(Available for HP-UX 11i v3 and later.) For per-group capping, use the MAX field in the FSS PRM group record (discussed in the section “Group/CPU record syntax” (page 55) ) for only those groups you want to cap.

•For all FSS PRM groups in the configuration

The CPUCAPON mode, enabled through the prmconfig -Moption discussed below, treats the FSS PRM group’s minimum allocation as its maximum allocation.

When CPUCAPON mode is enabled, CPU capping is in effect for all user-configured FSS PRM groups on a system—regardless of CPU load. Each FSS PRM group takes its entire CPU allocation. Thus, no group can obtain more CPU resources.

The PRM_SYS group, however, is exempt from capping. If it gets CPU time and has no work, the PRM scheduler immediately goes to the next FSS PRM group.

NOTE: Capping based on the CPUCAPON mode overrides per-group capping; however, using both forms of capping at the same time is not recommended.

For PSET PRM groups, capping is a result of the number of cores assigned to the group.

Capping CPU usage can be a good idea when migrating users and applications to a new system. When the system is first introduced, the few users on the system may become accustomed to having all of the machine’s resources. However, by setting CPU caps early after the system’s introduction, you can simulate the performance of the system under heavier use. Consequently, when the system becomes more heavily used, performance is not noticeably less. For information on capping CPU resource use, see “Specifying PRM groups/controlling CPU resource use” (page 54).

24 Understanding how PRM manages resources