•Allows continued use of percentages – If you prefer using percentages instead of shares, you can assign each level in the hierarchy only 100 resource shares.

•Facilitates giving equal access – If you want each PRM group to have equal access to a resource, simply assign each group the same number of shares. When you add a group, you do not have to recalculate resources and divide by the new number of groups; just assign the new group the same number of shares as the other groups. Similarly, removing a group does not require a recalculation of resources; just remove the group.

•Allows for more intuitive groups – Hierarchies enable you to place similar items together, such as all databases or a business entity/goal, and assign them resources as a single item.

•Enables making higher-level policy decisions – By placing groups in a hierarchy, you can implement changes in policy or funding at a higher level in a configuration without affecting all elements of the configuration.

•Facilitates system upgrades, capacity planning, and partitioning – If you are moving from a two-core system to a four-core system, you can reserve the two additional cores by adding a place-holder group at the top level in the hierarchy, assigning it shares equal to 50% of the CPU resources, and enabling capping. This place-holder prevents users from getting a boost in performance from the new cores, then being frustrated by poor performance when more applications are added to the system.

The syntax for hierarchical groups is explained in “Group/CPU record syntax” (page 55).

By default, PRM utilities (prmconfig, prmlist, prmmonitor) include only leaf groups in their output. Use the -hoption to display information for parent groups as well.

How PRM manages CPU resources

This section describes how PRM manages CPU resources. To understand PRM’s CPU management, it is useful to know how the standard HP-UX scheduler works.

The HP-UX scheduler chooses which process to run based on priority. Except for real-time processes, the system dynamically adjusts the priority of a process based on resource requirements and resources used. In general, when processes are not running, the HP-UX scheduler raises their priorities; and while they are running, their priorities are lowered. The rate at which priority declines during execution is linear. The rate at which priority increases while waiting is exponential, with the rate of increase fastest when the CPU load is low and slowest when the CPU load is high. When a process other than the current process attains a higher priority, the scheduler suspends the current process and starts running the higher priority process.

Because the rate at which the priority increases is slowest when CPU load is high, the result is that a process with a heavy demand for CPU time is penalized by the standard HP-UX scheduler as its CPU resource use increases.

With PRM, you can reverse the effects of the standard scheduler. By placing users with greater demands for CPU resources in an FSS PRM group with a higher relative number of CPU shares than other groups, you give them a higher priority for CPU time. In a similar manner, you can assign an application to an FSS PRM group with a higher relative number of shares. The application will run in its assigned FSS PRM group, regardless of which user invokes it. This way you can ensure that critical applications have enough CPU resources. You can also isolate applications and users with greater demands for CPU resources by placing them in a PSET PRM group and assigning the desired number of cores to the group. The applications and users will have dedicated access to the cores in the PSET PRM group, ensuring CPU cycles when needed. This method of isolating applications and users effectively creates a partition on your system.

PRM manages CPU resources by using the fair share scheduler (FSS) for FSS PRM groups. When the PRM CPU manager is enabled, FSS runs for FSS PRM groups instead of the HP-UX standard scheduler. When PSET PRM groups are configured, FSS still runs for FSS PRM groups, but the standard HP-UX scheduler is used within PSET PRM groups.

22 Understanding how PRM manages resources