The Intelligent Resource Director (IRD) component of Workload Manager (WLM) involves CPU management, dynamic Channel Path Identifier (CHPID) management, and channel subsystem priority queueing. This article explores the effects of IRD on the CPU management portion. The CPU management part of IRD is further divided into two functions: Logical Partition (LPAR) CPU management and weight management. For a more extensive examination of these functions, refer to the Redbook z/OS Intelligent Resource Director (SG24-5952).
In IRD CPU management, two actions may occur. The number of logical processors defined to a particular LPAR may be changed based on the weight assigned. The objective is to have as few Logical Processors (LPs) in use as the weight will allow and to increase the number of LPs only to accommodate the weight requirements of the partition.
Weights also may be changed based on the performance objectives of service classes in the partition. The effect is that, if a more important service class is missing its goals, CPU resource (LPAR weight) may be dynamically taken away from the less important LPAR and assigned to the more important. This weight change may require changing the number of LPs; so the two actions are related in this aspect of IRD CPU management.
The SRM Constant
The SRM constant is used to normalize the service charged to a unit of work to make it independent of time. Service policy definitions and performance decisions shouldn’t have to be reevaluated every time the processor configuration changes.
The SRM constant is used to take the service time (in seconds) and multiply it by the processor constant (service units per second), which should result in a value that’s relatively uniform across different processor models and configurations.
For example, on a 2084-302, the SRM constant is about 20,752 service units (SU)/sec while on the 2084-323 the SRM constant is 14,171 SU/sec. By using this value on a unit of work running in each environment that consumes one second of CPU time, the SUs charged would be 20,752 and 14,171, respectively.
IRD and the SRM Constant
When an LP is added to an LPAR, the SRM constant is recalculated to reflect the new configuration. However, this recalculation does not occur when IRD adds or removes the LP.
Since the SRM constant is used to normalize CPU time based on the processor model, the effect of IRD actions is that of dynamically changing the CPU model based on service class goals. This can have a significant impact on calculations and measurements if it’s not taken into account.