Large Systems Performance Reference
Two important capacity metrics are defined in this section before we describe the use of the LSPR tables: external throughput rate and internal throughput rate.
External throughput rate
For a CICS workload, units of work are normally expressed as the number of CICS transactions completed. To be useful, the units of work that are measured must represent a large and repeatable sample of the total workload to best represent the average. Elapsed time is normally expressed in seconds.
ETR characterizes system capacity because it is an elapsed time measurement (system capacity encompasses the performance of the processor and all of its external resources, considered together). As such, ETR lends itself to the system comparison methodology. This methodology requires the data processing system to be configured with all intended resources, including the processor, with appropriate amounts of central storage, expanded storage, channels, control units, I/O devices, TP network, and so on.
After the system is configured, the goal is to determine how much work the system, as a whole, can process over time. To accomplish this goal, the system is loaded with the appropriate workload until it cannot absorb work at any greater rate. The highest ETR achieved is the processing capability of the system.
When you make a system measurement of this type, all resources on the system are potential capacity inhibitors. If a resource other than the processor is, in fact, a capacity inhibitor, the processor is likely to be running at something less than optimal utilization.
This system comparison methodology is a legitimate way to measure when the intent is to assess the capacity of the system as a whole. For online systems, response time also becomes an important system-related metric because poor response times inhibit the ability of users to work. Therefore, system measurements for online work usually involve some type of response time criteria. If the response time criteria is not met, what ETR can be realized does not matter.
Internal throughput rate
As with ETR, units of work are normally expressed as jobs (or job-steps) for batch workloads, and as transactions or commands for online workloads. System control programs (SCPs) and most major software products have facilities to provide this information. To be useful, the units of work that are measured must represent a large and repeatable sample of the total workload to best represent the average. Processor busy time is normally expressed in seconds.
ITR characterizes processor capacity because it is a CPU busytime measurement. As such, ITR lends itself to the processor comparison methodology. Because the focus of LSPR is on a single resource (the processor), you must modify the measurement approach from the approach that you use for a system comparison.
To ensure that the processor is the primary point of focus, you must configure it with all necessary external resources (including central storage, expanded storage, channels, control units, and I/O devices) in adequate quantities so that they do not become constraints. You must avoid the use of processor cycles to manage external resource constraints to assure consistent and comparable measurement data across the spectrum of processors being tested.
Many acceptance criteria for LSPR measurements can help assure that external resources are adequate. For example, internal response times should be sub second; if they are not, some type of resource constraint must be resolved. For various DASD types, expected nominal service times are known. If the measured service times are high, some type of queuing is occurring, which indicates a constrained resource. When unexpected resource constraints are detected, they are fixed and the measurement is redone.
Because the processor is also a resource that must be managed by the SCP, steps must be taken to ensure that excess queuing on it does not occur. The way to avoid this type of constraint is to make the measurements at preselected utilization levels that are less than 100%. Because the LSPR is designed to relate processor capacity, measurements must be made at reasonably high utilization, but without causing uncontrolled levels of processor queuing. Typically, LSPR measurements for online workloads are made at a utilization level of approximately 90%. Batch workloads are always measured with steady-state utilizations above 90%. Mixed workloads that contain an online and batch component are measured at utilizations near 99%.
One other point must be made about processor utilization. Whenever two processors are to be compared for capacity purposes, they should both be viewed at the same loading point, which means at equal utilization. Assessing relative capacity when one processor is running at low utilization and the other is running at high utilization is imprecise. The LSPR methodology mandates that processor comparisons be made at equivalent utilization levels.
Relationship between internal throughput rate (ITR) and external throughput rate (ETR)
LSPR internal throughput rate (ITR) ratios
LSPR capacity data is presented in the form of ITR ratios for IBM processors where each model is configured with multiple z/OS images that are based on an average LPAR profile of client systems. All capacity numbers are relative to the IBM 2094-701 running multi-image z/OS image.
Comparing ITR ratios for two processor configurations allows a capacity planner to predict the effects of modifying hardware configuration at a high level. However, the most accurate sizings require the use of the LPAR Configuration Capacity Planning function of the zPCR tool, which can be customized to match a specific multi-image configuration rather than the average configurations that are reflected in the multi-image LSPR table.