This article discusses Workload Manager’s (WLM’s) approach to goal management in a Sysplex environment so you can better understand and justify the need to sometimes define system-specific service classes.
Unfortunately, a commonly misunderstood WLM recommendation is: “Try not to have more than 30 service classes defined.” This recommendation is incorrect. The true recommendation, which is greatly influenced by WLM Sysplex management, should actually be stated as: “Try to have less than 30 active service class periods on any given system in the Sysplex during intervals of time that matter.”
Key points of this recommendation include:
• Periods: You should care about the number of periods since it’s the periods that have goals that need to be managed.
• Active: You shouldn’t consider periods that have no activity since they aren’t selected by WLM to manage and they can’t donate resources.
• Any given system in the Sysplex: If workload A is running in its own service class on SYS1 and the workload doesn’t exist on SYS2, then A’s service class shouldn’t be counted toward total active periods on SYS2.
• Intervals of time that matter: If a set of workloads is active during the day, and a different set is active at night, each should be counted separately.
The basis of this WLM guideline has little to do with alleviating WLM overhead and more to do with improving WLM responsiveness. If too many periods on a z/OS system are active, WLM responsiveness decreases since WLM on each system can help, at most, only one goal period every 10 seconds. The more active periods that need help, the more 10-second intervals need to go by for WLM to have a chance to assist all the goal periods.
Mistakenly, many installations work hard to keep the total number of defined service class periods to a minimum. This approach may be sufficient for installations with relatively similar workloads across the systems in the Sysplex. However, for larger installations with Sysplexes composed of systems of widely varying capacity running disparate workloads, this may not be the best approach. Because of WLM Sysplex management, sometimes defining more goal periods will be desired so certain systems can have a unique set of service classes optimal for the capacity and requirements of that system.
WLM Sysplex Management