A job with SYSSTC service class is getting less CPU

mighty · New User Joined: 21 May 2008 Posts: 26 Location: chennai

Jobs with less priority service class(PRODBAT) is getting CPU compared to the job with high priority service class(SYSSTC). I have seen in WLM definition and SYSSTC dispatching priorities are higher then the PRODBAT.

Can someone please explain me how this is possible.

steve-myers · Posted: Fri Apr 10, 2020 5:15 pm

Robert Sample · Posted: Fri Apr 10, 2020 9:39 pm

What was the CPU utilization when you observed this difference? What was the performance index for PRODBAT and SYSSTC when you observed this difference? WLM uses the goals of the policy to determine where to give resources. It does NOT guarantee that a service class with a higher priority will always get more resources than a service class with a lower priority. You also have to consider the importance of the workload of PRODBAT versus SYSSTC. There are a lot of other factors that influence WLM's resource allocations (including what is happening on every system in a sysplex), so merely stating that one service class is getting less CPU than another service class does not mean anything -- if the CPU is not running at or very near 100% used then you should not consider this an issue at all; if it is running at 100% it is entirely possible that other factors were causing waits and that would impact which service class got CPU time.

mighty · New User Joined: 21 May 2008 Posts: 26 Location: chennai

Thank you Steve and Robert for your replies.
CPU Utilisation is not at 100% and this issue occurs in almost all the accounts. SYSSTC is mostly given to system started tasks. And when we analyse, the delay caused is due to CPU delay.

Robert Sample · Posted: Sun Apr 12, 2020 12:17 am

You didn't tell us what the PI for PRODBAT and SYSSTC were. If the service classes are meeting their goals, then you -- despite posting here -- do not really have any concerns that are worth worrying about.

Also, remember that WLM manages to goals and goals are NOT guaranteed to work like you think they do. A high priority service class will NOT get all the CPU time since WLM is also managing the goal for the lower priority service class and hence ensuring that it does get resources as well. The only time this won't be true is when work is DISCRETIONARY, in which case it will only get resources when no higher priority service class can use them.

Factors that impact WLM which you have not mentioned: number of processors in the sysplex, the importance of PRODBAT, how many service classes and periods you have defined, neither service classes' PI, and how the policy is defined in WLM. There are other factors as well, but these are some of the important ones.

The bottom line is that if you think a lower priority service class will not get resources even if a higher priority service class has delays, you are flat-out wrong and need to adjust your thinking about WLM.