2. PERFORMANCE REPORTING ANALYSIS › 2.1 ILM Analysis › 2.1.4 ILM Concepts

2.1.4 ILM Concepts

z/OS has introduced the concept of (ILM) IBM License Manager.
ILM allows PR/SM partitions (LPs) to be configured with a
defined capacity, expressed in MSUs per hour.  IBM's Workload
Manager (WLM) monitors the actual LP MSU utilization using a
rolling four-hour average.  If the MSU four-hour rolling
average utilization exceeds the defined capacity, WLM
instructs PR/SM to "soft cap" the LP, effectively restricting
processor consumption until the four-hour average drops below
the defined capacity. Thus, with ILM, an LP can effectively
deal with short-term spikes in processor utilization far
above the defined capacity, as long as the four-hour rolling
average remains below the defined capacity.

Some software licenses are based on the defined capacity of
the machine.  Sites using ILM should closely track their peak
four-hour rolling average values as well as the WLM capping
percentage to correctly size their LP's capacity limits and,
hence, their ability to achieve savings on software license
charges by tailoring their license certificates.

Correctly sizing your LPARs capacity limits (and your ability
to achieve savings on software license charges by tailoring
your license certificates) requires the tracking of your peak
four-hour rolling average values as well as the WLM capping
percentage.  The CA MICS data elements that report these
values are, respectively, CPUMXLAC and CPUPCNSW.

If the MSU consumption rate is constantly lower than the
defined limit, you may consider decreasing the LPAR capacity
until WLM begins capping the partition (a capping percentage
of 1% to 5% ensures the work is done optimally at the lowest
cost).

Capping affects all workloads running in the partition and
can result in elongated response times and generally
unacceptable performance.

If the peak value exceeds the defined capacity for long
periods (high WLM capping percentage reported), you may
consider increasing the LPAR capacity limit until the WLM
capping percentage falls to within an acceptable range.

If the peak value occasionally exceeds the defined capacity
(low WLM capping percentage), you may consider time-shifting
the workload.

Ideally, your peak value should always be as close as
possible to the defined capacity, without exceeding it, while
your WLM capping percentage should always be near to zero.