6. DATA SOURCES › 6.8 PR/SM LPAR Concepts › 6.8.2 How PR/SM Works › 6.8.2.2 Introduction to LPAR Soft-Capping

6.8.2.2 Introduction to LPAR Soft-Capping

Workload charging enables data centers to use both hardware
and software to manage costs and system capacity:  customers
determine the portion of processing capacity needed to run
their workloads; and, for the selected products used to run
these workloads, charges are based on a site-defined
capacity, rather than on the entire CPC capacity.

The measurement unit used to define and monitor an LPAR
capacity is the MSU (millions of service units per hour),
which is calculated as follows:

MSU = ( su/sec * Nb_of_CPs * 3600 ) / 1000000

where su/sec is the physical CPU adjustment factor.  Its
value does not correlate to the adjustment factor from the
SMF type 72 Workload Activity record. The value in the type
72 record is based on the number of logical processors
assigned to the partition.  In the type 70 record, it is
based on the number of physical processors in the CPC and is
constant from LPAR to LPAR.

It is important to understand that, depending on the way you
size your logical partitions, both software charges and
system performance will be impacted.  When under WLM control,
the system will continuously monitor the actual MSU
consumption of an LPAR, and, depending on the LPAR size, will
restrict the partition from using more CPU resource than its
allowed capacity.

The method of assigning a defined capacity to an LPAR, also
called a soft cap, overcomes the traditional capping methods
limitations.  It also allows spikes in your workload:  when
an LPAR has a defined capacity, the Workload Manager (WLM)
compares this limit to the average MSUs consumed during a
four-hour rolling interval.  As long as the four-hour rolling
average stays below the defined capacity limit for the LPAR,
utilization is allowed to spike.  As soon as this average
rises above the defined capacity limit, WLM restricts the
partition from using more CPU resource than its defined
capacity limit, and the LPAR remains capped (restricted)
until the four-hour rolling average drops below the defined
capacity limit.

This is the recommended method for setting up Variable
Workload License Charges (VWLC) for a product:  charges are
based on the defined capacity; spikes in your workload, up
to the total LPAR size, are allowed.  However, correctly
sizing your LPARs capacity limits (and hence, 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.

If the peak value 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 shifting some
workloads to a time of day when the LPAR is less busy.

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.