IBM Z9, Planning Manual

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Suitable Workloads
Workloads best suited for logical partitions that use dedicated processors are those
that maintain a fairly even external throughput rate (ETR) while using most of the
capacity of the logical partition. Logical partitions with timing dependencies might
require dedicated processors.
The installation goals for a logical partition that uses dedicated processors should
be similar to the goals of the processor complex. For example, if the goal is an
average processor utilization rate of 70%–85%, then the same goal should be used
for a logical partition that uses dedicated processors.
Shared Central Processors
LPs can share CPs with other LPs. A sharing LP does not have exclusive use of
the physical CPs. There is no correspondence between the logical CPs in a sharing
LP and the physical CPs on which the logical CPs are dispatched (except on a
one-way CPC). A logical CP can be dispatched on any physical CP and, on
successive dispatches, the logical CP can be dispatched on different physical CPs.
The number of CPs available for use by sharing LPs is determined by adding the
number of CPs already assigned to active, dedicated LPs and subtracting that sum
from the total number of physical CPs available.
The total of all logical CPs for all sharing LPs can be larger than the number of
physical CPs serving the sharing LPs. For example, if four LPs are active on a
six-way CPC and each LP is defined to have four CPs, the total number of online
logical CPs is 16.
For coupling facility considerations, see “Processor Considerations for Coupling
Facility LPs” on page 3-30.
Use the Customize/Delete Activation Profiles task available from the CPC
Operational Customization Tasks list to open a reset or image profile to define
shared CPs for an LP. The Not dedicated field is located in the Logical processor
assignment group box on the Processor page for the LP.
Suitable Workloads
Workloads best suited for sharing logical partitions are those that have a widely
fluctuating ETR or would not fit well into the capacity of a dedicated logical partition.
If a workload can use only a small portion of the capacity of a dedicated logical
partition, redefine the logical partition to use shared processors to free the available
capacity for use by other logical partitions.
A workload with a widely fluctuating ETR would experience peaks and valleys in its
processor-utilization curve. Such fluctuations can occur over extremely short periods
of time (minutes or seconds). This type of workload could take advantage of the
time- and event-driven dispatching available. With event-driven dispatching, a
sharing logical partition receives the resources required as needed and leaves the
capacity free for other logical partitions when not needed. See “Wait Completion” on
page 3-44.
When combining workloads on a processor complex by means of logical partitions,
examine their average and peak requirements carefully. If the workloads fluctuate
over very short intervals, the total capacity of the system must meet the sum of the
average requirements for each workload. If processor utilization fluctuates over
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PR/SM Planning Guide