IBM Z9, Planning Manual

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longer periods, and the peak utilization periods for these workloads occurs
simultaneously, then the total capacity of the logical partitions must meet the sum of
the peak requirements for each workload.
Sharing logical partitions that use event-driven dispatching are better able to
maintain high transaction rates with fluctuating demand while being responsive.
However, the ITR for a sharing logical partition is lower than the ITR for a dedicated
logical partition.
The capability to limit CPU usage for any or all logical partitions with shared
processors is provided by the PR/SM capping function. The capping function
enhances LPAR workload balancing controls for environments with a requirement
that the CPU resources for a logical partition be limited. Capped logical partitions
are recommended for use when CPU resources must be limited for business
reasons (in accordance with a contract), or when the impact that one logical
partition can have on other logical partitions needs to be limited.
Processing Weights
An LP with dedicated CPs is not affected by processing weights.
Processing weights are used to specify the portion of the shared CP resources
allocated to an LP. Although PR/SM always manages sharing LPs according to the
specified processing weights, there are times when an LP will receive either more
or less than its processing share:
v
An LP will receive more than its processing share when there is excess CP
capacity, provided it has work to do and other LPs are not using their share.
v An LP will receive less than its processing share when its workload demand
drops below the capacity specified by its weight.
v
An LP will not receive more than its processing share when the CP resources for
that LP are capped.
The recommended procedure is to specify processing weights to satisfy the peak
requirements of the LPs.
Use the Customize/Delete Activation Profiles task available from the CPC
Operational Customization Tasks list to open a reset or image profile to define the
processing weight for a shared LP. The Processing weight field is located in the
Logical processor assignment group box on the Processor page for the LP.
Use of Processing Weights: As an example, consider a system with 6 CPs and
3 LPs defined as follows:
LP Name Logical CPs Weight
------- ----------- ------
ZVSE 1 300
ZOSTEST 6 100
ZVM 2 900
Processing weights can range from 1 to 999 (weights of less than 2% difference are
not considered significant) and are used as follows:
v
The processing weights for all active, sharing LPs are added together. This total
is considered to be 100% of the processing resource available to shared CPs.
v The share of processing resources for each LP is calculated by dividing the
processing weight for each sharing LP by the total processing weight. For
example, at peak CP utilization levels, the dispatcher allocates shared processing
resources to each of the LPs as follows:
Chapter 3. Determining the Characteristics of Logical Partitions
3-37
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