6947ch07.fm Draft Document for Review April 7, 2004 6:15 pm
178 IBM eServer zSeries 990 Technical Guide

Value of CPU management

The benefits of CPU management include the following:
򐂰Logical CPs perform at the fastest uniprocessor speed available.
This results in the number of logical CPs tuned to the number of physical CPs of service
being delivered by the logical partition current weight. If the logical partition is getting 4
equivalent physical CPs of service and has 8 logical CPs online to z/OS, then each logical
CP only gets half of an equivalent physical CP. For example, if a CP delivers 200 MIPS,
half of it will deliver 100 MIPS. This occurs because each logical CP gets fewer time slices.
򐂰Reduced PR/SM overhead.
There is a PR/SM overhead for managing a logical CP. The higher the number of logical
CPs in relation to the number of equivalent physical CPs, the higher the PR/SM overhead.
This is because PR/SM has to do more processing to manage the number of logical CPs
that exceeds the number of equivalent physical CPs.
򐂰z/OS gets more control over how CP resources are distributed.
Using CPU management, z/OS is able to manage CP resources in relation to WLM goals
for work. This was not possible in the past when a logical partition had CP resources
assigned and used these as best it could in one logical partition. Now, z/OS is able to
change the assigned CP resources (LPAR weights) and place them where they are
required for the work. CPU management does the following:
– Identifies what changes are needed and when.
– Projects the likely results on both the work it is trying to help and the work that it will be
taking the resources from.
– Performs the changes.
– Analyzes the results to ensure the changes have been effective.
There is also the question of the speed at which an operator can perform these actions.
WLM can perform these actions every Policy Adjustment interval, which is normally ten
seconds as determined by WLM. It is not possible for an operator to perform all the tasks
in this time.
For additional information on implementing LPAR CPU management under IRD see the
redbook: z/OS Intelligent Resource Director, SG24-5952.
7.5.2 Dynamic Channel Path Management
There is no such thing as a “typical” workload. The requirements for processor capacity, I/O
capacity, and other resources vary throughout the day, week, month, and year.
Dynamic Channel Path Management (DCM) provides the ability to have the system
automatically manage the number of paths available to disk subsystems. By making
additional paths available where they are needed, the effectiveness of your installed chann els
is increased, and the number of channels required to deliver a given level of service is
potentially reduced.
DCM also provides availability benefits by attempting to ensure that the paths it adds to a
control unit have as few points of failure in common with existing paths as possible, and
configuration management benefits by allowing the installation to define a less specific
configuration. On a z990 where paths can be shared by Multiple Image Facility (MIF), DCM
will coordinate its activities across logical partitions within a single Logical Channel
Subsystem on a server within a single sysplex.