6. DATA SOURCES › 6.8 PR/SM LPAR Concepts › 6.8.4 Interpreting PR/SM Data › 6.8.4.3 Logical Partition Interaction

6.8.4.3 Logical Partition Interaction

Prior to the introduction of the PR/SM LPAR environment, the
Virtual Machine Facility (VM) was commonly used to run
multiple SCPs in a single CPC.  LPAR mode makes this possible
without the additional VM system control program.  The fact
that RMF measures and records the activity of all the
partitions in a PR/SM LPAR complex simplifies the problems
encountered when analyzing MVS activity under VM.

In VM, collecting, synchronizing, and summarizing measurement
information from each guest with the measurements from a
system-wide VM monitor makes the analysis of this environment
extremely challenging.  With the LPAR mode measurements from
RMF, many, if not all, of the effects of sharing a physical
complex among multiple SCPs can be characterized with
measurement data from a single source.

Since memory and channel paths are dedicated to individual
partitions (except when MIF is used), the analysis of the
utilization of these resources is largely unchanged when
compared to LPAR operations.  The single exception is in the
measurements of the activity of the I/O Processor (IOP).
These are global to the IOP and, therefore, apply to the sum
of the activity of all active partitions.  There is no direct
way to determine the load on the IOP imposed by a specific
partition.

As long as the demand for processor resources is less than
the availability of central processors, the activity in one
logical partition will have little or no impact on other
partitions.  It is only during periods of contention for
processor resources, or when partition capping is in effect,
that inter-partition effects can be expected.  With capping,
it is the presence of other active partitions, not their
level of activity, that limits processor dispatching.

Because the LPAR dispatcher prioritizes access to central
processors based on the active partitions' relative weights,
the effect of contention or partition capping is less
processor time available for a logical partition than it
would have had in a non-LPAR complex.

The initial relative share of processor resources available
to each partition is controlled by the user.  The processing
weight for a partition is set when the partition is defined
and can be modified for an active partition at any time by
the console operator.  The actual numeric values are
meaningful only when compared to each other.  Before being
applied, the weight is divided by the number of active
logical processors for the partition.  The weights are then
applied to the sum of the activity of all logical processors
active in the partition.  Thus, during a period of high
contention for processor resources, the sum of the dispatch
times for all of the logical processors for a partition with
a relative weight of 200 will be twice the sum of the
dispatch times of a second partition with a weight of 100.

A partition with fewer logical processors defined will
receive a higher proportion of an individual logical
processor than a partition with more logical processors
defined.  For example, assume a triadic CPC with three
logical partitions is defined, all with equal relative
weights.  SYS1 is defined with one logical processor, SYS2
with two LPs, and SYS3 with 3 LPs.  During periods of
contention, all would accumulate approximately equal amounts
of dispatch time.  Ignoring the effects of LPAR management
time, SYS1's single processor would be 100% dispatched,
SYS2's two LPs would each be 50% dispatched, and SYS3's three
LPs would each be 33% dispatched.

An important consideration is that the number of logical
processors defined for a partition can limit its total
processor utilization to a value less than that implied by
its relative weight.  In other words, if a partition has only
one logical processor defined, it can never receive more than
50% of the resources of a dyadic CPC, even if it has a high
relative weight as compared to the weights of any other
active partition.

    1 - Dispatching Frequency
    2 - Logical Processor Dispatching and Execution Rate
    3 - Logical Processor Dispatching and I/O Operations