When the workload management dispatcher is enabled on multimember DB2® instances, CPU resource scheduling
operates across all of the members on a given host. Note
that the workload management dispatcher also supports shared LPAR
(micropartition) environments on AIX®.
Multimember database environments
On multimember DB2 instances, the dispatcher behaves
in the following ways:
- The relative quantity of CPU shares are evaluated and CPU resources
are allocated across all members in the instance based
on the work that is active in that instance on that host or LPAR as
a whole. For example, let's consider a two member database
with work being executed in two service classes, A and B. Service
class A is assigned 3500 soft CPU shares and service class B is assigned
6500 soft CPU shares. The CPU utilization for the entire host or LPAR
is 35% for service class A and 65% for service class B despite the
possibility that relatively more work is being executed on one member
than the other. The workload management dispatcher looks across both
members when making its decisions regarding which service class
should be getting more CPU resources at any given moment.
Figure 1. Multimember database environments: Soft
CPU shares allocations pie chart across 2 database members
- The value of the wlm_disp_concur database
manager configuration parameter applies to each host or LPAR. In multimember
databases, the dispatch concurrency level specified by the wlm_disp_concur configuration
parameter is applied across all of the members.
- For multimember databases, CPU limits are enforced on a per host
or LPAR basis - when multiple members are used, each partition
is allowed to use all of a specified CPU limit as long as the sum
across all members is less than or equal to the CPU limit.
If the CPU limit on the single service class A is 35% and there are
2 members, then member 1 can use up to 35% if
the sum of the CPU utilization across member 1 and 2 does
not exceed 35%.
Figure 2. Multimember database
environments: CPU limit pie chart across two database members
Micropartition (shared LPAR) environments
The
workload management dispatcher supports micropartition environments
in AIX. In micropartition environments,
the percentage CPU utilization for both controls and monitoring in
the workload management dispatcher is computed relative to the CPU
resources allocated to the LPAR by the operating system or hypervisor
over the most recent dispatcher CPU resource scheduling cycle, using
the entitled (guaranteed) CPU resources for the LPAR as a baseline
level. In cases when the CPU resource allocated to the LPAR is greater
than the entitled CPU resources, the dispatcher computes the CPU utilization
relative to the greater amount, and when the CPU resource allocation
is less than the entitled CPU resources, the dispatcher computes the
CPU utilization relative to the entitled CPU resources.
Note: The
workload management dispatcher behavior is consistent with the way AIX WLM performs its calculations.
For dedicated LPARs on AIX,
and for all non-AIX environments, the CPU utilization is calculated
relative to the full CPU capacity of the physical cores available
to the DB2 database manager.