194 IBM Enterprise Workload Manager
7.5 Where to send the work (Weight calculations)
This section describes the process in which weights are computed for the servers and
applications registered with the domain manager. A load balancer may register specific
applications or specific systems in the groups. Because different data is used to compute
weights in these cases, the current assumption is that instances in one group must be either
all system instances or all application instances.
As mentioned earlier, the domain manager is capable of two different types of load balancing.
If the load balancer registers system-level members, the domain manager will perform
system-level load balancing. If the load balancer registers application-level members, the
domain manager will attempt to perform ARM-instrumented application load balancing.
However, if any of the applications registered in the group are not ARM-instrumented, the
entire group will revert to system load balancing.
Initially, all instances in a group are assumed to be equivalent as far as available resources,
and start out with an equal number of credits to be used for weights. To maintain stability in
the weight generation algorithm, a finite number of “weight credits” will be used. An instance’s
weight is nothing but an accumulation of its weight credits. If one instance of the group is
awarded an extra credit, it must be taken from another instance. Likewise, if an instance is
punished and fined a credit, it must be given to another.
7.5.1 System-level balancing
The system-level balancing algorithm is best understood in two stages: the absolute stage
and the relative adjustment stage. The absolute stage has the highest priority and factors in
elements such as when instances are determined to be down or quiesced. The behavior of
the weight algorithm in these cases is predetermined, and doesn't depend on the condition of
any other systems. System-level balancing in the absolute stage occurs when:
? Instance is down: set weight = 0, and contact flag = 0
→ will receive no credits.
This may occur when the domain manager has missed the last several statistical update
messages or if the managed server hasn’t yet connected to the domain manager.
? Instance is quiesced: set weight = 0, and quiesced flag = 1
→ will receive no credits.
By setting the quiesced flag, the system is excluded from the group member weight
calculations.
Statistical metrics gathered about each group instance are used to form a preference of using
one instance relative to the other instances. The statistical data available to us at the system
level are:
? System CPU utilization: Total number of CPUs and the amount of CPU time used.
Utilization currently is determined by:
CPU Utilization=Total CPU Time used/(CPU Count * Time Interval)
? Importance level of work running on systems.
This is specified in the service policy for each service class as described in “Setting
service class goals” on page 139.
? Performance index (PI) of work running on systems.
The PI is shown on the EWLM Control Center if you view the performance data of the
service classes under the Monitor section (see “Service class details” on page 108).
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