Novell NETWARE 6-DOCUMENTATION Manual page 2771

Load Balancing
14
Server Operating System Administration Guide
Two important requirements of any load balancing scheme are stability (not
overreacting to small load imbalances) and the ability to distribute the
processing load quickly.
The NetWare scheduler handles the stability requirement by using a threshold.
The threshold determines how much load imbalance is permitted in the system
before the load balancing mechanism kicks in.
Because a low threshold value would have less tolerance for load imbalances
than a higher value, the lower value could lead to excessive thread movement
due to frequent load balancing. A higher value would have more tolerance for
load imbalances, with the result that the load balancing mechanism would be
triggered less often. However, an optimum value of the threshold would
prevent excessive thread migration while addressing load imbalance only as
needed.
The NetWare scheduler periodically calculates the system-wide load and the
mean load and uses the latter to compare loads and to determine whether an
individual processor is overloaded or underloaded.
The load balancing threshold and the calculated mean load are then used to
determine the high and low trigger loads.
A processor is overloaded when its load exceeds the high trigger load.
A processor is underloaded when it is below the low trigger load.
In this situation, the scheduler then moves threads from the overloaded
processor to the underloaded processor with the result that the loads are
balanced. See the following figure for an illustration of the relationship
between the mean load, the load balancing threshold, and the high and low
trigger loads.