Expand Configuration and Management Manual (H06.21+, J06.10+)

Tuning
Expand Configuration and Management Manual — 529522-013
19 - 15
Multi-CPU Paths
The main disadvantage of the Expand multi-CPU feature is that its advantages are
available only when traffic fits a certain pattern. For example, if most traffic occurs
between the same two nodes—or if these nodes are direct neighbors and traffic is sent
between the same two processors in one direction—then the Expand multi-CPU
feature cannot spread the load effectively. Other disadvantages include:
•
Increased processor overhead
•
The possibility of occasional disruptions during load balancing
The interaction of some elements of the system and of the Expand network determine
the degree of performance improvement that the Expand multi-CPU feature might
achieve. These are the elements that control how well the Expand multi-CPU feature
can spread traffic over its constituent paths.
These elements include:
•
Traffic Pattern
•
Load Balancing
Traffic Pattern
Unlike a multi-line path, which can spread traffic evenly over all of its lines regardless
of the traffic pattern, a single path within a multi-CPU path is assigned to traffic
between each pair of endpoints. This path assignment is fixed until traffic is rebalanced
over all the paths in the multi-CPU path. For this reason, the more that traffic is spread
across different endpoints, the better a multi-CPU path can spread the load across its
member paths.
For example, if nearly all traffic in an Expand network is sent between the same two
processes in one direction, then the multi-CPU path can only assign this traffic to one
path and the other paths will remain virtually idle.
In another scenario, the traf
fic pattern might not be optimal at first, but a change in
configuration could improve it; often this configuration change will benefit overall
performance in addition to multi-CPU path performance. For example, if nearly all
traffic is between the same two non-neighbor nodes but on different processors on
each node, the multi-CPU path can only assign the traffic to one path. However, if new
paths are configured directly between these nodes, making them neighbors, then the
multi-CPU path can spread the traffic over multiple paths.
You should be aware of the traffic pattern in your network before configuring multi-CPU
paths.
Note. Endpoints are considered to be different if they are on different nodes or, if the remote
node is a neighbor node, on different local and remote processors and different directions.