Expand Configuration and Management Manual (G06.24+)

Tuning
Expand Configuration and Management Manual—523347-008
20-16
Multi-CPU 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 traffic pattern might not be optimal at first, but a change in
configuration could improve it; often this configuration change will benefit overall
performance as well as 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.
Load Balancing
When a multi-CPU path initially assigns paths to each pair of endpoints, the traffic
pattern is usually not yet known. Load balancing is used to correct this problem as
more information is gathered by moving Expand line-handler process pairs from more
heavily loaded paths to more lightly loaded paths within the multi-CPU path. A slight
disruption occurs in message transfer occurs when pairs are changed. This disruption
is similar to what can occur when a better route is found in the Expand network and
connections are reestablished over the new best-path route.
You can schedule load balancing to occur automatically at periodic intervals or you can
initiate it manually. Exactly when you should rebalance a multi-CPU path depends on
the volatility of the traffic pattern. For example:
•
If the pattern is nearly constant, then load balancing can be initiated once after a
change in the status of the multi-CPU path.
•
If the pattern changes somewhat during the day, but slowly from day to day, then
load balancing should be done once a day during off-peak hours.
•
If the pattern changes radically, load balancing should be done an hour or so into
each new traffic pattern to establish new path assignments.
A maximum of 16 moves can be put on the output change list. All of the above stop
when that count is reached. Pairs on the change list are flagged with an anti-thrashing
bit; selection of those pairs for moving is avoided during the next one rebalance.
Because rebalancing is slightly disruptive, $NCP changes Expand line-handler process
pairs only at the following times:
•
When a new path comes up. (This is similar to what happens in normal paths when
a new path that has a lower TF is discovered.)
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.