Expand Configuration and Management Manual (G06.24+)

Subsystem Description
Expand Configuration and Management Manual—523347-008
18-33
Multi-CPU Paths
The three goals are handled in three separate steps.
1. First, CPU matching is done for each source/destination pair by looking for line
handlers that have better CPU matches than their current owner. If more than one
path has the best match, choose the one that yields the lowest predicted load-
factor spread. The pair is moved without regard for anti-thrashing bits (see below)
or possible increase in the load-factor spread.
2. Next, the load factors are balanced. The load-factor spread is the highest load
factor minus the lowest load factor; this step tries to minimize the load factor
spread until it is less than 0.5. To do this, calculate the sensitivity of each path's
load factor to its total traffic, assuming a linear relationship between average LF
and total traffic. This is used to predict the effect on the load factors of moving
traffic from one line handler to another.
Then consider moving each pair from each other line handler to the one with the
lowest load factor, and of moving each pair from the line handler with the highest
load factor to each other line handler and predict the resulting change in load
factors.
Choose the single move that results in the lowest predicted load factor spread, put
it on the output change list, update the load factors according to the predicted
changes, and check the new load factor spread value. This is continued until the
load factor spread is less than 0.5 or no moves can be found that improve the load
factor spread.
3. Lastly, the pair counts are balanced. Use the path selection algorithm described
above with current LF information to determine the goal number of pairs for each
line handler. To prevent new line handlers with low LFs and no current pairs from
taking on more pairs than they can actually handle, those line handlers with too few
pairs have their goals reduced by half their shortfall.
Then consider moving each pair from the line handler with the highest excess pairs
to each line handler with a dearth. Choose the move that results in the lowest
predicted load-factor spread with no increase from previous efforts. If more than
one path has the same lowest load-factor spread, choose the one with the largest
pair-count shortfall. This is continued until there are no excess pairs or all possible
moves increase the load-factor spread.
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 pairs
only at the following times:
Caution. A multi-CPU rebalance can introduce a temporary disruption in the network, similar
to but in general less than that caused by an Expand path change. For that reason, it is
recommended that rebalances be limited to off-peak hours unless an imbalance is clearly
causing immediate problems.