Expand Configuration and Management Manual (G06.24+)

Tuning
Expand Configuration and Management Manual—523347-008
20-20
Multi-CPU Paths
Superpath Rebalancing
Superpath rebalancing is run periodically to correct path selection as traffic patterns
change. It has three goals:
•
CPU Matching: Make sure all source/destination pairs are using a path with the
most CPU matches available (same local/remote CPU).
•
Load Factor Balancing: Try to make the load factors (LF = ETF / TF) of all paths
within 0.5 of each other.
•
Pair Count Balancing: Spread those pairs whose traffic have no adverse impact on
load factors (LFs) over all paths in inverse proportion to their effective time factors
(ETFs).
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 ETF
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 ETF information to determine the goal number of pairs for each
line handler. To prevent new line handlers with low ETFs 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