private double variance(double load, double weight, double average) {
    double deviation = (load / weight - average);
    return deviation * deviation;
  }
Calculate how much the variance in load will decrease for a given move.

@param sLoad the current load on the source member
@param sWeight the weight of the source member
@param tLoad the current load on the target member
@param tWeight the weight of the target member
@param bucketSize the size of the bucket we're considering moving
@param average the target weighted load for all members.
@return the change in variance that would occur by making this move. Essentially
        variance_before - variance_after, so a positive change is a means the variance is
        decreasing.
geode-core/src/main/java/org/apache/geode/internal/cache/partitioned/rebalance/model/PartitionedRegionLoadModel.java