[HBASE-26297] Balancer run is improperly triggered by accuracy error of double comparison

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 2.5.0, 3.0.0-alpha-2, 2.3.7, 2.4.7
• Component/s: Balancer

Environment

• Hadoop Flags: Reviewed
• Assignee: Clara Xiong
• Reporter: Clara Xiong
• Created: 24/Sep/21 19:09
• Updated: 30/Sep/21 04:25
• Resolved: 26/Sep/21 15:49

Description

protected synchronized boolean areSomeRegionReplicasColocated(Cluster c) {
  regionReplicaHostCostFunction.init(c);
  if (regionReplicaHostCostFunction.cost() > 0) {
    return true;
  }
  regionReplicaRackCostFunction.init(c);
  if (regionReplicaRackCostFunction.cost() > 0) {
    return true;
  }

The values are in double data type. Balancer could get stuck in constant runs and unnecessary moves.

2021-09-24 12:02:41,943 INFO org.apache.hadoop.hbase.master.balancer.StochasticLoadBalancer: Running balancer because at least one server hosts replicas of the same region.
2021-09-24 12:01:42,878 INFO org.apache.hadoop.hbase.master.balancer.StochasticLoadBalancer: Finished computing new moving plan. Computation took 2400001 ms to try 3048341 different iterations.  Found a solution that moves 81 regions; Going from a computed imbalance of 1.7429830473781883E-4 to a new imbalance of 1.6169961756947032E-4. 

 we should use COST_EPSILON instead of 0 for double comparison.

 

 

Issue Links

• is a child of: HBASE-25697 StochasticBalancer improvement for large scale clusters
• links to: GitHub Pull Request #3698