[jira] [Created] (HBASE-17565) StochasticLoadBalancer may incorrectly skip balancing due to skewed multiplier sum

["Ted Yu (JIRA)" <ji...@apache.org>]

Ted Yu created HBASE-17565:
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             Summary: StochasticLoadBalancer may incorrectly skip balancing due to skewed multiplier sum
                 Key: HBASE-17565
                 URL: https://issues.apache.org/jira/browse/HBASE-17565
             Project: HBase
          Issue Type: Bug
            Reporter: Ted Yu
            Assignee: Ted Yu


I was investigating why a 6 node cluster kept skipping balancing requests.

Here were the region counts on the servers:
449, 448, 447, 449, 453, 0
{code}
2017-01-26 22:04:47,145 INFO  [RpcServer.deafult.FPBQ.Fifo.handler=1,queue=0,port=16000] balancer.StochasticLoadBalancer: Skipping load balancing because balanced cluster; total cost is 127.0171157050385, sum multiplier is 111087.0 min cost which need balance is 0.05
{code}

The big multiplier sum caught my eyes. Here was what additional debug logging showed:

{code}
2017-01-27 23:25:31,749 DEBUG [RpcServer.deafult.FPBQ.Fifo.handler=9,queue=0,port=16000] balancer.StochasticLoadBalancer: class org.apache.hadoop.hbase.master.balancer.          StochasticLoadBalancer$RegionReplicaHostCostFunction with multiplier 100000.0
2017-01-27 23:25:31,749 DEBUG [RpcServer.deafult.FPBQ.Fifo.handler=9,queue=0,port=16000] balancer.StochasticLoadBalancer: class org.apache.hadoop.hbase.master.balancer.          StochasticLoadBalancer$RegionReplicaRackCostFunction with multiplier 10000.0
{code}
Note however, that no table in the cluster used read replica.

I can think of two ways of fixing this situation:

1. If there is no read replica in the cluster, ignore the multipliers for the above two functions.
2. When cost() returned by the CostFunction is 0 (or very very close to 0.0), ignore the multiplier.



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