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https://issues.apache.org/jira/browse/HBASE-17565?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15853430#comment-15853430
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Ted Yu commented on HBASE-17565:
--------------------------------
There're two categories of cost functions: one related to read replica
(RegionReplicaHostCostFunction and
RegionReplicaRackCostFunction) and the other cost functions which apply to
every cluster setup.
RegionReplicaRackCostFunction is a subclass of RegionReplicaHostCostFunction.
The cost() method of these two cost functions already has short circuit built
in.
For the other category, even if we add isNeeded() method, the method would
return true.
That was why adding such method is redundant.
> 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
> Priority: Critical
> Fix For: 2.0.0, 1.4.0
>
> Attachments: 17565.v1.txt, 17565.v2.txt, 17565.v3.txt
>
>
> 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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