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https://issues.apache.org/jira/browse/HBASE-16815?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15570507#comment-15570507
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Guanghao Zhang commented on HBASE-16815:
----------------------------------------

If queue's number is 0, skip startHandler() maybe better?
{noformat}
2016-10-12 12:50:27,311 DEBUG [main] ipc.RWQueueRpcExecutor: FifoRWQ.default 
writeQueues=7 writeHandlers=15 readQueues=8 readHandlers=14
{noformat}
I thought that we can change log level to INFO and log scan queues number, scan 
handlers count, too.

> Low scan ratio in RPC queue tuning triggers divide by zero exception
> --------------------------------------------------------------------
>
>                 Key: HBASE-16815
>                 URL: https://issues.apache.org/jira/browse/HBASE-16815
>             Project: HBase
>          Issue Type: Bug
>          Components: regionserver, rpc
>    Affects Versions: 2.0.0, 1.3.0
>            Reporter: Lars George
>
> Trying the following settings:
> {noformat}
> <property>
>   <name>hbase.ipc.server.callqueue.handler.factor</name>
>   <value>0.5</value>
> </property>
> <property>
>   <name>hbase.ipc.server.callqueue.read.ratio</name>
>   <value>0.5</value>
> </property>
> <property>
>   <name>hbase.ipc.server.callqueue.scan.ratio</name>
>   <value>0.1</value>
> </property>
> {noformat}
> With 30 default handlers, this means 15 queues. Further, it means 8 write 
> queues and 7 read queues. 10% of that is {{0.7}} which is then floor'ed to 
> {{0}}. The debug log confirms it, as the tertiary check omits the scan 
> details when they are zero:
> {noformat}
> 2016-10-12 12:50:27,305 INFO  [main] ipc.SimpleRpcScheduler: Using fifo as 
> user call queue, count=15
> 2016-10-12 12:50:27,311 DEBUG [main] ipc.RWQueueRpcExecutor: FifoRWQ.default 
> writeQueues=7 writeHandlers=15 readQueues=8 readHandlers=14
> {noformat}
> But the code in {{RWQueueRpcExecutor}} calls {{RpcExecutor.startHandler()}} 
> nevertheless and that does this:
> {code}
>     for (int i = 0; i < numHandlers; i++) {
>       final int index = qindex + (i % qsize);
>       String name = "RpcServer." + threadPrefix + ".handler=" + 
> handlers.size() + ",queue=" +
>           index + ",port=" + port;
> {code}
> The modulo triggers then 
> {noformat}
> 2016-10-12 11:41:22,810 ERROR [main] master.HMasterCommandLine: Master exiting
> java.lang.RuntimeException: Failed construction of Master: class 
> org.apache.hadoop.hbase.master.HMasterCommandLine$LocalHMaster
>         at 
> org.apache.hadoop.hbase.util.JVMClusterUtil.createMasterThread(JVMClusterUtil.java:145)
>         at 
> org.apache.hadoop.hbase.LocalHBaseCluster.addMaster(LocalHBaseCluster.java:220)
>         at 
> org.apache.hadoop.hbase.LocalHBaseCluster.(LocalHBaseCluster.java:155)
>         at 
> org.apache.hadoop.hbase.master.HMasterCommandLine.startMaster(HMasterCommandLine.java:222)
>         at 
> org.apache.hadoop.hbase.master.HMasterCommandLine.run(HMasterCommandLine.java:137)
>         at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:70)
>         at 
> org.apache.hadoop.hbase.util.ServerCommandLine.doMain(ServerCommandLine.java:126)
>         at org.apache.hadoop.hbase.master.HMaster.main(HMaster.java:2524)
> Caused by: java.lang.ArithmeticException: / by zero
>         at 
> org.apache.hadoop.hbase.ipc.RpcExecutor.startHandlers(RpcExecutor.java:125)
>         at 
> org.apache.hadoop.hbase.ipc.RWQueueRpcExecutor.startHandlers(RWQueueRpcExecutor.java:178)
>         at org.apache.hadoop.hbase.ipc.RpcExecutor.start(RpcExecutor.java:78)
>         at 
> org.apache.hadoop.hbase.ipc.SimpleRpcScheduler.start(SimpleRpcScheduler.java:272)
>         at org.apache.hadoop.hbase.ipc.RpcServer.start(RpcServer.java:2212)
>         at 
> org.apache.hadoop.hbase.regionserver.RSRpcServices.start(RSRpcServices.java:1143)
>         at 
> org.apache.hadoop.hbase.regionserver.HRegionServer.(HRegionServer.java:615)
>         at org.apache.hadoop.hbase.master.HMaster.(HMaster.java:396)
>         at 
> org.apache.hadoop.hbase.master.HMasterCommandLine$LocalHMaster.(HMasterCommandLine.java:312)
>         at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native 
> Method)
>         at 
> sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
>         at 
> sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
>         at java.lang.reflect.Constructor.newInstance(Constructor.java:422)
>         at 
> org.apache.hadoop.hbase.util.JVMClusterUtil.createMasterThread(JVMClusterUtil.java:140)
>         ... 7 more
> {noformat}
> That causes the server to not even start. I would suggest we either skip the 
> {{startHandler()}} call altogether, or make it zero aware.
> Another possible option is to reserve at least _one_ scan handler/queue when 
> the scan ratio is greater than zero, but only of there is more than one read 
> handler/queue to begin with. Otherwise the scan handler/queue should be zero 
> and share the one read handler/queue.
> Makes sense?



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