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https://issues.apache.org/jira/browse/HBASE-9321?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13750255#comment-13750255
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Jimmy Xiang commented on HBASE-9321:
------------------------------------

For this issue, how should be move ahead? To support impersonation, we can 
either create a new connection/hconnection per user, or reuse the connection 
among users if proxy user is supported.  If one connection per user, we may 
have many connections if there are many users.  To reuse the connection, we can 
either check the current user so that clients can use doAs, or we can use some 
client RPC context to pass the user info down with threadlocal variables.
                
> Contention getting the current user in RpcClient$Connection.writeRequest
> ------------------------------------------------------------------------
>
>                 Key: HBASE-9321
>                 URL: https://issues.apache.org/jira/browse/HBASE-9321
>             Project: HBase
>          Issue Type: Bug
>    Affects Versions: 0.95.2
>            Reporter: Jean-Daniel Cryans
>             Fix For: 0.98.0, 0.96.0
>
>         Attachments: trunk-9321.patch
>
>
> I've been running tests on clusters with "lots" of regions, about 400, and 
> I'm seeing weird contention in the client.
> This one I see a lot, hundreds and sometimes thousands of threads are blocked 
> like this:
> {noformat}
> "htable-pool4-t74" daemon prio=10 tid=0x00007f2254114000 nid=0x2a99 waiting 
> for monitor entry [0x00007f21f9e94000]
>    java.lang.Thread.State: BLOCKED (on object monitor)
>       at 
> org.apache.hadoop.security.UserGroupInformation.getCurrentUser(UserGroupInformation.java:466)
>       - waiting to lock <0x00000000fb5ad000> (a java.lang.Class for 
> org.apache.hadoop.security.UserGroupInformation)
>       at 
> org.apache.hadoop.hbase.ipc.RpcClient$Connection.writeRequest(RpcClient.java:1013)
>       at org.apache.hadoop.hbase.ipc.RpcClient.call(RpcClient.java:1407)
>       at 
> org.apache.hadoop.hbase.ipc.RpcClient.callBlockingMethod(RpcClient.java:1634)
>       at 
> org.apache.hadoop.hbase.ipc.RpcClient$BlockingRpcChannelImplementation.callBlockingMethod(RpcClient.java:1691)
>       at 
> org.apache.hadoop.hbase.protobuf.generated.ClientProtos$ClientService$BlockingStub.multi(ClientProtos.java:27339)
>       at 
> org.apache.hadoop.hbase.client.MultiServerCallable.call(MultiServerCallable.java:105)
>       at 
> org.apache.hadoop.hbase.client.MultiServerCallable.call(MultiServerCallable.java:43)
>       at 
> org.apache.hadoop.hbase.client.RpcRetryingCaller.callWithoutRetries(RpcRetryingCaller.java:183)
> {noformat}
> While the holder is doing this:
> {noformat}
> "htable-pool17-t55" daemon prio=10 tid=0x00007f2244408000 nid=0x2a98 runnable 
> [0x00007f21f9f95000]
>    java.lang.Thread.State: RUNNABLE
>       at java.security.AccessController.getStackAccessControlContext(Native 
> Method)
>       at java.security.AccessController.getContext(AccessController.java:487)
>       at 
> org.apache.hadoop.security.UserGroupInformation.getCurrentUser(UserGroupInformation.java:466)
>       - locked <0x00000000fb5ad000> (a java.lang.Class for 
> org.apache.hadoop.security.UserGroupInformation)
>       at 
> org.apache.hadoop.hbase.ipc.RpcClient$Connection.writeRequest(RpcClient.java:1013)
>       at org.apache.hadoop.hbase.ipc.RpcClient.call(RpcClient.java:1407)
>       at 
> org.apache.hadoop.hbase.ipc.RpcClient.callBlockingMethod(RpcClient.java:1634)
>       at 
> org.apache.hadoop.hbase.ipc.RpcClient$BlockingRpcChannelImplementation.callBlockingMethod(RpcClient.java:1691)
>       at 
> org.apache.hadoop.hbase.protobuf.generated.ClientProtos$ClientService$BlockingStub.multi(ClientProtos.java:27339)
>       at 
> org.apache.hadoop.hbase.client.MultiServerCallable.call(MultiServerCallable.java:105)
>       at 
> org.apache.hadoop.hbase.client.MultiServerCallable.call(MultiServerCallable.java:43)
>       at 
> org.apache.hadoop.hbase.client.RpcRetryingCaller.callWithoutRetries(RpcRetryingCaller.java:183)
> {noformat}

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