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https://issues.apache.org/jira/browse/ZOOKEEPER-1669?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16091403#comment-16091403
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Cheney Sun commented on ZOOKEEPER-1669:
---------------------------------------

Looks like the issue is already addressed in [ZOOKEEPER-1347] and a patch was 
available, but not fixed in 3.4, which is the latest stable version and the one 
we currently use. @Michael, is it meaningful to porting the fix to older 
version, 3.4 or 3.3?

> Operations to server will be timed-out while thousands of sessions expired 
> same time
> ------------------------------------------------------------------------------------
>
>                 Key: ZOOKEEPER-1669
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1669
>             Project: ZooKeeper
>          Issue Type: Improvement
>          Components: server
>    Affects Versions: 3.3.5
>            Reporter: tokoot
>            Assignee: Cheney Sun
>              Labels: performance
>
> If there are thousands of clients, and most of them disconnect with server 
> same time(client restarted or servers partitioned with clients), the server 
> will busy to close those "connections" and become unavailable. The problem is 
> in following:
>   private void closeSessionWithoutWakeup(long sessionId) {
>       HashSet<NIOServerCnxn> cnxns;
>           synchronized (this.cnxns) {
>               cnxns = (HashSet<NIOServerCnxn>)this.cnxns.clone();  // other 
> thread will block because of here
>           }
>       ...
>   }



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