Wei-Chiu Chuang commented on HDFS-11900:

Thanks for the patch John, I kicked off a precommit build for you. 

> Hedged reads thread pool creation not synchronized
> --------------------------------------------------
>                 Key: HDFS-11900
>                 URL: https://issues.apache.org/jira/browse/HDFS-11900
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: hdfs-client
>    Affects Versions: 2.8.0
>            Reporter: John Zhuge
>            Assignee: John Zhuge
>         Attachments: HDFS-11900.001.patch
> *Non-static* synchronized method initThreadsNumForHedgedReads can't 
> synchronize the access to the *static* class variable HEDGED_READ_THREAD_POOL.
> {code}
>   private static ThreadPoolExecutor HEDGED_READ_THREAD_POOL;
> ...
>   private synchronized void initThreadsNumForHedgedReads(int num) {
> {code}
> 2 DFS clients may update the same static variable in a race because the lock 
> is on each DFS client object, not on the shared DFSClient class object.
> There are 2 possible fixes:
> 1. "Global thread pool": Change initThreadsNumForHedgedReads to static
> 2. "Per-client thread pool": Change HEDGED_READ_THREAD_POOL to non-static
> From the description for property {{dfs.client.hedged.read.threadpool.size}}:
> {quote}
>     to a positive number. The threadpool size is how many threads to dedicate
>     to the running of these 'hedged', concurrent reads in your client.
> {quote}
> it seems to indicate the thread pool is per DFS client.
> Let's assume we go with #1 "Global thread pool". One DFS client has the 
> property set to 10 in its config, while the other client has the property set 
> to 5 in its config, what is supposed to the size of the global thread pool? 
> 5? 10? Or 15?
> The 2nd fix seems more reasonable to me.

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