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Date: Thu, 10 Aug 2017 19:30:00 +0000 (UTC)
From: "Wei-Chiu Chuang (JIRA)" <jira@apache.org>
To: hdfs-issues@hadoop.apache.org
Message-ID: <JIRA.13075560.1496051056000.3070.1502393400251@Atlassian.JIRA>
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Subject: [jira] [Commented] (HDFS-11900) Hedged reads thread pool creation
 not synchronized
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archived-at: Thu, 10 Aug 2017 19:30:04 -0000


    [ https://issues.apache.org/jira/browse/HDFS-11900?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16122172#comment-16122172 ] 

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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