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Date: Wed, 9 Dec 2009 17:11:18 +0000 (UTC)
From: "Jean-Daniel Cryans (JIRA)" <jira@apache.org>
To: hbase-dev@hadoop.apache.org
Subject: [jira] Commented: (HBASE-2023) Client sync block can cause 1 thread
 of a multi-threaded client to block all others
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    [ https://issues.apache.org/jira/browse/HBASE-2023?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12788165#action_12788165 ] 

Jean-Daniel Cryans commented on HBASE-2023:
-------------------------------------------

Yes region splits (takes at least 6 seconds in the pre-0.21 architecture) will generate NotServingRegionException from the RS that was holding the parent of the split so if 1 out of 10 threads (so in the same JVM) goes to write to that location then it will block all threads for that time.

> Client sync block can cause 1 thread of a multi-threaded client to block all others
> -----------------------------------------------------------------------------------
>
>                 Key: HBASE-2023
>                 URL: https://issues.apache.org/jira/browse/HBASE-2023
>             Project: Hadoop HBase
>          Issue Type: Bug
>    Affects Versions: 0.20.2
>            Reporter: ryan rawson
>
> Take a highly multithreaded client, processing a few thousand requests a second.  If a table goes offline, one thread will get stuck in "locateRegionInMeta" which is located inside the following sync block:
>         synchronized(userRegionLock){
>           return locateRegionInMeta(META_TABLE_NAME, tableName, row, useCache);
>         }
> So when other threads need to find a region (EVEN IF ITS CACHED!!!) it will encounter this sync and wait. 
> This can become an issue on a busy thrift server (where I first noticed the problem), one region offline can prevent access to all other regions!
> Potential solution: narrow this lock, or perhaps just get rid of it completely.

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