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From "Jean-Daniel Cryans (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-2023) Client sync block can cause 1 thread of a multi-threaded client to block all others
Date Thu, 10 Dec 2009 02:13:18 GMT

    [ https://issues.apache.org/jira/browse/HBASE-2023?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12788448#action_12788448
] 

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

So for this issue I see some kind of trade-off. 

 - If all threads synchronize before the method, stuff in cache won't be picked up until another
thread is done looking for a another row. On the plus side, that thread waiting in line could
be needing the new location that will be put in the cache by the thread holding the lock.

 - If the synchronize is more narrow eg after looking up the cache, the threads won't be blocked
but some threads looking for a location in .META. could be looking for the same row and yet
will all go through that code.

 - If no synchronization, it's like the previous situation but all threads will query .META.
around the same time.

I don't like putting more load on .META. and I don't like having clients waiting sometimes
for nothing.

> 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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