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https://issues.apache.org/jira/browse/HBASE-2023?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12788448#action_12788448
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Jean-Daniel Cryans commented on HBASE-2023:
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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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