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https://issues.apache.org/jira/browse/HBASE-4150?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13078574#comment-13078574
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Ted Yu commented on HBASE-4150:
-------------------------------
Looks like we should describe the above scenario in javadoc and book.
> Potentially too many connections may be opened if ThreadLocalPool or
> RoundRobinPool is used
> -------------------------------------------------------------------------------------------
>
> Key: HBASE-4150
> URL: https://issues.apache.org/jira/browse/HBASE-4150
> Project: HBase
> Issue Type: Bug
> Reporter: Ted Yu
> Fix For: 0.92.0
>
>
> See 'Problem with hbase.client.ipc.pool.type=threadlocal in trunk' discussion
> started by Lars George.
> From Lars Hofhansl:
> Looking at HBaseClient.getConnection(...) I see this:
> {code}
> synchronized (connections) {
> connection = connections.get(remoteId);
> if (connection == null) {
> connection = new Connection(remoteId);
> connections.put(remoteId, connection);
> }
> }
> {code}
> At the same time PoolMap.ThreadLocalPool.put is defined like this:
> {code}
> public R put(R resource) {
> R previousResource = get();
> if (previousResource == null) {
> ...
> if (poolSize.intValue() >= maxSize) {
> return null;
> }
> ...
> }
> {code}
> So... If the ThreadLocalPool reaches its capacity it always returns null and
> hence all new threads will create a
> new connection every time getConnection is called!
> I have also verified with a test program that works fine as long as the
> number of client threads (which include
> the threads in HTable's threadpool of course) is < poolsize. Once that is no
> longer the case the number of
> connections "explodes" and the program dies with OOMEs (mostly because each
> Connection is associated with
> yet another thread).
> It's not clear what should happen, though. Maybe (1) the ThreadLocalPool
> should not have a limit, or maybe
> (2) allocations past the pool size should throw an exception (i.e. there's a
> hard limit), or maybe (3) in that case
> a single connection is returned for all threads while the pool it over its
> limit or (4) we start round robin with the other
> connection in the other thread locals.
> For #1 means that the number of client threads needs to be more carefully
> managed by the client app.
> In this case it would also be somewhat pointless that Connection have their
> own threads, we just pass stuff
> between threads.
> #2 would work, but puts more logic in the client.
> #3 would lead to hard to debug performance issues.
> And #4 is messy :)
> From Ted Yu:
> For HBaseClient, at least the javadoc doesn't match:
> {code}
> * @param config configuration
> * @return either a {@link PoolType#Reusable} or {@link
> PoolType#ThreadLocal}
> */
> private static PoolType getPoolType(Configuration config) {
> return PoolType.valueOf(config.get(HConstants.HBASE_CLIENT_IPC_POOL_TYPE),
> PoolType.RoundRobin, PoolType.ThreadLocal);
> {code}
> I think for RoundRobinPool, we shouldn't allow maxSize to be
> Integer#MAX_VALUE. Otherwise connection explosion described by Lars may incur.
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