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From "Ted Yu (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-4150) Potentially too many connections may be opened if ThreadLocalPool or RoundRobinPool is used
Date Sun, 31 Jul 2011 21:37:09 GMT
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 George:
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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