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From "Ronald Tschalaer (JIRA)" <j...@apache.org>
Subject [jira] Created: (DERBY-4137) OOM issue using XA with timeouts
Date Wed, 01 Apr 2009 12:35:20 GMT
OOM issue using XA with timeouts
--------------------------------

                 Key: DERBY-4137
                 URL: https://issues.apache.org/jira/browse/DERBY-4137
             Project: Derby
          Issue Type: Bug
          Components: JDBC
    Affects Versions: 10.4.2.0
            Reporter: Ronald Tschalaer


When using JTA for transaction control and a transaction timeout is set,
EmbedXAResource ends up calling XATransactionState.scheduleTimeoutTask() which
in turn registers a timeoutTask with java.util.Timer. In the normal case where
the transaction finishes before the timeout, XATransactionState.xa_finalize()
then calls timeoutTask.cancel(). So far this so good. The problem, however, is
that java.util.TimerTask.cancel() does not actually remove the task from the
timer queue, meaning that a strong reference to the timeoutTask is kept (and
through that to XATransactionState, the EmbedConnection, etc). The reference
is not removed until the time at which the timeout would have fired, which can
be a long time. Under load this can quickly lead to an OOM situation.

A simple fix is to call Timer.purge() every so often. While the javadocs talk
about purge() being rarely needed and that it's not extremely cheap, I've
found that calling it after every cancel() is the best approach, for several
reasons: 1) the scenario here is that almost all tasks are cancelled, and
hence this somewhat fits the Timer.purge() description of an "application that
cancels a large number of tasks"; 2) there usually isn't a very large number
of simultaneous transactions, and hence purge() is actually quite cheap; 3)
this ensures the strong reference is immediately removed, allowing the GC to
do a better job. Interestingly enough, I've had this exact same issue on a
different type of db, and I had tested the purge() there and found it to be in
the sub-microsecond range for 100 transactions (or similar - I don't recall
the exact data), i.e. completely negligible.

In short, my suggestion is to change xa_finalize as follows:

    synchronized void xa_finalize() {
        if (timeoutTask != null) {
            timeoutTask.cancel();
            Monitor.getMonitor().getTimerFactory().
                    getCancellationTimer().purge();
        }
        isFinished = true;
    }

As a temporary workaround, applications can do this themselves, i.e.
add something like the following whenever they close a Connection:

  import org.apache.derby.iapi.services.monitor.Monitor;
  Monitor.getMonitor().getTimerFactory().getCancellationTimer().purge();



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