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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DERBY-4137) OOM issue using XA with timeouts
Date Wed, 08 Jun 2011 09:53:58 GMT

    [ https://issues.apache.org/jira/browse/DERBY-4137?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13045872#comment-13045872
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Knut Anders Hatlen commented on DERBY-4137:
-------------------------------------------

Thanks. The patch makes more sense to me now. :)

Does the OOME happen because there are too many CancelXATransactionTask objects, or because
the CancelXATransactionTask objects reference too many other objects via the XATransactionState?
If the latter, would it be sufficient to clear the reference to the XATransactionState object
when calling cancel()? I see that CancelQueryTask in GenericStatementContext uses that technique.
If possible, it would be good to avoid calling the purge() method on every XA commit/rollback,
both because of the complexity caused by reflection and because the javadoc warns about its
performance impact.

> 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
>            Assignee: Kristian Waagan
>              Labels: derby_triage10_5_2
>         Attachments: derby-4137-1a-purge_on_cancel.diff, derby-4137-1a-purge_on_cancel.stat,
derby-4137-1b-purge_on_cancel.diff
>
>
> 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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