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From "Ronald Tschalaer (JIRA)" <j...@apache.org>
Subject [jira] Updated: (DERBY-4137) OOM issue using XA with timeouts
Date Wed, 01 Apr 2009 14:01:14 GMT

     [ https://issues.apache.org/jira/browse/DERBY-4137?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Ronald Tschalaer updated DERBY-4137:
------------------------------------


I just noticed that GenericStatementContext has potentially the same issue (I haven't delved
deep enough to understand the lifecyle of instances of that class, so it may or may not be
an
issue there).

> 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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