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From "Kristian Waagan (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (DERBY-4137) OOM issue using XA with timeouts
Date Fri, 10 Jun 2011 13:38:59 GMT

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

Kristian Waagan updated DERBY-4137:
-----------------------------------

    Attachment: derby-4137-2a-reduce_memory_footprint.diff

Attaching patch 2a, which reduces the footprint of the cancellation task object.

I made the cancellation task class static, and made it accept a reference to the XAState object.
When the task is canceled, which happens when the XA transaction succeeds within the timeout
value, the reference to the XAState object is cleared.

As mentioned earlier, the memory usage when using XA timeouts is a function of the transaction
rate and the timeout value. Setting the timeout value very high is not a good idea (i.e. several
hours or days).

If this fix is insufficient, we can as already mentioned add the purge functionality.

suites.All and 'ant junit-lowmem' passed.
Patch ready for review.

> 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, derby-4137-2a-reduce_memory_footprint.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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