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From "Chris Berry (JIRA)" <j...@apache.org>
Subject [jira] Created: (ABDERA-153) Serious memory leak in ExtensionFactoryMap
Date Mon, 28 Apr 2008 17:45:55 GMT
Serious memory leak in ExtensionFactoryMap

                 Key: ABDERA-153
                 URL: https://issues.apache.org/jira/browse/ABDERA-153
             Project: Abdera
          Issue Type: Bug
    Affects Versions: 0.3.0
         Environment: N/A
            Reporter: Chris Berry
            Priority: Critical

Some time ago we reported a very serious memory leak in Abdera 0.3.0.
It was reported to the abdera users list on Nov 7, 2007 as "memory leak".
That message is replayed below...
At the time we could workaround the bug in our client, but over time this became impossible,
and we had to hack ExtensionFactoryMap.

Attached is a patch to org.apache.abdera.factory.ExtensionFactoryMap
This patch simply removes the problem -- the WeakHashMap.
This change has a negligible effect on performance (as measured in load testing) 
But without it, abdera 0.3.0 will get OutOfMemoryExceptions quite quickly under load, rendering
it unusable.
BTW: this code has been in Production for many weeks...

We were hoping that you could apply this patch to 0.3.0??
Or better;  produce 0.3.1

We are not able to upgrade to 0.4.0 yet, and would rather not have a hacked copy of 0.3.0.
Also, others not able to upgrade yet (since the upgrade is not entirely transparent) may benefit
from the patch

I will also open a JIRA on this.

-- Chris 

	From: 	bryon@jacob.net
	Subject: 	memory leak
	Date: 	November 5, 2007 3:49:24 PM CST
	To: 	abdera-user@incubator.apache.org
	Reply-To: 	abdera-user@incubator.apache.org

                    Hey All -

I'm working on a data service based on Abdera (working with Chris Berry, who's a regular on
these lists...)  When we were running our first battery of serious load testing on our system,
we encountered memory-leaky behavior, and a profiler showed us that we were indeed leaking
hundreds of megabytes a minute, all traceable back to the wrappers field on org.apache.abdera.factory.ExtensionFactoryMap.
 This field is a map from elements to their wrappers, if any.  At first, I was puzzled by
the memory leak, as the field is initialized thusly:

	this.wrappers = Collections.synchronizedMap( new WeakHashMap<Element,Element>());

clearly, the implementor took care to make sure that this cache would not leak by making it
a WeakHashMap, which generally guarantees that the map itself will not keep a key and its
corresponding entry from being garbage collected.  I dug throughout our application code to
find if we were actually holding other references to these objects, and I googled for anyone
having problems with esoteric interactions between Collections.synchronizedMap and WeakHashMaps
- found nothing there.  Then I went back to square one and re-read the WeakHashMap javadoc
very carefully.  Here's the relevant section:

	Implementation note: The value objects in a WeakHashMap are held by ordinary strong references.
Thus care should be taken to ensure that value objects do not strongly refer to their own
keys, either directly or indirectly, since that will prevent the keys from being discarded.
Note that a value object may refer indirectly to its key via the WeakHashMap itself; that
is, a value object may strongly refer to some other key object whose associated value object,
in turn, strongly refers to the key of the first value object. One way to deal with this is
to wrap values themselves within WeakReferences before inserting, as in: m.put(key, new WeakReference(value)),
and then unwrapping upon each get.

This is why there is a memory leak - the map is a mapping from elements to their wrappers
- by the very nature of the object being a wrapper of the element, it will usually have a
strong reference to the element itself, which is the key! You can verify that Abdera wrappers,
in general, will do this by looking at org.apache.abdera.model.ElementWrapper, which takes
the element being wrapped as its constructor argument, and holds a strong reference to it
as an instance variable.

This map is an optimization to memoize the calls to getElementWrapper() and not reconstruct
them more than is necessary - it is not needed for abdera to function properly, so we have
temporarily worked around the problem in our own application like so - we used to acquire
our FOMFactory by calling abdera.getFactory() on our org.apache.abdera.Abdera instance, and
re-using that singleton throughout our application.  Now we construct a new FOMFactory with
new FOMFactory(abdera) once per request to the server, and since the only appreciable state
on the factory is this map itself, this is a valid work-around.

I'd initially planned to really fix this issue and submit a patch along with this message,
but digging a little deeper, I'm not sure that the correct fix is crystal clear...  We could
do as the javadoc above suggests, and wrap the values with WeakReferences to plug the leak,
or we could use a LinkedHashMap configured as an LRU cache to just bound the cache, so it
can't grow out of control - but right now, I don't think that either of those solutions would
be correct, because it seems to me that none of the objects in the hierarchy rooted at FOMElement
define equals() and/or hashCode() methods, so all of the objects are cached based on their
actual object identity.  It seems that in the all likely use cases, instances of FOMElement
and its descendants are re-parsed on every request to a server running abdera, and so what
we will see is cache misses virtually 100% of the time, so even though we'd have plugged the
leak, strictly speaking, we would have ignored the underlying issue that we're caching data
on every request that will be fundamentally unable to be retrieved on subsequent requests.
 This is based only on my reading over the code for a few hours, so I could be missing something,
and I also might be forgetting about a use case that demands and makes proper use of this
memoization, but as it stands right now, my recommended fix would probably be to just cut
out the cache altogether, and allow for wrappers to get constructed fresh every time they
are requested.  One more possibility is that the cache is actually a useful optimization,
but only during the scope of one request - in which case the "work-around" we are using now
is actually the best practice, and the fix would be to remove the factory instance on the
Abdera class...

I'd like to hear from the Abdera developers what their thoughts are on this issue, and what
the best resolution is likely to be.  This is no longer a pressing issue for our team, but
it is potentially a time bomb waiting to blow up for any project dependent on Abdera.

thanks!  (and thanks for Abdera, generally - we're easily a year ahead of where we'd be on
this project without it!)

-Bryon (long-time listener, first-time caller)

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