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From Bryon Jacob <br...@jacob.net>
Subject memory leak
Date Mon, 05 Nov 2007 21:49:24 GMT
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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