RE: PATCH for deadlocking issues with LoadedObjectsRegistry use o f WeakHashMap

[Mahler Thomas <th...@itellium.com>]

Hi Galvin,

Joe already posted his patch. I'll review it in the next few days.
thanks for your help!

Thomas

> -----Original Message-----
> From: Galvin Hsiu [mailto:gkh4w@yahoo.com]
> Sent: Thursday, June 26, 2003 8:43 AM
> To: ojb-dev@db.apache.org
> Cc: jfe@pobox.com
> Subject: PATCH for deadlocking issues with 
> LoadedObjectsRegistry use of
> WeakHashMap
> 
> 
> Hi Thomas,
> 
> Both Joe Elliott and I did encounter this issue and
> investigated it at length by examining the source code
> to the WeakHashMap.  What we discovered was a resource
> contention issue in the WeakHashMap implementation on
> JDK 1.4.1_03 (not sure if this was fixed in the 
> previous JDK versions or versions after) - but
> whenever puts are getting called concurrently on the
> same WeakHashMap, the while loop in the put function
> call of the Map interface gets stuck since the said
> iterator is modified by another thread.  Let me know
> if you want further explanation and I will try to post
> up the said code snippet.
> 
> The patch is as follows - synchronize using the weak
> hash map on puts, remove and clears, but not on the
> gets. These critical sections will ensure only 1
> thread can modify this weak hash map at a time; I
> personally think the performance impact on this is
> minimal since the LoadedObjectRegistry seems to be
> utilized PRIMARILY (aka explicit lookups) for the
> frameworks utilizing the OTM layer (JDO, ODMG) as
> opposed to the PB layer, which we are using right now.
> Regardless, I'd rather trade a little performance hit
> as opposed to ballooning CPU usage / thrashing due to
> this interesting bug.
> 
> // BEGIN PATCHED FILE
> 
> package org.apache.ojb.broker.core;
> 
> /*
> ====================================================================
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>  * are met:
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> the software itself,
>  *    if and wherever such third-party acknowledgments
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>  * 4. The names "Apache" and "Apache Software
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>  *    derived from this software without prior written
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>  * 5. Products derived from this software may not be
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> 
> import java.util.WeakHashMap;
> 
> /**
>  * This is a helper class which registers all objects
> loaded
>  * from database. It is used by ODMG layer to
> determine the state of
>  * objects: if an object was not loaded from database
> then it is new.
>  * Note: objects remain registered even after they are
> deleted. This
>  * is necessary to prevent creation of deleted objects
> by another
>  * thread, see <a
> href="http://archives.apache.org/eyebrowse/ReadMsg?listId=106&
> msgNo=1382">this<a>
>  * for details.
>  *
>  * @author <a href="mailto:olegnitz@apache.org">Oleg
> Nitz<a>
>  * @version $Id: LoadedObjectsRegistry.java,v 1.1
> 2003/04/26 23:18:25 arminw Exp $
>  */
> public class LoadedObjectsRegistry
> {
>     private static WeakHashMap registry = new
> WeakHashMap();
> 
>     private static final Object DUMMY = new Object();
> 
>     public static void register(Object object)
>     {
>         // Patch: critical section on the put of the
> registry
>         synchronized(registry)
>         {
>         registry.put(object, DUMMY);
>         }
>     }
> 
>     public static boolean isRegistered(Object object)
>     {
>         return (registry.get(object) != null);
>     }
> 
>     public static void clear()
>     {
>         // Patch: critical section on the clear of the
> registry
>         synchronized(registry)
>         {
>         registry.clear();
>         }
>     }
> 
>     public static void remove(Object object)
>     {
>         if(object != null)
>         {
>         // Patch: critical section on the remove of
> the registry
>             synchronized(registry)
>             {
>             registry.remove(object);
>             }
>         }
>     }
> }
> 
> 
> 
> Hi Joseph,
> 
> To include your changes into the codebase please post
> your patch (or 
> patched files).
> YOu can post it as a zipped file either to the list or
> directly to me.
> Of course we want to get OJB free of such issues!
> 
> cheers,
> Thomas
> 
> Joseph Elliott wrote:
> > has anybody else encountered deadlocking issues
> under load with the ojb
> > library?  we started seeing frequent situations with
> pegged cpu usage and
> > stuck threads testing a large web application
> (running under tomcat on duel
> > cpu linux boxes)  a thread dump showed numerous
> threads all stuck in the
> > WeakHashMap.put's called from
> LoadedObjectsRegistry.java
> > 
> > adding synchronize blocks around the
> LoadedObjectsRegistry.java put and
> > clear calls has seemed to have stabilized things. 
> does any body have any
> > opinions about the performance impact of this and if
> there are any other
> > alternatives to making the system safe under highly
> concurrent usage?
> > 
> > we're currently working with a patched version of
> ojb for our application,
> > but it would be nice if this could be addressed in
> the official code base
> > for the next rc.
> > 
> > thanks much,
> >  - joe
> 
> 
> 
> 
> 
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