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From Alain Stalder <astal...@span.ch>
Subject Re: Improve Groovy class loading performance and memory management
Date Thu, 19 May 2016 08:23:41 GMT
The version with a WeakHashMap and first getting ClassInfo without 
synchronization (which would could not be used in the end anyway) 
appeared in some quick (probably not representative) tests with 
concurrent access to be about 10-20% slower than the current 
implementation. Similar result also when I split up into 4096 
WeakHashMap instances in an array of clazz.hashCode()%4096 and always 
synchronizing on get().

I looked into the source of ClassValue. I had naively expected it to 
contain some native code or otherwise interact more directly with the 
VM, but there is only a new field in the Class class that it uses. So 
there is a hard link from the class via that field to ClassInfo in the 
case of Groovy, which I guess prevents "on-the-fly" garbage collection 
because ClassInfo contains lots of stuff with hard references to the 
class. And with the pre Java 7 implementation I presume the situation is 
similar. (With a WeakHashMap that is different because the value does 
not count, the entry can be collected independently of the value and it 
could be argued that ClassValue should have been implemented in a way 
that would have had the same behavior, if I my reasoning is correct.)

So maybe refactoring to always using WeakReference<Class> in all objects 
stored in ClassInfo (meta classes, caches, ...) would be sufficient to 
get "on-the-fly" garbage collection (i.e. before the maximum on 
Metaspace or Heap is reached)? And would the additional weakRef.get() 
calls maybe have again a noticeable effect on performance? I won't try 
this refactoring myself, but if someone else wants to try this?

Well, at least I think I might start to understand the problem... ;)

Alain

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