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From Robin Garner <robin.gar...@anu.edu.au>
Subject Re: [drlvm] Class unloading support - tested one approach
Date Wed, 08 Nov 2006 12:21:47 GMT
Robin Garner wrote:
> Aleksey Ignatenko wrote:
>> Robin.
>>> OK, well how about keeping a weak reference to the >j.l.ClassLoader
>>> object instead of a strong one.  When the reference >becomes (strong)ly
>>> unreachable, invoke the class-unloading phase.
>> If you have weak reference to j.l.Classloader - GC will collect it 
>> (with all
>> appropriate jlClasses) as soon as there are no references to
>> j.l.Classloaderand appropriate classes. But there is possible
>> situation when there are some
>> live objects of that classes and no references to jlClassloader and
>> jlClasses. This will lead to unpredictable consequences (crash, etc).
>> I want to remind that there 3 mandatory conditions of class unloading:
>> 1. j.l.Classloader instance is unreachable.
>> 2. Appropriate j.l.Class instances are unreachable.
>> 3. No object of any class loaded by appropriate class loader exists.
> Let me repeat.  I offer an efficient solution to (3).  I don't purport 
> to have a solution to (1) and (2).

Let me just add:  This is because I don't think (1) or (2) are 
particularly difficult from a performance point of view, although I'm 
happy to accept that there may still be some subtle engineering challenges.

Now this is just off the top of my head, but what about this for a design:
- A j.l.ClassLoader maintains a collection of each of the classes it has 
- A j.l.Class contains a pointer to its j.l.ClassLoader
- A j.l.Class maintains a collection of its vtable(s) (or a pointer if 1:1).
The point of this is that a class loader and its classes are a 'self 
sustaining' data structure - if one element in it is reachable the whole 
thing is reachable.

The VM maintains a weak reference to all its j.l.ClassLoader instances, 
and maintains a ReferenceQueue for weakly-reachable classloaders. 
ClassLoaders are placed on the ReferenceQueue if and only if they are 
unreachable from the heap (including via their j.l.Class objects).  Note 
this is an irreversible condition: objects that are unreachable can 
never become reachable again, except through very specific methods.

When it sweeps the ReferenceQueue for unreachable classloaders, the VM 
places the unreachable classloaders in a queue of classloaders that are 
candidates for unloading.  This queue is part of the root set of the VM. 
  A classloader in this queue is unreachable from the heap, and can be 
unloaded when there are no objects of any class it has loaded.

This is where my mechanism comes into play.

If an object executes getClass() then its classloader is removed from 
the unloadable classloader queue, its weak reference gets recreated  and 
we're back at the initial state.  My guess is that this is a pretty 
infrequent method call.

I think this stage of the algorithm is easy in performance terms - 
difficult in terms of proving correctness, but if you have an efficient 
reachability mechanism for classes I think the building blocks are 
there, and the subtleties are nothing that a talented engineer can't solve.

I'm not 100% sure what your counter-proposal is: I recall 2 approaches 
from the mailing list:
1) Each object has an additional word in its header that points back to
    its j.l.Class object, and we proceed from here.

Given that the mean object size is ~28 bytes, this proposal adds 14% to 
each object size.  This increases the frequency of GC by 14% and incurs 
a 14% slowdown.  Of course this is an oversimplification but a 14% 
slowdown is a pretty lousy starting point to argue from.

2) The existing pointer in the GC header is traced during GC time.

The average number of pointers per object (excluding the vtable) is 
between 1.5 and 2 for the majority of benchmarks I have looked at 
(footnote: if you know something different, drop me a line) (geometric 
mean 1.78 for {specJVM, pseudoJBB and DaCapo 20051009}).  Tracing one 
additional reference per object will therefore increase the cost of GC 
by ~60% on average.  Again oversimplification but indicative.  If we 
assume that GC accounts for 10% of runtime (more or less depending on 
heap size), this is a runtime overhead of 6%.

My proposal has been measured at ~1% overhead in GC time, or 0.1% in 
execution time (caveats as above).  If there is some complexity in 
establishing classloader reachability from this basis, I would assume it 
can easliy be absorbed.

Therefore I think my proposal, while not complete, can form the basis of 
an efficient complete system for class unloading.

(PS: I'd *love* to be proven wrong)


> Regards,
> Robin
>> Aleksey.
>> On 11/8/06, Robin Garner <robin.garner@anu.edu.au> wrote:
>>> Pavel Pervov wrote:
>>> > Robin,
>>> >
>>> > The kind of model I had in mind was along the lines of:
>>> >> - VM maintains a linked list (or other collection type) of the
>>> currently
>>> >> loaded classloaders, each of which in turn maintains the 
>>> collection of
>>> >> classes loaded by that type.  The sweep of classloaders goes 
>>> something
>>> >> like:
>>> >>
>>> >> for (ClassLoader cl : classLoaders)
>>> >>   for (Class c : cl.classes)
>>> >>     cl.reachable |= c.vtable.reachable
>>> >
>>> >
>>> > This is not enough. There are may be live j/l/Class'es and
>>> > j/l/Classloader's
>>> > in the heap. Even though no objects of any classes loaded by a 
>>> particual
>>> > class loader are available in the heap, if we have live reference to
>>> > j/l/ClassLoader itself, it just can't be unloaded.
>>> OK, well how about keeping a weak reference to the j.l.ClassLoader
>>> object instead of a strong one.  When the reference becomes (strong)ly
>>> unreachable, invoke the class-unloading phase.
>>> To me the key issue from a performance POV is the reachability of
>>> classes from objects in the heap.  I don't pretend to have an answer to
>>> the other questions---the performance critical one is the one I have
>>> addressed, and I accept there may be many solutions to this part of the
>>> question.
>>> > I believe that a separate heap trace pass, different from the standard
>>> >> GC, that visited vtables and reachable resources from there would 
>>> also
>>> >> be a viable solution.  As mentioned in an earlier post, writing 
>>> this in
>>> >> MMTk (where a heap trace operation is a class that you can easily
>>> >> subtype to do this) would be easy.
>>> >>
>>> >> One of the advantages of my other proposal is that it can be
>>> implemented
>>> >> in the VM independent of the GC to some extent.  This additional
>>> >> mark/scan phase may or may not be easy to implement, depending on the
>>> >> structure of DRLVM GCs, which is something I haven't explored.
>>> >
>>> >
>>> > DRLVM may work with (potentially) any number of GCs. Designing class
>>> > unloading the way, which would require mark&scan cooperation from 
>>> GC, is
>>> > not
>>> > generally a good idea (from my HPOV).
>>> That's what I gathered.  hence my proposal.
>>> cheers
>>> -- 
>>> Robin Garner
>>> Dept. of Computer Science
>>> Australian National University

Robin Garner
Dept. of Computer Science
Australian National University

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