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From "Pavel Afremov" <pavel.n.afre...@gmail.com>
Subject Re: [drlvm][gcv5] finalizer design
Date Mon, 18 Dec 2006 16:33:28 GMT
Hi

Native finalizer threads really has following advantages:

   - No java code should be called in vm_hint_finalaze. It's a main
   advantage. Potentially it's more secure.
   - Less on 1% calls via native-java-native boundaries. ;)

>From other side java thread can be improved by "magic" technology in future,
for extra performance.



Following native thread advantages is fake or disputable:

   - "Extra java threads maps on native". Why it should be bad? Also
   finalizers of some objects can ask VM about current java thread. So mapping
   java threads to native should be done in any case.
   - "Common pool for VM native threads". In the case of java threads
   (maybe include GC java threads) VM will be able to have one common pool of
   the user and system threads. So I don't see any advantages here.
   - "Native threads easy can be scheduled". Current implementation of GC
   v5 finalization support doesn't use Work Balance Subsystem. The high
   priority for finalizer threads forever is not good example of scheduling.
   Also all java thread based on native, so priority for java thread can be
   changed too. It's not implemented now for java thread, but it's not
   advantage of native thread in any case. In current "temporary"
   implementation of GC v5 finalization support, the scheduling is worse than
   in "java thread" implementation. But is can be improved in future. So from
   this point of view native and java threads are equal.
   - "C++ interfaces is more natural for DRLVM". It's very disputable. I
   think that java interfaces is the best interface solution on this moment,
   especially for java machines and java applications.
   - "Potential circularity dependence with VM bootstrapping". Xiao-Feng
   could present a test which shows it or call path. If you mean Fake
   Circularity Error, that there are two "impossible" tests which shows that
   it's not finalization system faults (see HARMONY-1945), and additional check
   to avoid it was added to finalization system in HARMONY-2230. Maybe you mean
   that java thread is not so secure as native, because depends on the other
   components and can be affects by errors in them?



I agree that support both native and java threads is a good solution. The
main target for "native" implementation is support of WBS (Work Balance
Subsystem), for "java" implementation is support of thread priority
changing. Both this implementation should be unified to one common
interface. Now this interface contains vm_hint_finalaze function only.



Thanks.

Pavel Afremov.


On 12/16/06, Xiao-Feng Li <xiaofeng.li@gmail.com> wrote:
>
> All. It's good to see this discussion initiated. That's exactly what
> we want. It would be a little frustrating to see our solution
> committed without any response from the community. :-)  Anyway, we
> submitted GCv5 finalizer solution for three reasons:
>
> 1. We don't want GCv5 submission to break any existing functionality,
> and to keep the impact to minimum.
> 2 . We think it might be good idea to have more than one Finalizer
> solutions in a JVM, as we have multiple JITs, multiple GCs, and
> probably multiple other components as well.
> 3 . We think a native thread Finalizer solution is better than a Java
> thread solution. Since the Java thread actually runs in a native
> thread, we don't need the extra wrapper.
>
> Explanations to 2.
> - It doesn't matter to have multiple solutions for one component
> except for very strong reason.  Multiple solutions can be measured and
> studied for different scenarios.
> - A Java thread finalizer solution is not a bad idea, and it would
> be also desirable to have a native thread finalizer if it has many
> advantages as described below. Different GCs can invoke different
> finalizer solutions for its best performance, or same GC can invoke
> different finalizer solutions at different times. This is feature
> rather than a bug.
> - It is probably good for DRLVM to have the two finalizer solutions
> to exercise its modularity design in the finalization subsystem. This
> is one of the goals of DRLVM design.
> - It might be unreasonable to say one solution is absolutely better
> than the other one, hence it is uncessary to quickly decide to keep
> one and reject another one. The VM is evolving, so are its subsystems.
>
> Explanations to  3 .
> - In Java finalizer thread implementation, there exists potential
> circular dependence between the Java thread startup and JVM
> bootstrapping. The bootstrapping issues or bugs with Java code in VM
> were discussed more than once.
> - In Java finalizer thread implementation, there are rounds of
> redundant steps to do finalization with Java thread. In existing Java
> thread implementation, to execute the finalizers, VM native calls Java
> method startFinalization to wakeup finalizer threads. The finalizer
> Java threads call a native method doFinalization to excute the
> finalizers. This native method accesses native queue and calls Java
> finalizer method again. With a native thread finalizer, it simply
> calls the Java finalizer directly without all other boundary
> crossings.
> - A java finalizer thread finally maps to a native thread managed by
> VM. This extra mapping is unnecessary.
> - Finalizer threads are VM internal entities. VM may want to
> schedule it as it wants for load balance or helper threading. This is
> much easier with the direct native threads.
> - With native thread finalizer, we can share the same thread pool
> with other VM components such as GC, etc. This helps to manage the
> system overall performance and scalability, and it's easier.
> - DRLVM is in written in C++, its components interact through native
> interfaces. It is natural for VM core components written in native
> code.
>
> It would be good to continue this discussion to reach an agreement on
> the design. We probably can converge on one solution, or agree to keep
> two solutions.
>
> Thanks,
> xiaofeng
>
>
> On 12/16/06, Weldon Washburn <weldonwjw@gmail.com> wrote:
> > On 12/15/06, Pavel Afremov <pavel.n.afremov@gmail.com> wrote:
> > >
> > > Hi.
> > >
> > >
> > >
> > > Weldon.
> > >
> > > Could You be so kind to read mail list <<[DRLVM][GCv5] patch for new
> LOS
> > > collector and finalizer/weakref support>>.
> >
> >
> > Yes, I recall your comments.  I decided that since GCV5 finalizer does
> not
> > disrupt existing finalizer to go ahead and do the commit.  Its is fairly
> > simple to commit compensating changes if we decide to eliminate GCV5
> > finalizer.
> >
> > I tried to start discussion in it, during patch review process. But it
> was
> > > stopped  by Ligang Wang, because, as I understand, the "new" solution
> is
> > > not
> > > "real" solution (Work Balance Subsystem was turned of there, for
> > > example), "it's
> > > only a start, and at the moment only for GCv5". So it's a temporary
> > > solution.
> >
> >
> > The discussion needs to be restarted.  Xiao Feng, can you reply to the
> > questions?
> >
> > So I have several question for "new" scheme, and when it will be solved,
> we
> > > will be ready to discuss a future development of finalization system.
> I
> > > will
> > > be glad to share my ideas in this area.
> > >
> > >
> > >
> > > Salikh.
> > >
> > > You can find prepared by my patch
> > > HARMONY-2230<https://issues.apache.org/jira/browse/HARMONY-2230>,
> > > which contains described by you feature, implemented in more correct
> way,
> > > as
> > > I think.
> > >
> > >
> > >
> > > Thanks.
> > >
> > > Pavel Afremov.
> > >
> > >
> > >
> > >
> > > On 12/15/06, Salikh Zakirov <Salikh.Zakirov@intel.com> wrote:
> > > >
> > > > Weldon Washburn wrote:
> > > > > Harmony-2560 adds a second finalizer implementation to the Apache
> code
> > > > > base.
> > > >
> > > > Speaking of finalizers design, I would like to note that
> HARMONY-1952
> > > > has a proof-of-concept idea of refining the current weak references
> > > > design to prevent running java code from a vm_hint_finalize()
> callback,
> > > > which may happen anywhere in the user code.
> > > >
> > > > > But "build
> > > > > test" does not exercise finalizers to any degree.
> > > >
> > > > Not true.
> > > > $ grep -lr finalize vm/tests/
> > > > vm/tests/kernel/java/lang/RuntimeAdditionalSupport2.java
> > > > vm/tests/kernel/java/lang/RuntimeAdditionalTest40.java
> > > > vm/tests/kernel/java/lang/RuntimeAdditionalTest43.java
> > > > vm/tests/kernel/java/lang/RuntimeTest2.java
> > > > vm/tests/kernel/java/lang/SystemExtensionTest.java
> > > > vm/tests/smoke/exception/FinalizeStackTest.java
> > > > vm/tests/smoke/gc/Finalizer.java
> > > > vm/tests/smoke/gc/FinAlloc.java
> > > > vm/tests/smoke/gc/RunFinalizersOnExitTest.java
> > > > vm/tests/smoke/gc/SynchronizedFinilazersTest.java
> > > > vm/tests/smoke/stress/Finalizer.java
> > > > vm/tests/smoke/thread/InfiniteFinalizer.java
> > > >
> > > > But then, I believe that 'build test' run without any additional
> > > switches
> > > > will not exercise any of GCv5.
> > > >
> > > > > In any case, long term we need just one finalizer design and
> > > > > implementation.  I would like to see a discussion on the merits of
> > > each
> > > > of
> > > > > the finalizer approaches.  It would be good to pick one approach
> > > within
> > > > one
> > > > > week.  Then we can clean up the code to reduce confusion.
> > > >
> > > > I guess you mean "it would be good if someone submits a _cleaned
> code_
> > > > within one week so that we could make a decision" ?
> > > >
> > > >
> > >
> > >
> >
> >
> > --
> > Weldon Washburn
> > Intel Enterprise Solutions Software Division
> >
> >
>

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