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From "Weldon Washburn" <weldon...@gmail.com>
Subject Re: [drlvm] finalizer design questions
Date Tue, 26 Dec 2006 23:43:33 GMT
On 12/26/06, Pavel Afremov <pavel.n.afremov@gmail.com> wrote:
>
> Hi.
>
> On 12/19/06, in "[drlvm][gcv5] finalizer design" thread I wrote,
> that  "WBS
> should increase relative finalization performance by following steps:
>
>   - Increase number of finalizer threads while them quantity is less
>   then processors number.
>   - Using locks for stopping user threads. I have some ideas how to do
>   this without deadlock.
>   - Increase priority of finalizer thread. Or reduce priority of user
>   threads which generate finalizable objects, because finalization
> activities
>   shouldn't stop "good" threads which don't create fianalizabe object."


hmm.... I am not sure how a JVM would determine which java app
thread(s) will generate finalizable objects at specific rates.  In other
words, how would the JVM anticipate which thread(s) in the future are "good"
and which one(s) are bad??  I vote for a simple design (with less deadlocks
to debug and dodge.)

As I understand "independent" investigation provided by Weldon, which
> discussed here, reaches the same results. So this design can be considered
> as approved by Harmony community.


hmmm.... OK.   Just to confirm we are on the same page.  The design I
suggest is to have exactly one finalizer thread for each CPU in the box.
These finalizer threads are to be created during JVM initialization.
No finalizer threads are added or killed once JVM initialization
completes. The priority of each of these finalizer threads is set slightly
above the highest priority of the Java app threads.  The finalizer
queues have synchronized access.  All threads must grab the queue's lock to
enqueue/dequeue objects.  Is this what you are thinking?

BR
>
> Pavel Afremov.
>
>
>
>
> On 12/26/06, Gregory Shimansky <gshimansky@gmail.com> wrote:
> >
> > 2006/12/26, Weldon Washburn <weldonwjw@gmail.com>:
> > >
> > > On 12/25/06, Gregory Shimansky <gshimansky@gmail.com> wrote:
> > > >
> > > > Weldon Washburn wrote:
> > > > > On 12/24/06, Gregory Shimansky <gshimansky@gmail.com> wrote:
> > > > >>
> > > > >> On Sunday 24 December 2006 16:23 Weldon Washburn wrote:
> > > > [snip]
> > >
> > >
> > >
> > >
> > > This is a very good investigation, looks like this is exactly the way
> > > > that production VMs follow with finalizers.
> > > >
> > > > One last thing that is unclear to me is if an application creates
> many
> > > > objects of a class which have long running (never ending) finalize
> > > > method, it may cause out of memory condition because objects
> wouldn't
> > be
> > > > removed from heap. Is it true with this approach?
> > >
> > >
> > > I ran some more tests.  I created 10,000 finalizable objects.  Then I
> > > "newed" enough empty arrays to cause the GC to shove the 10,000
> > > finalizable
> > > objects into finalization queue.  The finalize() method contains
> exactly
> > > the
> > > same while(true){ busy} workload as before.
> > >
> > > As far as I can tell, only the first object on the finalize queue is
> > ever
> > > executed. The other 9,999 objects remain in the queue until the first
> > > object's finalizer returns (which it never does.)   Supporting
> evidence
> > is
> > > that when I change the workload so that the finalize() method finishes
> > > after
> > > 100M loops, the JVM then move onto the next finalizable object in the
> > > queue.
> > >
> > > In other words, as far as the JVM is concerned the Java app programmer
> > can
> > > legitimately create a situation where the JVM is flooded with objects
> > > requiring finalization and these objects chew up all availiable Java
> > heap.
> > > And the Java app programmer can legitimately create a situation where
> a
> > > finalizer never finishes and it hogs 97% of available CPU.  It looks
> to
> > me
> > > like one of those unavoidable situations where the java app will crash
> > and
> > > burn.  Its much the same situation as intentionally creating an
> infinite
> > > link-list of live objects.  Its easy to build an app that does
> > this.  And,
> > > yes, the JVM will run out of heap and exit.  Its probably best to tell
> > the
> > > app programmer, "just don't do this".
> >
> >
> > Thanks for an interesting investigation, it is really valuable. It shows
> > that production VM which you've used does not create any new threads for
> > finalization. I was thinking about some legitimate scenario which could
> > lead
> > to a jammed finalizer queue and therefore may lead to OOME eventually.
> But
> > later I realized that the whole java process would just hang.
> >
> > The scenario which I was thinking of is if an application has a class
> with
> > a
> > finalizer that deals with file IO, like closing a file. On Unixes when a
> > file is located on NFS filesystem, and this filesystem is disconnected,
> > then
> > file IO with such file (usually, depending on NFS mount options) stops
> the
> > process. I think it is a whole process, not just one thread (correct me
> if
> > I
> > am wrong), so it wouldn't be just finalization queque which would wait
> for
> > NFS IO, it would be all VM threads, so OOME would not happen.
> >
> > --
> > Gregory
> >
> >
>
>


-- 
Weldon Washburn
Intel Enterprise Solutions Software Division

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