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From "Weldon Washburn" <weldon...@gmail.com>
Subject Re: [drlvm] finalizer design questions
Date Thu, 28 Dec 2006 02:34:52 GMT
On 12/27/06, Geir Magnusson Jr. <geir@pobox.com> wrote



> [snip]





> Why can't we simply mimic the rational behavior of the RI and other
> production VMs and leave it at that?
>
> geir


I agree.  To discover what other JVMs do, I created a very simple finalizer
probe then put it in JIRA HARMONY-2908.  I ran this probe on Sun JVM
1.5.0_07.  Below are the results.   It would be good if someone can run this
probe on other JVM/OS combinations. The probe is single threaded and has
three different execution modes ("java Finx 0", "java Finx 1" and "java Finx
2").  By running each of the modes on WindowsXP and using Microsoft's
Process Viewer, one can learn what the different JVM threads are doing.
More on this later.

Mode 0
This mode intentionally does not create any finalizable objects.  The main()
method simply runs a cpu intensive workload forever.  After every 1000000
loops main() will print a distinctive string that includes a loop count.

Mode 1
main() creates 100K finalizable objects that are intentionally shoved into a
state where their finalizer needs to be called.  main() then proceeds to run
the same cpu intensive workload as above.  The finalize() method will
execute just one call of the same cpu intensive workload then returns.  This
simulates a short running finalizer.  finalize() prints a distinctive string
to make it easy to quickly read the output which is comingled with main().

Mode 2
This mode is identical to Mode 1 except the finalize() method calls the cpu
intensive workload endlessly.

The above describes how the probe is constructed.  Below are observations
from running this probe on Sun 1.5.0 JVM.

Mode 0

There are seven threads.  Thread 0 consumes 99% of the total cpu time.  And
is executing in user mode 100% of the time.  Most likely this is the java
app thread running main().  All the remaining threads do not accumulate any
significant cpu time.



Mode 1

There are seven threads.  Thread 0 accumulates roughly 2% of total cpu
time.  Thread 3 accumulates the other 98%.  Process Viewer reports Thread 3
having "above normal" priority.  (I have not chased down the mapping from
Process Viewer priority to win API priority).  At the top of the finalize()
method a static variable is incremented then printed out.  This allows us to
watch a rolling count of how many objects have been finalized.  Watching the
console output for a few minutes, it looks like about 9000 objects are
finalized in the same time period that main() completes 100 loops.  Since
both main() and finalize() are running the same workload, it looks
like Thread 3 with  "above normal" priority is the thread running the
finalizers.  The disparity between 90:1 on console output and 50:1 in
Process Viewer is probably sampling noise.  Also, it looks like Thread 0 is
running main() just like it was in Mode 0.



Mode 2

Again the JVM is running exactly 7 threads.  Only the one object's
finalize() method is ever called.  Process Viewer shows Thread 3 has "above
normal" priority and accumulating roughly 99% of the cpu time.  Watching the
console output for a few minutes, it look like 2600 finalize() loops to 15
main loops.  It appears that no additional threads are created to handle to
remaining 99,999 waiting finalizable objects.   These objects appear to be
blocked waiting for the first object to finish. Also it looks like Thread 0
is running main() just like Mode 0 and 1.  Given that Process Viewer shows
that Thread 0 continuously and slowly accumulates CPU time, it appears that
main() is not suspended but continues to make forward progress.

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