On Dec 26, 2006, at 10:53 AM, Weldon Washburn wrote:
On 12/26/06, Gregory Shimansky <[EMAIL PROTECTED]> wrote:
2006/12/26, Weldon Washburn <[EMAIL PROTECTED]>:
>
> On 12/25/06, Gregory Shimansky <[EMAIL PROTECTED]> wrote:
> >
> > Weldon Washburn wrote:
> > > On 12/24/06, Gregory Shimansky <[EMAIL PROTECTED]> wrote:
> > >>
> > >> On Sunday 24 December 2006 16:23 Weldon Washburn wrote:
[snip]
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.
hmm... there are probably several orthogonal issues in what you
describe
above:
1)
Would the above issues be any different if finalizers were not
involved?
For example, the app manually determines when to close a file on an
NFS
mount. I suspect the answer is no.
Right.
2)
Does it make sense for a particular java app to be dependent upon
an NFS
mount? This is something that is best addressed by someone who
knows the
app environment. Stuff like service uptime agreements, network
topology/traffic , how loaded servers are, etc. In other words,
the app
programmer. The JVM designer should be aware of these issues and
know what
falls in the domain of the JVM vs. the app.
3)
What would an app do if an NFS mount point suddenly disappeared
while file
I/O is in flight? If its really mission critical, often times the
file I/O
is part of a two-phase commit transaction. At a higher level, the
transaction aborts thus no damage is done.
This is dependent upon app design.
4)
What do other JVMs do when an NFS mount point disappears?
Obviously this
far away from the original topic, finalizer design. But at some
point,
someone needs to run controlled tests and tell dev list what the
results
are.
It's not clear that the VM has a clue that a mount point is NFS vs
local disk. It will probably get some fault from the OS and that
would be that. Same thing if you unplug a disk drive...
5)
How are real java apps designed? What do they really use finalizer
for?
I don't believe that people use them anymore. When I was writing
real things in Java, I never used them - I made sure that resource
management and such was deliberate, and not dependent upon when and
if (and even where - it can run in any thread, right?) the finalizer
ran.
geir
