On 01/20/2014 09:00 AM, Peter Levart wrote:
On 01/20/2014 02:51 AM, David Holmes wrote:
Hi Peter,
On 17/01/2014 11:24 PM, Peter Levart wrote:
On 01/17/2014 02:13 PM, Peter Levart wrote:
// Fast path for cleaners
boolean isCleaner = false;
try {
isCleaner = r instanceof Cleaner;
} catch (OutofMemoryError oome) {
continue;
}
if (isCleaner) {
((Cleaner)r).clean();
continue;
}
Hi David, Kalyan,
I've caught-up now. Just thinking: is "instanceof Cleaner" throwing
OOME as a result of loading the Cleaner class? Wouldn't the above
code then throw some error also in ((Cleaner)r) - the checkcast,
since Cleaner class would not be successfully initialized?
Well, no. The above code would just skip Cleaner processing in this
situation. And will never be doing it again after the heap is freed...
So it might be good to load and initialize Cleaner class as part of
ReferenceHandler initialization to ensure correct operation...
Well, yes and no. Let me try once more:
Above code will skip Cleaner processing if the 1st time "instanceof
Cleaner" is executed, OOME is thrown as a consequence of full heap
while
loading and initializing the Cleaner class.
Yes - I was assuming that this would not fail the very first time and
so the Cleaner class would already be loaded. Failing to be able to
load the Cleaner class was one of the potential issues flagged
earlier with this problem. I was actually assuming that Cleaner would
be loaded already due to some actual Cleaner subclasses being used,
but this does not happen as part of the default initialization. :(
The irony being that if the Cleaner class is not loaded then r can
not be an instance of Cleaner and so we would fail to load the class
in a case where we didn't need it anyway.
What I wanted to focus on here was an OOME from the instanceof
itself, but as you say that might trigger classloading of Cleaner
(which is not what I was interested in).
The 2nd time the "instanceof
Cleaner" is executed after such OOME, the same line would throw
NoClassDefFoundError as a consequence of referencing a class that
failed
initialization. Am I right?
instanceof is not one of the class initialization triggers, so we
should not see an OOME generated due to a class initialization
exception and so the class will not be put into the Erroneous state
and so subsequent attempts to use the class will not automatically
trigger NoClassdefFoundError.
If OOME occurs during actual loading/linking of the class Cleaner it
is unclear what would happen on subsequent attempts. OOME is not a
LinkageError that must be rethrown on subsequent attempts, and it is
potentially a transient condition, so I would expect a re-load
attempt to be allowed. However we are now deep into the details of
the VM and it may well depend on the exact place from which the OOME
originates.
The bottom line with the current problem is that there are multiple
non-obvious paths by which the ReferenceHandler can encounter an
OOME. In such cases we do not want the ReferenceHandler to terminate
- which implies catching the OOME and continuing. However we also do
not want to silently skip Cleaner processing or reference queue
processing - as that would lead to hard to diagnoze bugs. But trying
to report the problem may not be possible due to being out-of-memory.
It may be that we need to break things up into multiple try/catch
blocks, where each catch does a System.gc() and then reports that the
OOME occurred. Of course the reporting must still be in a try/catch
for the OOME. Though at some point letting the ReferenceHandler die
may be the only way to "report" a major memory problem.
David
Hm... If I give -verbose:class option to run a simple test program:
public class Test { public static void main(String... a) {} }
I see Cleaner class being loaded before Test class. I don't see by
which tread or if it might get loaded after main() starts, but I
suspect that loading of Cleaner is not a problem here. Initialization
of Cleaner class is not performed by ReferenceHandler thread as you
pointed out. The instanceof does not trigger it and if it returns true
then Cleaner has already been initialized. So there must be some other
cause for instanceof throwing OOME...
What do you say about this variant of ReferenceHandler.run() method:
public void run() {
for (;;) {
Reference r;
Cleaner c;
synchronized (lock) {
r = pending;
if (r != null) {
// instanceof operator might throw OOME
sometimes. Just retry after
// yielding - might have better luck next time...
try {
c = r instanceof Cleaner ? (Cleaner) r :
null;
} catch (OutOfMemoryError x) {
Thread.yield();
continue;
}
pending = r.discovered;
r.discovered = null;
} else {
// The waiting on the lock may cause an OOME
because it may try to allocate
// exception objects, so also catch OOME here
to avoid silent exit of the
// reference handler thread.
//
// Explicitly define the order of the two
exceptions we catch here
// when waiting for the lock.
//
// We do not want to try to potentially load
the InterruptedException class
// (which would be done if this was its first
use, and InterruptedException
// were checked first) in this situation.
//
// This may lead to the VM not ever trying to
load the InterruptedException
// class again.
try {
try {
lock.wait();
} catch (OutOfMemoryError x) { }
} catch (InterruptedException x) { }
continue;
}
}
// Fast path for cleaners
if (c != null) {
c.clean();
continue;
}
ReferenceQueue q = r.queue;
if (q != ReferenceQueue.NULL) q.enqueue(r);
}
}
... it tries to not consume and skip Cleaner instances when OOME is
caught.
I don't think ReferenceHandler is to make responsible for reporting
OOMEs. Full heap is a global condition and ReferenceHandler is the
last to accuse for it.
Regards, Peter
Hi David,
I think the following variation is even better. It executes
Thread.yield() after catching OOME but outside synchronized block so
that given CPU slice can be used by GC threads to make progress
enqueueing pending References (they are not able to enqueue them while
ReferenceHandler is holding the lock):
public void run() {
for (;;) {
Reference r;
Cleaner c;
try {
try {
synchronized (lock) {
r = pending;
if (r != null) {
// 'instanceof' might throw OOME
sometimes so do this before
// unlinking 'r' from the 'pending'
chain...
c = r instanceof Cleaner ? (Cleaner) r
: null;
// unlink 'r' from 'pending' chain
pending = r.discovered;
r.discovered = null;
} else {
// The waiting on the lock may cause an
OOME because it may try to allocate
// exception objects.
lock.wait();
continue;
}
}
} catch (OutOfMemoryError x) {
// Catch OOME from 'r instanceof Cleaner' or
'lock.wait()' 1st so that we don't
// try to potentially load the
InterruptedException class
// (which would be done if this was its first
use, and InterruptedException
// were checked first) in this situation.
// Give other threads CPU time so they
hopefully release some objects and GC
// clears some heap.
// Also prevent CPU intensive spinning in case
'r instanceof Cleaner' above
// persistently throws OOME for some time...
Thread.yield();
// retry
continue;
}
} catch (InterruptedException x) {
// Catch InterruptedException from 'lock.wait()'
and retry
continue;
}
// Fast path for cleaners
if (c != null) {
c.clean();
continue;
}
ReferenceQueue q = r.queue;
if (q != ReferenceQueue.NULL) q.enqueue(r);
}
}
Regards, Peter
