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) {

if (isCleaner) {

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.



Regards, Peter

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