ResourceExhausted JVMTI event and -XX:*OnOutOfMemoryError

[David Holmes]

On 23/07/2019 6:16 pm, Yasumasa Suenaga wrote:
> Sorry, the format is broken... I re-sent the email:
> 
> 
> There are not documented the relation between -XX:*OnOutOfMemoryError 
> and ResourceExhausted,
> but JVMTI spec says ResourceExhausted is sent when VM resource are 
> exhausted.
> 
> https://docs.oracle.com/en/java/javase/12/docs/specs/jvmti.html#ResourceExhausted 

The JVMTI spec can't say anything about -XX:*OnOutOfMemoryError as they 
are a hotspot implementation detail. If you use the -XX flag you get 
whatever the hotspot implementation does. That may override normal 
specified behaviour. For example, if you use -XX:+ExitOnOutOfMemoryError 
you can't then complain that the VM never executes a catch block for 
OutOfMemoryError because it exited first! It's the same for the event 
processing - we're exiting before we reach that code.

> As you said, we should not do other things when OutOfMemoryError is 
> occurred.
> But I think it should be compliant when we will use Java related code in 
> the event handler.
> If we do with C/C++ only (e.g. call printf()), I think we can do that.

But we don't know what the handler will do. Nor do we know why the -XX 
flag was used, or even what it may do in some cases.

> If all codes should not be run when OutOfMemoryError is occurred,
> ResourceExhausted should be obsoleted, or we need to add the note for 
> JVMTI document.

There is no logical argument there for obsoleting the ResourceExhausted 
event.

I understand you want the event to be posted before the flag is 
processed, but somebody else may want it exactly as it is - e.g. dumping 
the heap before their event handler does some cleanup/recovery work.

And as I said the JVM TI spec knows nothing of this XX flag so cannot 
say anything about it.

David
-----

> 
> Thanks,
> 
> Yasumasa
> 
> 
> On 2019/07/23 16:43, David Holmes wrote:
>> Hi Yasumasa,
>>
>> On 23/07/2019 5:24 pm, Yasumasa Suenaga wrote:
>>> Hi all,
>>>
>>> I tried to use JVMTI agent which hooks ResourceExhausted event and 
>>> -XX:ExitOnOutOfMemoryError together.
>>> The process was exited when OutOfMemoryError was occurred, but 
>>> ResourceExhausted event was not fired.
>>>
>>> I checked HotSpot implementation, -XX:*OutOfMemoryError is processed 
>>> before JVMTI event processing.
>>>
>>> http://hg.openjdk.java.net/jdk/jdk/file/d999a1a11485/src/hotspot/share/gc/shared/memAllocator.cpp#l125 
>>>
>>>
>>> IMHO ResourceExhausted should be called before -XX:*OutOfMemoryError.
>>> If it is a bug, I will file it to JBS and fix it.
>>
>> I don't think it is a bug. First, there's no specification for how 
>> these two things interact - -XX:*OnOutOfMemoryError is just a VM 
>> specific debugging/diagnostic hook. Second, if we're out of memory 
>> then we don't want to be doing other things which may need to also 
>> acquire memory. Third, no matter which way you code this someone may 
>> in some case want it the other way - and it's not worth coding to 
>> allow such flexibility IMO.
>>
>> Just my $2. :)
>>
>> Cheers,
>> David
>>
>>>
>>> Thanks,
>>>
>>> Yasumasa