Synchronization between JNI & GC

[Jungwoo Ha]

Thanks a lot! It really helped! --Jungwoo


On Wed, Apr 2, 2014 at 2:40 PM, Krystal Mok <rednaxelafx at gmail.com> wrote:

> Hi Jungwoo,
>
> To my knowledge, the only way JNI synchronizes with GC in HotSpot is by
> the means of the GC locker and safepoint checks upon entry to VM.
>
> When JNI code enters a critical section (e.g. after
> calling GetPrimitiveArrayCritical() but before calling
> ReleasePrimitiveArrayCritical()), HotSpot will activate the GC locker,
> meaning it does not allow a GC at the moment. If a GC is requested when the
> GC locker is active, HotSpot will try to expand the heap to fulfill the
> allocation request instead of doing a GC.
> When JNI exits all critical sections, the GC locker is inactivated. If
> there had been a GC request when the GC locker was active, then a major GC
> will be started at GC locker inactivation time.
> -XX:+GCLockerInvokesConcurrent overrides that behavior to start a
> concurrent GC cycle instead of a full GC.
>
> In the general case, JNI synchronizes with the GC by safepoints. When JNI
> calls into the VM (via JNI Invocation API) or returns to the VM, there are
> safepoint checks at the entry point, so that if a GC safepoint is active,
> it shouldn't run past the safepoint until GC completes. That's how the
> assert you mentioned could work.
>
> BTW, Universe::heap()->is_gc_active() only checks if there's a
> stop-the-world GC activity happening right now. It doesn't care about the
> concurrent phases.
>
> Hope it helps,
> Kris
>
>
> On Wed, Apr 2, 2014 at 2:00 PM, Jungwoo Ha <jwha at google.com> wrote:
>
>> Is there a good write up on how the synchronization happen between JNI
>> and GC?
>> I am looking at the NewGlobalRef code but it has assert statement like
>> this:
>>
>> assert(!Universe::heap()->is_gc_active(), "can't extend the root set
>> during GC");
>>
>> But how is this enforced by the JNI code calling NewGlobalRef?
>>
>> Thanks,
>> Jungwoo
>>
>
>
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