OpenJDK 17: Loitering AbstractQueuedSynchronizer$ConditionNode instances

[Thomas Schatzl]

Hi,

   since this looks like a suggestion for a change to the libraries 
similar to the mentioned JDK-6805775, and not actually GC, cc'ing the 
core-libs-dev mailing list.

Hth,
   Thomas

On 07.02.24 15:20, Frank Kretschmer wrote:
> Hi Java GC-experts,
> 
> I'm facing an interesting G1 garbage collector observation in OpenJDK
> 17.0.9+9, which I would like to share with you.
> 
> My application runs many asynchronous tasks in a fixed thread pool,
> utilizing its standard LinkedBlockingQueue. Usually, it generates just a
> little garbage, but from time to time, I observed that the survivor
> spaces grow unexpectedly, and minor collection times increase.
> 
> This being the case, many
> java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionNode
> instances can be found on the heap. In fact, the whole heap (rank 1 as
> shown in jmap) was filled up with ConditionNode instances after a while.
> 
> After some tests, I figured out that G1 seems to be able to collect
> “dead” ConditionNode instances during minor collections only if no
> formerly alive ConditionNode instances were promoted to the old
> generation and died there.
> 
> To illustrate that, I've attached a “G1LoiteringConditionNodes” class
> that can be run for demo purposes, e.g. under Linux with OpenJDK
> 17.0.9+9 (VM options see comments within the class), and its gc-log
> output. It shows that during the first two minutes, everything is fine,
> but after a promotion to the old generation, survivors grow and minor
> pause time increase from 3 to 10ms.
> 
> For me, it looks like an issue similar to
> https://bugs.openjdk.org/browse/JDK-6805775 “LinkedBlockingQueue Nodes
> should unlink themselves before becoming garbage”, which was fixed in
> OpenJDK 7.
> 
> What’s your opinion about that? Wouldn’t it be worth to enable G1 to
> collect those AbstractQueuedSynchronizer$ConditionNode instances during
> minor collections, as it is done for LinkedBlockingQueue Nodes?
> 
> Best regards
> 
> Frank Kretschmer, Java Realtime Application Developer