MemorySegments allocated from shared Arena from java.lang.foreign.Arena.ofShared() have their lifecycle controlled by jdk.internal.foreign.SharedSession. This class ensures that the MemorySegments can't be freed until after a thread has called Arena.close().
This is implemented using a counter that is atomically incremented when used, and decremented when not used, on every invocation of a downcall. While shared Arenas allow any thread to use it and to close it, this tracking has a cost when multiple threads are contended on it.
This patch changes the implementation to use multiple counters to reduce contention. sun.nio.ch.IOUtil, java.nio.Buffer and sun.nio.ch.SimpleAsynchronousFileChannelImpl are modified as they have threads releasing the scope different from the ones that allocated them, so a ticket that tracks the counter has to be passed over.


The microbenchmark org.openjdk.bench.java.lang.foreign.CallOverheadConstant.panama_identity_memory_address_shared_3
was used to generate results. The scalability was checked on a number of platforms with the JMH parameter "-t" specifying the number of threads.

Results have been attached to results.txt.