[foreign-memaccess+abi] RFR: 8270851: Logic for attaching/detaching native threads could be improved

[Sebastian Stenzel]

On Fri, 16 Jul 2021 15:59:18 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> For a complete description of the issue, please refer to:
> 
> https://bugs.openjdk.java.net/browse/JDK-8270851
> 
> This patch makes the logic for detaching native threads more lazy - by only doing the detach when a native thread has completed. This is achieved by using some thread local storage, which is used to keep track of the Java thread associated to a given native thread. If, by the time the thread local storage is destroyed, we see that a Java thread has been attached, we do a detach operation. This trick effectively minimizes the number of Thread instances created when interacting with multi-threaded native code.
> 
> This patch also tweaks the logic for attaching native threads to the VM by using the "daemon" attach variant. That is, native threads registered against the VM (because of Panama upcalls) should not prevent the JVM to shut down in an orderly fashion (in cases where the native threads might outlive the JVM).

I just checked out this branch and ran some tests and the results are great. Not only did performance in multithreaded tests increase significantly (outperforming JNR) and memory usage dropped to a fraction, it is also way easier to run profiling tools due to having longer living threads. ��

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PR: https://git.openjdk.java.net/panama-foreign/pull/570