SelectorManager threads are discriminated in race to get threads_lock mutex

[Kern, Joachim]

Hi guys,

The ContinuationFrameTest.java<https://github.com/openjdk/jdk/blob/master/test/jdk/java/net/httpclient/http2/ContinuationFrameTest.java> on AIX fails to end within the gracedelay period.
The gracedelay period function in ReferenceTracker.java <https://github.com/openjdk/jdk/blob/master/test/jdk/java/net/httpclient/ReferenceTracker.java#L270> effectively does the following:
for (i = 0; i < 50; i++) {
            if (!TRACKERS.stream().anyMatch(hasOutstanding))
                break;
            System.gc();
            Thread.sleep(10);
}

During this grace period there are at least 3 thread categories fighting for the threads_lock mutex.

1.       The SafepointSynchronize<http://ccwdfgl9733.glds.c.eu-de-1.cloud.sap:8080/source/s?defs=SafepointSynchronize&project=openjdk-jdk>::begin<http://ccwdfgl9733.glds.c.eu-de-1.cloud.sap:8080/source/s?refs=begin&project=openjdk-jdk>() locks the threads_lock mutex for the gc. It holds the mutex for about 300ms.

2.       The wait_until_not_protected() function, which is called a dozen times during one loop iteration. It holds the mutex for about 0.4ms.

3.       And the SelectorManager threads which need the mutex one time to complete their shutdown. They need the mutex for about 0.5ms.



The threads of all three categories are running with identical native thread priority .



On the various Linux derivates(x86 and ppc64) you can see, that the SelectorManager threads are finding slots between the other mutex consumers, so that all threads are shutdown within about 7 loop iterations.

On AIX the SelectorManager threads can’t make it or less often. So, some of them are still alive after the complete grace period, although they were queued up even before the grace period. This means in total 50 iterations where their requests were ignored and all later coming requests from 1.+2. were satisfied.



So, how are the thread priorities handled in hotspot. Why are the AIX SelectorManager threads discriminated.

Can you please help me with an idea.



Greetings

Joachim

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