Reduce lock contention in Thread.getAndClearInterrupt

[babsingh]

Lock contention in Thread.getAndClearInterrupt is reduced by
acquiring the lock only when the value of the Thread.interrupted
field is true.

Related: eclipse-openj9/openj9#20414

[ThanHenderson]

In the equivalent VirtualThread.java method, the synchronized block is just within the if-statement:

openj9-openjdk-jdk21/src/java.base/share/classes/java/lang/VirtualThread.java

Lines 885 to 895 in cd40810

	boolean getAndClearInterrupt() {
	assert Thread.currentThread() == this;
	boolean oldValue = interrupted;
	if (oldValue) {
	synchronized (interruptLock) {
	interrupted = false;
	carrierThread.clearInterrupt();
	}
	}
	return oldValue;
	}

Is there a reason as to why that approach wouldn't work here?

[babsingh]

Functionally, no difference will be seen. In the case, where there is high contention i.e. multiple threads attempt to enter the synchronized block and clear the interrupt, only one thread will invoke the native method (Thread.interruptedImpl) due to the extra if statement. Otherwise, all threads that enter the synchronized block will invoke the native method; and this will lower the throughput when resources are scarce. At the cost of an extra if statement, we lower the invocations of the native method in the worst case to improve perf.

[ThanHenderson]

If there is such a performance benefit, we should likely apply this to the VirtualThread.java method too.

[babsingh]

Waiting for perf data. Will apply the change to VirtualThread.java after reviewing perf data.

[babsingh]

• Related lock contention is completely removed, and there is a 15% perf improvement.
• Also, VirtualThread.java has been updated.

[babsingh]

jenkins test sanity alinux64 jdk21

[babsingh]

jenkins test extended.system zlinux jdk21

[babsingh]

jenkins test extended.functional plinux jdk21

[babsingh]

• JDK23 port: Reduce lock contention in Thread.getAndClearInterrupt openj9-openjdk-jdk23#72
• JDK-next port: Reduce lock contention in Thread.getAndClearInterrupt openj9-openjdk-jdk#877

fyi @tajila