Jvmti GetThreadGroupChildren allocates more memory than necessary

[EricYangIBM]

GetThreadGroupChildren returns two arrays that may allocate more memory than needed.

openj9/runtime/jvmti/jvmtiThreadGroup.c

Lines 379 to 383 in 92da2c6

	/* The groups array may have unused space as weakGroup may not be added. */
	if (NULL != weakGroup) {
	groups[i] = (jthreadGroup)vmFuncs->j9jni_createLocalRef((JNIEnv *)currentThread, weakGroup);
	i += 1;
	}

openj9/runtime/jvmti/jvmtiThreadGroup.c

Lines 411 to 415 in 92da2c6

	/* The threads array will have unused space since many threads will not belong to the given threadgroup. */
	if (threadGroup == threadGroupObject) {
	*currentThreadPtr++ = (jthread)vmFuncs->j9jni_createLocalRef((JNIEnv *)currentThread, threadObject);
	nLiveThreads += 1;
	}

See #15539 (comment)

[babsingh]

Another place where more memory is used. Only live threads are stored in the array. But, the array size is based upon all threads in the threadGroupObject.

openj9/runtime/jvmti/jvmtiThreadGroup.c

Lines 295 to 308 in 92da2c6

	jint i = 0;
	j9object_t childrenThreads = (j9object_t)J9VMJAVALANGTHREADGROUP_CHILDRENTHREADS(currentThread, threadGroupObject);
	jint numLiveThreads = 0;
	/* Include only live threads in the result */
	numLiveThreads = 0;
	for (i = 0; i < numThreads; ++i) {
	j9object_t thread = J9JAVAARRAYOFOBJECT_LOAD(currentThread, childrenThreads, i);
	J9VMThread *targetThread = NULL;
	if (JVMTI_ERROR_NONE == getVMThread(currentThread, (jthread)&thread, &targetThread, FALSE, TRUE)) {
	threads[numLiveThreads++] = (jthread)vmFuncs->j9jni_createLocalRef((JNIEnv *)currentThread, thread);
	releaseVMThread(currentThread, targetThread);
	}

[babsingh]

Another location. Only live threads are stored in the array. But, the array size is based on vm->totalThreadCount.

openj9/runtime/jvmti/jvmtiThread.c

Lines 173 to 178 in 92da2c6

	/* Only count live threads */
	if (threadObject != NULL) {
	if (J9VMJAVALANGTHREAD_STARTED(currentThread, threadObject) && (J9VMJAVALANGTHREAD_THREADREF(currentThread, threadObject) != NULL)) {
	*currentThreadPtr++ = (jthread)vm->internalVMFunctions->j9jni_createLocalRef((JNIEnv *)currentThread, (j9object_t)threadObject);
	++threadCount;
	}

[babsingh]

More memory is used. Tradeoff for reducing memory usage will be more memory operations and processing time.