捂脸斋

unsigned int VM_Version::calc_parallel_worker_threads() {
  unsigned int result;
  if (is_M_series()) {
    // for now, use same gc thread calculation for M-series as for niagara-plus
    // in future, we may want to tweak parameters for nof_parallel_worker_thread
    result = nof_parallel_worker_threads(5, 16, 8);
  } else if (is_niagara_plus()) {
    result = nof_parallel_worker_threads(5, 16, 8);
  } else {
    result = nof_parallel_worker_threads(5, 8, 8);
  }
  return result;
}
unsigned int Abstract_VM_Version::parallel_worker_threads() {
  if (!_parallel_worker_threads_initialized) {
    if (FLAG_IS_DEFAULT(ParallelGCThreads)) {
      _parallel_worker_threads = VM_Version::calc_parallel_worker_threads();
    } else {
      _parallel_worker_threads = ParallelGCThreads;
    }
    _parallel_worker_threads_initialized = true;
  }
  return _parallel_worker_threads;
}
unsigned int Abstract_VM_Version::calc_parallel_worker_threads() {
  return nof_parallel_worker_threads(5, 8, 8);
}
unsigned int Abstract_VM_Version::nof_parallel_worker_threads(
  unsigned int num,
  unsigned int den,
  unsigned int switch_pt) {
  if (FLAG_IS_DEFAULT(ParallelGCThreads)) {
    assert(ParallelGCThreads == 0, "Default ParallelGCThreads is not 0");
    // For very large machines, there are diminishing returns
    // for large numbers of worker threads.  Instead of
    // hogging the whole system, use a fraction of the workers for every
    // processor after the first 8.  For example, on a 72 cpu machine
    // and a chosen fraction of 5/8
    // use 8 + (72 - 8) * (5/8) == 48 worker threads.
    unsigned int ncpus = (unsigned int) os::active_processor_count();
    return (ncpus <= switch_pt) ?
      ncpus : (switch_pt + ((ncpus - switch_pt) * num) / den);
  } else {
    return ParallelGCThreads;
  }
}

# 指定GC线程数量，启动JVM
java -XX:ParallelGCThreads=3 -jar thirdparty.jar

# 查看JVM启动参数
jcmd <processId> VM.flags

# 查看所有GC线程
jstack <processId> | grep GC

# 实时查看GC情况 intervalMS: 采样间隔，毫秒
jstat -gc <processId> <intervalMS>

public static void boostMem(int seconds, int threadCount) {
    for (int i = 0; i < threadCount; i++) {
        new Thread(() -> {
            List<Object> objects = new ArrayList<>();
            LocalDateTime endTime = LocalDateTime.now().plusSeconds(seconds);
            while (LocalDateTime.now().isBefore(endTime)) {
                objects.add(new TestObject());
            }
        }).start();
    }
}