pool.apply_async 为何是串行，并不是异步并行运行？

# from multiprocessing import Process,JoinableQueue,Manager,Queue,Semaphore,Pool,freeze_support
import multiprocessing
import threading

def prt(flag):
    for i in range(10):
        print('aaaa ' + flag)
        # time.sleep(2)

def test(count):
    t1=threading.Thread(target=prt('a'+ str(count)))
    t1.start()
    # t1.join()

    for i in range(10):
        print("-- " + str(i))
        # time.sleep(1)

if __name__ == '__main__':
    multiprocessing.freeze_support()

    pool = multiprocessing.Pool(2)
    # 不会异步执行
    pool.apply_async(test(1))
    pool.apply_async(test(2))
    # 异步执行
    # p=Process(target=test)
    # p.start()
    # 也不异步扫执行
    # t=threading.Thread(target=test())
    # t.start()

    print("main end...");