def pymysql_test():
    connection = pymysql.connect(
        'database', 
        'webim_service',
         'password'
    )
    try:
        with connection.cursor() as cur:
            cur.execute('large select')
            cur.fetchall()
    finally:
        connection.close()

    res = None
    connection = None
    cur = None

    return None

def loop():
    q = queue.Queue()
    q.put(pymysql_test)
    q.put(pymysql_test)
    q.put(pymysql_test)
    q.put(pymysql_test)
    q.put(pymysql_test)
    q.put(pymysql_test)
    q.put(pymysql_test)
    q.put(pymysql_test)
    q.put(pymysql_test)
    q.put(pymysql_test)
    q.put(pymysql_test)
    q.put(pymysql_test)
    q.put(pymysql_test)
    q.put(pymysql_test)
    q.put(pymysql_test)
    q.put(pymysql_test)
    q.put(pymysql_test)
    q.put(pymysql_test)

    while True:
        target = q.get()
        print('run target')
        target()
        time.sleep(0.1)

    z = threading.Thread(target=loop)
    z.start()