global interpreter lock（cpython）

同一时刻只有一个线程运行在一个cpu上执行字节码（无法将多个线程映射到多个cpu上）

import dis

def add(a):
 a = a + 1
 return a
print(dis.dis(add))

GIL在某些情况下会释放

每次的结果都不一样 线程之间的安全问题

GIL会根据执行的直接码行数或者时间片释放GIL

遇到IO操作时主动释放

total = 0

def add():
 #1. dosomething1
 #2. io操作
 # 1. dosomething3
 global total
 for i in range(1000000):
 total += 1
def desc():
 global total
 for i in range(1000000):
 total -= 1

import threading
thread1 = threading.Thread(target=add)
thread2 = threading.Thread(target=desc)
thread1.start()
thread2.start()

thread1.join()
thread2.join()
print(total)

多线程编程

操作系统能够调度的的最小单位是进程，因为进程对系统的资源消耗非常大，所以后期就演变成了线程，线程实际上是依赖于我们的进程（任务管理器中我们实际上能看到的其实是进程 ），操作系统能调度的最小单元是线程。

对于io操作为主的编程来说，多进程和多先产出的性能差别不大，甚至多线程比多进程的性能还高，因为多线程编程更加轻量级。

简单的线程

import time
from threading import Thread


def get_detail_html(url):
 print("get detail html started")
 time.sleep(2)
 print("get detail html end")


def get_detail_url(url):
 print("get detail url started")
 time.sleep(4)
 print("get detail url end")


if __name__ == '__main__':
 thread1 = Thread(target=get_detail_html, args=("",))
 thread2 = Thread(target=get_detail_url, args=("",))
 # 设置为守护线程 当主线程运行完时 子线程被kill掉
 thread1.setDaemon(True)
 thread2.setDaemon(True)
 start_time = time.time()
 thread1.start()
 thread2.start()

 # 设置为阻塞 等待线程运行完再关闭主线程
 thread1.join()
 thread2.join()
 # 默认情况下 主线程退出与时 子线程不会被kill掉
 print("last time: {}".format(time.time() - start_time))

重载线程实现多线程

import time
import threading

def get_detail_html(url):
 print("get detail html started")
 time.sleep(2)
 print("get detail html end")


def get_detail_url(url):
 print("get detail url started")
 time.sleep(4)
 print("get detail url end")


#2. 通过集成Thread来实现多线程
class GetDetailHtml(threading.Thread):
 def __init__(self, name):
 super().__init__(name=name)

 def run(self):
 print("get detail html started")
 time.sleep(2)
 print("get detail html end")


class GetDetailUrl(threading.Thread):
 def __init__(self, name):
 super().__init__(name=name)

 def run(self):
 print("get detail url started")
 time.sleep(4)
 print("get detail url end")

if __name__ == "__main__":
 thread1 = GetDetailHtml("get_detail_html")
 thread2 = GetDetailUrl("get_detail_url")
 start_time = time.time()
 thread1.start()
 thread2.start()

 thread1.join()
 thread2.join()

 #当主线程退出的时候， 子线程kill掉
 print ("last time: {}".format(time.time()-start_time))

多线程之间的通信

使用queue

# filename: thread_queue_test.py
# 通过queue的方式进行线程间同步
from queue import Queue
import time
import threading


def get_detail_html(queue):
 # 死循环 爬取文章详情页
 while True:
 url = queue.get()
 # for url in detail_url_list:
 print("get detail html started")
 time.sleep(2)
 print("get detail html end")


def get_detail_url(queue):
 # 死循环 爬取文章列表页
 while True:
 print("get detail url started")
 time.sleep(4)
 for i in range(20):
 # put 等到有空闲位置 再放入
 # put_nowait 非阻塞方式
 queue.put("http://projectsedu.com/{id}".format(id=i))
 print("get detail url end")


# 1. 线程通信方式- 共享变量
if __name__ == "__main__":
 detail_url_queue = Queue(maxsize=1000)

 thread_detail_url = threading.Thread(target=get_detail_url, args=(detail_url_queue,))
 for i in range(10):
 html_thread = threading.Thread(target=get_detail_html, args=(detail_url_queue,))
 html_thread.start()

 start_time = time.time()
 # 调用task_down从主线程退出
 detail_url_queue.task_done()
 # 从queue的角度阻塞
 detail_url_queue.join()

 print("last time: {}".format(time.time() - start_time))

线程的同步问题

在多线程编程中必须要面对的问题

无锁不安全的原因

# 没有锁
def add1(a):
 a += 1

def desc1(a):
 a -= 1

"""add
1. load a a = 0
2. load 1 1
3. + 1
4. 赋值给a a=1
"""

"""add
1. load a a = 0
2. load 1 1
3. - 1
4. 赋值给a a=-1
"""
import dis
print(dis.dis(add1))
print(dis.dis(desc1))

普通锁(Lock)

用锁会影响性能，锁会引起死锁（两次获取锁，获取锁之后不释放，互相等待（a需要b的资源 b需要a的资源））

import threading
from threading import Lock

total = 0
# 定义一把锁
lock = Lock()
def add():
 global total
 global lock
 for i in range(1000000):
 # 获取锁
 lock.acquire()
 total += 1
 # 释放锁
 lock.release()

def desc():
 global total
 for i in range(1000000):
 lock.acquire()
 total -= 1
 lock.release()

thread1 = threading.Thread(target=add)
thread2 = threading.Thread(target=desc)
thread1.start()
thread2.start()

thread1.join()
thread2.join()
print(total)

相互等待（资源竞争）

"""
A(a、b)
acquire (a)
acquire (b)

B(a、b)
acquire (b)
acquire (a)

# 解决办法
B(a、b)
acquire (a)
acquire (b)
"""

可重入锁（Rlock）

import threading
from threading import RLock

total = 0
# 可重入锁 可以在同一个线程中可载入多次
lock = RLock()
def add(lock):
 global total
 for i in range(1000000):
 # 获取锁
 lock.acquire()
 lock.acquire()
 total += 1
 do_something(lock)
 # 释放锁
 lock.release()
 lock.release()

def desc():
 global total
 for i in range(1000000):
 lock.acquire()
 total -= 1
 lock.release()

def do_something(lock):
 lock.acquire()
 # do something
 lock.release()

thread1 = threading.Thread(target=add)
thread2 = threading.Thread(target=desc)
thread1.start()
thread2.start()

thread1.join()
thread2.join()
print(total)

条件变量锁（condition）

用于复杂的线程间同步

# 没有条件锁 不能实现对话
import threading


class XiaoAi(threading.Thread):
 def __init__(self, lock):
 super().__init__(name="小爱")
 self.lock = lock

 def run(self):
 self.lock.acquire()
 print("{} : 在 ".format(self.name))
 self.lock.release()

 self.lock.acquire()
 print("{} : 好啊 ".format(self.name))
 self.lock.release()


class TianMao(threading.Thread):
 def __init__(self, lock):
 super().__init__(name="天猫精灵")
 self.lock = lock

 def run(self):
 self.lock.acquire()
 print("{} : 小爱同学 ".format(self.name))
 self.lock.release()

 self.lock.acquire()
 print("{} : 我们来对古诗吧 ".format(self.name))
 self.lock.release()


if __name__ == "__main__":
 cond = threading.Condition()
 xiaoai = XiaoAi(cond)
 tianmao = TianMao(cond)

 xiaoai.start()
 tianmao.start()

# 条件锁
import threading


class XiaoAi(threading.Thread):
 def __init__(self, cond):
 super().__init__(name="小爱")
 self.cond = cond

 def run(self):
 with self.cond:
 self.cond.wait()
 print("{} : 在 ".format(self.name))
 self.cond.notify()

 self.cond.wait()
 print("{} : 好啊 ".format(self.name))
 self.cond.notify()

 self.cond.wait()
 print("{} : 君住长江尾 ".format(self.name))
 self.cond.notify()

 self.cond.wait()
 print("{} : 共饮长江水 ".format(self.name))
 self.cond.notify()

 self.cond.wait()
 print("{} : 此恨何时已 ".format(self.name))
 self.cond.notify()

 self.cond.wait()
 print("{} : 定不负相思意 ".format(self.name))
 self.cond.notify()

class TianMao(threading.Thread):
 def __init__(self, cond):
 super().__init__(name="天猫精灵")
 self.cond = cond

 def run(self):
 with self.cond:
 print("{} : 小爱同学 ".format(self.name))
 self.cond.notify()
 self.cond.wait()

 print("{} : 我们来对古诗吧 ".format(self.name))
 self.cond.notify()
 self.cond.wait()

 print("{} : 我住长江头 ".format(self.name))
 self.cond.notify()
 self.cond.wait()

 print("{} : 日日思君不见君 ".format(self.name))
 self.cond.notify()
 self.cond.wait()

 print("{} : 此水几时休 ".format(self.name))
 self.cond.notify()
 self.cond.wait()

 print("{} : 只愿君心似我心 ".format(self.name))
 self.cond.notify()
 self.cond.wait()



if __name__ == "__main__":
 from concurrent import futures
 cond = threading.Condition()
 xiaoai = XiaoAi(cond)
 tianmao = TianMao(cond)

 # 启动顺序很重要
 # 在调用with cond之后才能调用wait或者notify方法
 # condition有两层锁， 一把底层锁会在线程调用了wait方法的时候释放，
 # 上面的锁会在每次调用wait的时候分配一把并放入到cond的等待队列中，
 # 等到notify方法的唤醒
 xiaoai.start()
 tianmao.start()