目錄

• 一、同步鎖
  • 1.1 多個(gè)線程搶占資源的情況
    • 1.1.1 對(duì)公共數(shù)據(jù)的操作
  • 1.2 同步鎖的引用
  • 1.3 互斥鎖與join的區(qū)別
• 二、死鎖與遞歸鎖
  • 2.1 死鎖
  • 2.2 遞歸鎖RLock
• 三、典型問(wèn)題：科學(xué)家吃面
  • 3.1 死鎖問(wèn)題
  • 3.2 遞歸鎖解決死鎖問(wèn)題

一、同步鎖

1.1 多個(gè)線程搶占資源的情況

from threading import Thread import os,time def work():global ntemp=ntime.sleep(0.1)n=temp-1 if __name__ == '__main__':n=100l=[]for i in range(100):p=Thread(target=work)l.append(p)p.start()for p in l:p.join()print(n) #結(jié)果可能為99

1.1.1 對(duì)公共數(shù)據(jù)的操作

import threading R=threading.Lock() R.acquire() ''' 對(duì)公共數(shù)據(jù)的操作 ''' R.release()

1.2 同步鎖的引用

from threading import Thread,Lock import os,time def work():global nlock.acquire()temp=ntime.sleep(0.1)n=temp-1lock.release() if __name__ == '__main__':lock=Lock()n=100l=[]for i in range(100):p=Thread(target=work)l.append(p)p.start()for p in l:p.join()print(n) #結(jié)果肯定為0，由原來(lái)的并發(fā)執(zhí)行變成串行，犧牲了執(zhí)行效率保證了數(shù)據(jù)安全

1.3 互斥鎖與join的區(qū)別

#不加鎖:并發(fā)執(zhí)行,速度快,數(shù)據(jù)不安全 from threading import current_thread,Thread,Lock import os,time def task():global nprint('%s is running' %current_thread().getName())temp=ntime.sleep(0.5)n=temp-1if __name__ == '__main__':n=100lock=Lock()threads=[]start_time=time.time()for i in range(100):t=Thread(target=task)threads.append(t)t.start()for t in threads:t.join()stop_time=time.time()print('主:%s n:%s' %(stop_time-start_time,n))''' Thread-1 is running Thread-2 is running ...... Thread-100 is running 主:0.5216062068939209 n:99 '''#不加鎖:未加鎖部分并發(fā)執(zhí)行,加鎖部分串行執(zhí)行,速度慢,數(shù)據(jù)安全 from threading import current_thread,Thread,Lock import os,time def task():#未加鎖的代碼并發(fā)運(yùn)行time.sleep(3)print('%s start to run' %current_thread().getName())global n#加鎖的代碼串行運(yùn)行l(wèi)ock.acquire()temp=ntime.sleep(0.5)n=temp-1lock.release()if __name__ == '__main__':n=100lock=Lock()threads=[]start_time=time.time()for i in range(100):t=Thread(target=task)threads.append(t)t.start()for t in threads:t.join()stop_time=time.time()print('主:%s n:%s' %(stop_time-start_time,n))''' Thread-1 is running Thread-2 is running ...... Thread-100 is running 主:53.294203758239746 n:0 '''# 有的同學(xué)可能有疑問(wèn):既然加鎖會(huì)讓運(yùn)行變成串行,那么我在start之后立即使用join,就不用加鎖了啊,也是串行的效果啊# 沒(méi)錯(cuò):在start之后立刻使用jion,肯定會(huì)將100個(gè)任務(wù)的執(zhí)行變成串行,毫無(wú)疑問(wèn),最終n的結(jié)果也肯定是0,是安全的,但問(wèn)題是# start后立即join:任務(wù)內(nèi)的所有代碼都是串行執(zhí)行的,而加鎖,只是加鎖的部分即修改共享數(shù)據(jù)的部分是串行的# 單從保證數(shù)據(jù)安全方面,二者都可以實(shí)現(xiàn),但很明顯是加鎖的效率更高. from threading import current_thread,Thread,Lock import os,time def task():time.sleep(3)print('%s start to run' %current_thread().getName())global ntemp=ntime.sleep(0.5)n=temp-1if __name__ == '__main__':n=100lock=Lock()start_time=time.time()for i in range(100):t=Thread(target=task)t.start()t.join()stop_time=time.time()print('主:%s n:%s' %(stop_time-start_time,n))''' Thread-1 start to run Thread-2 start to run ...... Thread-100 start to run 主:350.6937336921692 n:0 #耗時(shí)是多么的恐怖 '''）

二、死鎖與遞歸鎖

進(jìn)程也有死鎖與遞歸鎖，在進(jìn)程那里忘記說(shuō)了，放到這里一起說(shuō)了。

所謂死鎖：是指兩個(gè)或兩個(gè)以上的進(jìn)程或線程在執(zhí)行過(guò)程中，因爭(zhēng)奪資源而造成的一種互相等待的現(xiàn)象，若無(wú)外力作用，它們都將無(wú)法推進(jìn)下去。此時(shí)稱系統(tǒng)處于死鎖狀態(tài)或系統(tǒng)產(chǎn)生了死鎖，這些永遠(yuǎn)在互相等待的進(jìn)程稱為死鎖進(jìn)程，如下就是死鎖

2.1 死鎖

from threading import Lock as Lock import time mutexA=Lock() mutexA.acquire() mutexA.acquire() print(123) mutexA.release() mutexA.release()

解決方法：遞歸鎖，在Python中為了支持在同一線程中多次請(qǐng)求同一資源，python提供了可重入鎖RLock。

這個(gè)RLock內(nèi)部維護(hù)著一個(gè)Lock和一個(gè)counter變量，counter記錄了acquire的次數(shù)，從而使得資源可以被多次require。直到一個(gè)線程所有的acquire都被release，其他的線程才能獲得資源。上面的例子如果使用RLock代替Lock，則不會(huì)發(fā)生死鎖。

2.2 遞歸鎖RLock

from threading import RLock as Lock import time mutexA=Lock() mutexA.acquire() mutexA.acquire() print(123) mutexA.release() mutexA.release()

三、典型問(wèn)題：科學(xué)家吃面

3.1 死鎖問(wèn)題

import time from threading import Thread,Lock noodle_lock = Lock() fork_lock = Lock() def eat1(name):noodle_lock.acquire()print('%s 搶到了面條'%name)fork_lock.acquire()print('%s 搶到了叉子'%name)print('%s 吃面'%name)fork_lock.release()noodle_lock.release()def eat2(name):fork_lock.acquire()print('%s 搶到了叉子' % name)time.sleep(1)noodle_lock.acquire()print('%s 搶到了面條' % name)print('%s 吃面' % name)noodle_lock.release()fork_lock.release()for name in ['哪吒','nick','tank']:t1 = Thread(target=eat1,args=(name,))t2 = Thread(target=eat2,args=(name,))t1.start()t2.start()

3.2 遞歸鎖解決死鎖問(wèn)題

import time from threading import Thread,RLock fork_lock = noodle_lock = RLock() def eat1(name):noodle_lock.acquire()print('%s 搶到了面條'%name)fork_lock.acquire()print('%s 搶到了叉子'%name)print('%s 吃面'%name)fork_lock.release()noodle_lock.release()def eat2(name):fork_lock.acquire()print('%s 搶到了叉子' % name)time.sleep(1)noodle_lock.acquire()print('%s 搶到了面條' % name)print('%s 吃面' % name)noodle_lock.release()fork_lock.release()for name in ['哪吒','nick','tank']:t1 = Thread(target=eat1,args=(name,))t2 = Thread(target=eat2,args=(name,))t1.start()t2.start()

轉(zhuǎn)載于:https://www.cnblogs.com/nickchen121/p/11141739.html

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