1、问题:
群中有同学贴了如下一段代码,问为何 list 最后打印的是空值?
from multiprocessing import Process, Manager import os manager = Manager() vip_list = [] #vip_list = manager.list() def testFunc(cc): vip_list.append(cc) print 'process id:', os.getpid() if __name__ == '__main__': threads = [] for ll in range(10): t = Process(target=testFunc, args=(ll,)) t.daemon = True threads.append(t) for i in range(len(threads)): threads[i].start() for j in range(len(threads)): threads[j].join() print "------------------------" print 'process id:', os.getpid() print vip_list
其实如果你了解 python 的多线程模型,GIL 问题,然后了解多线程、多进程原理,上述问题不难回答,不过如果你不知道也没关系,跑一下上面的代码你就知道是什么问题了。
python aa.py process id: 632 process id: 635 process id: 637 process id: 633 process id: 636 process id: 634 process id: 639 process id: 638 process id: 641 process id: 640 ------------------------ process id: 619 []
将第 6 行注释开启,你会看到如下结果:
process id: 32074 process id: 32073 process id: 32072 process id: 32078 process id: 32076 process id: 32071 process id: 32077 process id: 32079 process id: 32075 process id: 32080 ------------------------ process id: 32066 [3, 2, 1, 7, 5, 0, 6, 8, 4, 9]
2、python 多进程共享变量的几种方式:
(1)Shared memory:
Data can be stored in a shared memory map using Value or Array. For example, the following code
http://docs.python.org/2/library/multiprocessing.html#sharing-state-between-processes
from multiprocessing import Process, Value, Array def f(n, a): n.value = 3.1415927 for i in range(len(a)): a[i] = -a[i] if __name__ == '__main__': num = Value('d', 0.0) arr = Array('i', range(10)) p = Process(target=f, args=(num, arr)) p.start() p.join() print num.value print arr[:]
结果:
3.1415927 [0, -1, -2, -3, -4, -5, -6, -7, -8, -9]
(2)Server process:
A manager object returned by Manager() controls a server process which holds Python objects and allows other processes to manipulate them using proxies.
A manager returned by Manager() will support types list, dict, Namespace, Lock, RLock, Semaphore, BoundedSemaphore, Condition, Event, Queue, Value and Array.
代码见开头的例子。
http://docs.python.org/2/library/multiprocessing.html#managers
3、多进程的问题远不止这么多:数据的同步
看段简单的代码:一个简单的计数器:
from multiprocessing import Process, Manager import os manager = Manager() sum = manager.Value('tmp', 0) def testFunc(cc): sum.value += cc if __name__ == '__main__': threads = [] for ll in range(100): t = Process(target=testFunc, args=(1,)) t.daemon = True threads.append(t) for i in range(len(threads)): threads[i].start() for j in range(len(threads)): threads[j].join() print "------------------------" print 'process id:', os.getpid() print sum.value
结果:
------------------------ process id: 17378 97
也许你会问:WTF?其实这个问题在多线程时代就存在了,只是在多进程时代又杯具重演了而已:Lock!
from multiprocessing import Process, Manager, Lock import os lock = Lock() manager = Manager() sum = manager.Value('tmp', 0) def testFunc(cc, lock): with lock: sum.value += cc if __name__ == '__main__': threads = [] for ll in range(100): t = Process(target=testFunc, args=(1, lock)) t.daemon = True threads.append(t) for i in range(len(threads)): threads[i].start() for j in range(len(threads)): threads[j].join() print "------------------------" print 'process id:', os.getpid() print sum.value
这段代码性能如何呢?跑跑看,或者加大循环次数试一下。。。
4、最后的建议:
Note that usually sharing data between processes may not be the best choice, because of all the synchronization issues; an approach involving actors exchanging messages is usually seen as a better choice. See also Python documentation: As mentioned above, when doing concurrent programming it is usually best to avoid using shared state as far as possible. This is particularly true when using multiple processes. However, if you really do need to use some shared data then multiprocessing provides a couple of ways of doing so.
5、Refer:
http://stackoverflow.com/questions/14124588/python-multiprocessing-shared-memory
http://eli.thegreenplace.net/2012/01/04/shared-counter-with-pythons-multiprocessing/
http://docs.python.org/2/library/multiprocessing.html#multiprocessing.sharedctypes.synchronized
免责声明:本站资源来自互联网收集,仅供用于学习和交流,请遵循相关法律法规,本站一切资源不代表本站立场,如有侵权、后门、不妥请联系本站删除!
更新日志
- 【原神手游】5.1版本更新维护预告
- 李锦华.1987-日与夜(LP版)【SOLO】【WAV+CUE】
- 伍珂玥.2014-伍珂玥【湾彩星辰】【FLAC分轨】
- 吕方.2001-旧情人·老情歌精选2CD【华纳】【WAV+CUE】
- 《星际战士2》¥219,《100% 鲜橙汁》¥7
- 手办鉴赏室:少女眼神颇有杀意!连衣裙勾勒傲人轮廓
- 未来可期的二次元大逃杀游戏——《白厄战境》简评
- 炉石传说死亡骑士卡组怎么搭配 死亡骑士最新卡组2024推荐
- 魔兽世界地下堡珍玩怎么升级 地下堡珍玩升级方法攻略分享
- 魔兽世界怎么获得泽克维尔的残血珍玩 泽克维尔的残血珍玩获取方式
- FrankSinatra-StrangersInTheNight(1966Jazz)[Flac24-96]
- 大原樱子《CAMON+5thAnniversaryBest~》2CD[24-96]flac
- Isgaard2003-GoldenKey(LimitedEdition)【WAV+CUE】
- 《合金装备3RE》伊娃定妆照公布:没被政确太好了
- 石家庄新增《黑神话:悟空》墙绘:小狐狸萍萍