Pythonについて学ぶマルチプロセッシング（からPMOTWの記事) であり、このメソッドが具体的に何を行っているのかを明確にしていただければ幸いですjoin()。
で2008年の古いチュートリアルp.join()以下のコードで呼び出しを行わないと、「子プロセスはアイドル状態のまま終了せず、手動で強制終了する必要があるゾンビになる」と記載されています。
from multiprocessing import Process
def say_hello(name='world'):
print "Hello, %s" % name
p = Process(target=say_hello)
p.start()
p.join()
PIDテストのために とのプリントアウトを追加しましたtime.sleepが、私の知る限り、プロセスは自動的に終了します。
from multiprocessing import Process
import sys
import time
def say_hello(name='world'):
print "Hello, %s" % name
print 'Starting:', p.name, p.pid
sys.stdout.flush()
print 'Exiting :', p.name, p.pid
sys.stdout.flush()
time.sleep(20)
p = Process(target=say_hello)
p.start()
# no p.join()
20秒以内に:
936 ttys000 0:00.05 /Library/Frameworks/Python.framework/Versions/2.7/Reso
938 ttys000 0:00.00 /Library/Frameworks/Python.framework/Versions/2.7/Reso
947 ttys001 0:00.13 -bash
after 20 seconds:
947 ttys001 0:00.13 -bash
Behavior is the same with p.join() added back at end of the file. Python Module of the Week offers a very readable explanation of the module; "To wait until a process has completed its work and exited, use the join() method.", but it seems like at least OS X was doing that anyway.
Am also wondering about the name of the method. Is the .join() method concatenating anything here? Is it concatenating a process with it's end? Or does it just share a name with Python's native .join() method?
ベストアンサー1
The join() method, when used with threading or multiprocessing, is not related to str.join() - it's not actually concatenating anything together. Rather, it just means "wait for this [thread/process] to complete". The name join is used because the multiprocessing module's API is meant to look as similar to the threading module's API, and the threading module uses join for its Thread object. Using the term join to mean "wait for a thread to complete" is common across many programming languages, so Python just adopted it as well.
Now, the reason you see the 20 second delay both with and without the call to join() is because by default, when the main process is ready to exit, it will implicitly call join() on all running multiprocessing.Process instances. This isn't as clearly stated in the multiprocessing docs as it should be, but it is mentioned in the Programming Guidelines section:
Remember also that non-daemonic processes will be automatically be joined.
You can override this behavior by setting the daemon flag on the Process to True prior to starting the process:
p = Process(target=say_hello)
p.daemon = True
p.start()
# Both parent and child will exit here, since the main process has completed.
If you do that, the child process will be terminated as soon as the main process completes:
daemon
The process’s daemon flag, a Boolean value. This must be set before start() is called.
The initial value is inherited from the creating process.
When a process exits, it attempts to terminate all of its daemonic child processes.