out_queue.get() 阻塞，直到默认情况下有可用的结果。因此，您实际上是在启动一个流程并等待它完成，然后再开始下一个流程。相反，启动所有进程，然后获得所有结果。
例子：
#!python2
import multiprocessing as mp
from random import uniform, randrange
import time
def flop_no(rand_nos, a, b):
cals = []
for r in rand_nos:
cals.append(r + a * b)
return cals
def flop(val, a, b, out_queue):
cals = []
for v in val:
cals.append(v + a * b)
out_queue.put(cals)
# time.sleep(3)
def concurrency():
out_queue = mp.Queue()
a = 3.3
b = 4.4
rand_nos = [uniform(1, 4) for i in range(1000000)]
print len(rand_nos)
# for i in range(5):
start_time = time.time()
p1 = mp.Process(target=flop, args=(rand_nos[:250000], a, b, out_queue))
p2 = mp.Process(target=flop, args=(rand_nos[250000:500000], a, b, out_queue))
p3 = mp.Process(target=flop, args=(rand_nos[500000:750000], a, b, out_queue))
p4 = mp.Process(target=flop, args=(rand_nos[750000:], a, b, out_queue))
p1.start()
p2.start()
p3.start()
p4.start()
print len(out_queue.get())
print len(out_queue.get())
print len(out_queue.get())
print len(out_queue.get())
p1.join()
p2.join()
p3.join()
p4.join()
print "Running time parallel: ", time.time() - start_time, "secs"
def no_concurrency():
a = 3.3
b = 4.4
rand_nos = [uniform(1, 4) for i in range(1000000)]
start_time = time.time()
cals = flop_no(rand_nos, a, b)
print "Running time serial: ", time.time() - start_time, "secs"
if __name__ == '__main__':
concurrency()
no_concurrency()
# print "Program over"
Output:
1000000
250000
250000
250000
250000
Running time parallel: 3.54999995232 secs
Running time serial: 0.203000068665 secs
Note that parallel time is still slower. This is due to the overhead of starting 4 other Python processes. Your processing time for the whole job is only .2 seconds. The 3.5 seconds for parallel is mostly just starting up the processes. Note the commented out `# time.sleep(3)` above in `flop()`. Add that code in and the times are:
1000000
250000
250000
250000
250000
Running time parallel: 6.50900006294 secs
Running time serial: 0.203000068665 secs
The overall time only got 3 seconds faster (not 12) because they were running in parallel. You need a lot more data to make parallel processing worthwhile.
Here's a version where you can visually see how long it takes to start the processes. "here" is printed as each process begins to run `flop()`. An event is used to start all threads at the same time, and only the processing time is counted:
#!python2
import multiprocessing as mp
from random import uniform, randrange
import time
def flop_no(rand_nos, a, b):
cals = []
for r in rand_nos:
cals.append(r + a * b)
return cals
def flop(val, a, b, out_queue, start):
print 'here'
start.wait()
cals = []
for v in val:
cals.append(v + a * b)
out_queue.put(cals)
time.sleep(3)
def concurrency():
out_queue = mp.Queue()
start = mp.Event()
a = 3.3
b = 4.4
rand_nos = [uniform(1, 4) for i in range(1000000)]
print len(rand_nos)
# for i in range(5):
p1 = mp.Process(target=flop, args=(rand_nos[:250000], a, b, out_queue, start))
p2 = mp.Process(target=flop, args=(rand_nos[250000:500000], a, b, out_queue, start))
p3 = mp.Process(target=flop, args=(rand_nos[500000:750000], a, b, out_queue, start))
p4 = mp.Process(target=flop, args=(rand_nos[750000:], a, b, out_queue, start))
p1.start()
p2.start()
p3.start()
p4.start()
time.sleep(5) # Wait for processes to start. See Barrier in Python 3.2+ for a better solution.
print "go"
start.set()
start_time = time.time()
print len(out_queue.get())
print len(out_queue.get())
print len(out_queue.get())
print len(out_queue.get())
print "Running time parallel: ", time.time() - start_time, "secs"
p1.join()
p2.join()
p3.join()
p4.join()
def no_concurrency():
a = 3.3
b = 4.4
rand_nos = [uniform(1, 4) for i in range(1000000)]
start_time = time.time()
cals = flop_no(rand_nos, a, b)
print "Running time serial: ", time.time() - start_time, "secs"
if __name__ == '__main__':
concurrency()
no_concurrency()
# print "Program over"
输出：
1000000
here # note these print about a second apart.
here
here
here
go
250000
250000
250000
250000
Running time parallel: 0.171999931335 secs
Running time serial: 0.203000068665 secs
现在，处理时间变快了。不是很多......可能是由于进程间通信以获得结果。