我不明白为什么这个 MPI 代码中的 while 循环没有中断

Question

I'm doing a parallelization exercise using mpi4py where 2 dice are thrown a defined number of times (divided by the processes, ie, npp ) and the dots are counted. The results are stored in a dictionary, the mean deviation is calculated and until the condition of mean_dev being less than 0.001 the number of throws is doubled.

All of this works as expected, the problem is that the code doesn't quit. The condition is met, there's no more outputs, but the code hangs.

from ctypes.wintypes import SIZE
from dice import * #This is just a class that creates the dictionaries 
from random import randint
import matplotlib.pyplot as plt
import numpy as np
from mpi4py import MPI
from math import sqrt

def simulation(f_events, f_sides, f_n_dice):
    f_X = dice(sides, n_dice).myDice() #Nested dictionary composed of dices (last dict stores the sum)
    for j in range(f_events): #for loop to handle all the dice throwings aka events
        n = [] #List to store index respective to number on each dice
        for i in range(1, f_n_dice+1): #for cycle for each dice
            k = randint(1, f_sides) #Random number
            n.append(k)
            f_X[i][k] += 1 #The index (k) related to each throw is increased for the dice (i)
        sum_throw = sum(n) #Sum of the last throw
        f_X[f_n_dice+1][sum_throw] += 1 #Sum dictionary "increases" the index respective to the sum of the last throw
    return f_X

npp = int(4)//4 #Number of events divided by the number of processes
sides = 6 #Number of sides per dice
n_dice = 2 #Number of dices

comm = MPI.COMM_WORLD #Communicator to handle point-to-point communication
rank = comm.Get_rank() #Hierarchy of processes
size = comm.Get_size() #Number of processes

#-------------------- Parallelization portion of the code --------------------#

seq = (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12)
AUX = dict.fromkeys(seq, 0)
mean_dev = 1
while True:
    msg = comm.bcast(npp, root = 0)
    print("---> msg: ", msg, " for rank ", rank)
    print("The mean dev for %d" %rank + " is: ", mean_dev)

    D = simulation(npp, sides, n_dice)
    
    Dp = comm.gather(D, root = 0)
    print("This is Dp: ", Dp)
    
    summ = 0
    prob = [1/36, 2/36, 3/36, 4/36, 5/36, 6/36, 5/36, 4/36, 3/36, 2/36, 1/36]

    if rank==0:
        for p in range(0, size): 
                for n in range(dice().min, dice().max+1): #Range from minimum sum possible to the maximum sum possible depending on the number of dices used
                    AUX[n] += Dp[p][n_dice+1][n] #Adds the new data to the final sum dictionary 
                                                                #of the previously initiated nested dictionary
                print(Dp[p][n_dice+1])
    
        print("The final dictionary is: ", AUX, sum(AUX[j] for j in AUX))

        for i in range(dice().min, dice().max+1):
            exp = (prob[i-2])*(sum(AUX[j] for j in AUX))
            x = (AUX[i]-exp)/exp
            summ = summ + pow(x, 2)

        mean_dev = (1/11)*sqrt(summ)
        print("The deviation for {} is {}.".format(sum(AUX[j] for j in AUX), mean_dev))

    if mean_dev > 0.001:
        npp = 2*npp
        # new_msg = comm.bcast(npp, root = 0)
        # print("---> new_msg: ", new_msg, " for rank ", rank)
    else:
        break
        

I'm stumped on this one. Thanks in advance for any input!

---

The new code with the solution proposed by @victor-eijkhout:

from ctypes.wintypes import SIZE
from dice import *
from random import randint
import matplotlib.pyplot as plt
import numpy as np
from mpi4py import MPI
from math import sqrt

def simulation(f_events, f_sides, f_n_dice):
    f_X = dice(sides, n_dice).myDice() #Nested dictionary composed of dices (last dict stores the sum)
    for j in range(f_events): #for loop to handle all the dice throwings aka events
        n = [] #List to store index respective to number on each dice
        for i in range(1, f_n_dice+1): #for cycle for each dice
            k = randint(1, f_sides) #Random number
            n.append(k)
            f_X[i][k] += 1 #The index (k) related to each throw is increased for the dice (i)
        sum_throw = sum(n) #Sum of the last throw
        f_X[f_n_dice+1][sum_throw] += 1 #Sum dictionary "increases" the index respective to the sum of the last throw
    return f_X

npp = int(4)//4 #Number of events divided by the number of processes
sides = 6 #Number of sides per dice
n_dice = 2 #Number of dices

comm = MPI.COMM_WORLD #Communicator to handle point-to-point communication
rank = comm.Get_rank() #Hierarchy of processes
size = comm.Get_size() #Number of processes

#-------------------- Parallelization portion of the code --------------------#

seq = (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12)
AUX = dict.fromkeys(seq, 0)
mean_dev = 1
while True:
    msg = comm.bcast(npp, root = 0)
    #print("---> msg: ", msg, " for rank ", rank)
    
    D = simulation(npp, sides, n_dice)
        
    Dp = comm.gather(D, root = 0)
    #if Dp != None: print("This is Dp: ", Dp)

    
    #print("The mean dev for %d" %rank + " is: ", mean_dev)

    if rank==0:
        
        summ = 0
        prob = [1/36, 2/36, 3/36, 4/36, 5/36, 6/36, 5/36, 4/36, 3/36, 2/36, 1/36]

        for p in range(0, size): 
                for n in range(dice().min, dice().max+1): #Range from minimum sum possible to the maximum sum possible depending on the number of dices used
                    AUX[n] += Dp[p][n_dice+1][n] #Adds the new data to the final sum dictionary 
                                                                #of the previously initiated nested dictionary
                print(Dp[p][n_dice+1])
    
        print("The final dictionary is: ", AUX, sum(AUX[j] for j in AUX))

        for i in range(dice().min, dice().max+1):
            exp = (prob[i-2])*(sum(AUX[j] for j in AUX))
            x = (AUX[i]-exp)/exp
            summ = summ + pow(x, 2)

        mean_dev = (1/11)*sqrt(summ)
        print("The deviation for {} is {}.".format(sum(AUX[j] for j in AUX), mean_dev))

    #new_mean_dev = comm.gather(mean_dev, root = 0)
    new_mean_dev = comm.bcast(mean_dev, root = 0)
    print("---> msg2: ", new_mean_dev, " for rank ", rank)

    if new_mean_dev < 0.001:
        break
        # new_msg = comm.bcast(npp, root = 0)
        # print("---> new_msg: ", new_msg, " for rank ", rank)
        
    else:
        npp = 2*npp
        print("The new npp is: ", npp)

Answer 1

You are computing the mean deviation only on process zero, so that process will exit. However, the other processes do not get the value and so they never quit. You should broadcast the value after you compute it.

Answer 2

You are breaking out of your if statement. Just replace while True: with while mean_dev > 0.001: and you should be good. You can also just do an assignment at the end rather than wrapping it in the if .

If that doesn't work it simply means mean_dev is always greater than 0.001. You calculate mean_dev as (1/11)*sqrt(sum …) . Not under the whole algorithm, if the minimum sum of 2 dice is 2, then mean_dev will not drop below 0.14 or so. Try putting in a print statement and print out mean_dev each time through the loop and see if it's working as expected. Should you be dividing mean_dev by npp each time or something like that?

As a general rule, these kinds of problems where one is iterating to find a closer approximation generally terminate when the change in the estimate becomes very small. Should you be stopping when the change in mean_dev is less than 0.001? You would need to do something like abs(last_mean_dev-mean_dev)<0.001.