python - How to parallelize access to a class?

Question

I've implemented a simple direct Nbody simulation in python. I'm looking to parallelize it as we are doing the same operation again and again. In C++, I would have use openmp, but python doesn't have it.

So I was thinking to use the multiprocessing module. From what I understand, I would need a manager to manage the class (and the list particles?) and I was thinking of using a starmap pool.

I'm quite lost on how to use these function to achieve any semblance of parallelization, so any help is appreciated.

PS: I'm open to use other module too, the easier the better. The class is ditchable if using numpy array (for position velocity mass) solves the problem, I'll go with it.

Code:

import numpy as np
import matplotlib.pyplot as plt
import multiprocessing as mp


class particle:
    def __init__(self, xy, uv, m):
        self.xy =xy # position
        self.uv = uv # velocity
        self.m = m # mass
        self.force = np.zeros([2]) # at t=0s, force =0

    def update(self,dt):
        self.uv += self.force/self.m * dt
        self.xy += self.uv*dt
        self.force=np.zeros([2])

    def interaction(self,p,jj,eps):
        dr = p[jj].xy - self.xy
        dr_norm = np.linalg.norm(dr + eps)
        self.force += G*self.m*p[jj].m/(dr_norm**2) * dr/dr_norm
        p[jj].force -= G*self.m*p[jj].m/(dr_norm**2) * dr/dr_norm


def init_world(n_part):
    p=[]
    for ii in range(n_part):
        p.append(particle(np.random.uniform(0,50,size=(2))*1e15,np.random.uniform(-10,10,size=(2))*0,np.random.uniform(2,25)*1e28))
    return p

G = 6.67e-11 # in SI units
dt= 1e5 # in second, 86400s = one day
niter = 10000
n_part = 300
eps = 1e8 #softening to avoid infinite force a small distance

p = init_world(n_part)

xy = np.asarray([p[ii].xy for ii in range(n_part)])

fig, ax1 = plt.subplots()
im = ax1.scatter(xy[:,0],xy[:,1])
plt.show()

for tt in range(niter):
    for ii in range(n_part):
        for jj in range(ii+1,n_part):
            p[ii].interaction(p,jj,eps)

    for ii in range(n_part):
            p[ii].update(dt)

    xy = np.asarray([p[ii].xy for ii in range(n_part)])

    ax1.set_title(tt)
    im.set_offsets(xy)
    plt.pause(0.01)

question from:https://stackoverflow.com/questions/65929422/how-to-parallelize-access-to-a-class

Answer 1

If you want to share a list of custom objects (such as particle in the question) among processes, you can consider a simplified example here:

import multiprocessing
from multiprocessing.managers import BaseManager


TOTAL_PROCESS = 3

class Particle():
    def __init__(self, x, y):
        self.x = x
        self.y = y
        
    def multiply(self, z):
        self.x *= z
        self.y *= z

    def __repr__(self):
        return f'(x={self.x},y={self.y})'


def worker(sharedList, ix):
    # Call multiply() for the specific item in the list
    sharedList[ix].multiply(2)


def main():
    BaseManager.register('Particle', Particle)  # Register your custom class
    clsManager = BaseManager()  # A manager to manage Particle objects
    clsManager.start()
    manager = multiprocessing.Manager()  # Another manager to manage a shared list

    # Create a list of Particle objects
    sharedList = manager.list([clsManager.Particle(x, x+1) for x in range(0, TOTAL_PROCESS)])
    
    # See the origina list
    for x in sharedList:
        print(x, end=' ')
    else:
        print()

    # Run multiple processes and to make each of them them to work on a specific object only
    processes = []
    for i in range(TOTAL_PROCESS):
        p = multiprocessing.Process(target=worker, args=[sharedList, i])
        p.start()
        processes.append(p)

    for p in processes:
        p.join()
    
    # See the updated list of Pariticle objects
    for x in sharedList:
        print(x, end=' ')
    else:
        print()
    
if __name__ == '__main__':
    main()