python simpy memory usage with large numbers of objects/processes

Question

I am using simpy to create a DES with a very large numbers of objects (many millions). I am running into memory issues and have being trying to figure out how to address this. It is possible to work out which objects will not undergo anymore interactions with other processes and so I can delete these objects from the simulation in theory freeing up memory. I created the below test this.

import psutil as ps
import simpy
import random


class MemoryUse(object):
    """a class used to output memory usage at various times within the sim"""

    def __init__(self, env, input_dict):

        self.env = env
        self.input_dict = input_dict

        self.env.process(self.before())
        self.env.process(self.during())
        self.env.process(self.after_sr())
        self.env.process(self.after())

    def before(self):

        yield self.env.timeout(0)
        print("full object list and memory events at time: ", self.env.now, " ", ps.virtual_memory())
        print(len(self.input_dict), len(self.env._queue))

    def during(self):
        yield self.env.timeout(2)
        print("full object list and events ar time: ", self.env.now, " ", ps.virtual_memory())
        print(len(self.input_dict), len(self.env._queue))

    def after_sr(self):

        yield self.env.timeout(4)
        print("reduced object list and reduced events at time: ", self.env.now, " ", ps.virtual_memory())
        print(len(self.input_dict), len(self.env._queue))

    def after(self):
        yield self.env.timeout(6)
        print("no objects and no events at time: ", self.env.now, " ", ps.virtual_memory())
        print(len(self.input_dict), len(self.env._queue))


class ExObj(object):
    """a generic object"""

    def __init__(self, env, id, input_dict):

        self.env = env
        self.id = id
        self.input_dict = input_dict

        if random.randint(0, 100) < 70:
            # set as SR
            self.timeout = 2
        else:
            self.timeout = 4

    def action(self):

        yield self.env.timeout(self.timeout)

        del self.input_dict[self.id]


class StartObj(object):
    """this enables me to create the obj events after the sim has started so as to measure memory usage before the events
    associated with the object exists"""

    def __init__(self, env, input_dict):

        self.env = env
        self.input_dict = input_dict

        self.env.process(self.start_obj())

    def start_obj(self):

        yield self.env.timeout(1)

        for k, v in self.input_dict.items():
            self.env.process(v.action())

        yield self.env.timeout(0)

# memory usage before we do anything
print("before all: ", ps.virtual_memory())

# create simpy env
env = simpy.Environment()
obj_dict = {}

# create memory calculation events
memory = MemoryUse(env, obj_dict)

# create objects
for i in range(2500000):
    obj_dict[i] = ExObj(env, i, obj_dict)


# create process that will itself start events associated with the objects
start = StartObj(env, obj_dict)

# run
env.run()

# clear the dict if not already clear
for j in range(2500000):
    obj_dict.clear()

# final memory check
print("after all: ", ps.virtual_memory())
print(len(obj_dict))

I was expecting memory usage to drop by time 4, as many objects have been removed and processes completed (around 70%). However memory usage appears to stay the same (See below). Why is this so? What is using this memory? Do completed processes stay in the simulation?

before all:  svmem(total=42195423232, available=39684155392, percent=6.0, used=2246373376, free=38884859904, active=2390749184, inactive=441712640, buffers=263155712, cached=801034240, shared=28721152)
full object list and memory events at time:  0   svmem(total=42195423232, available=38834251776, percent=8.0, used=3096276992, free=38035181568, active=3241959424, inactive=441466880, buffers=263159808, cached=800804864, shared=28721152)
2500000 4
full object list and events ar time:  2   svmem(total=42195423232, available=35121584128, percent=16.8, used=6808891392, free=34322219008, active=6947561472, inactive=441761792, buffers=263163904, cached=801148928, shared=28774400)
2500000 2500002
reduced object list and reduced events at time:  4   svmem(total=42195423232, available=35120973824, percent=16.8, used=6809530368, free=34321600512, active=6948368384, inactive=441737216, buffers=263168000, cached=801124352, shared=28745728)
767416 767417
no objects and no events at time:  6   svmem(total=42195423232, available=38448134144, percent=8.9, used=3482365952, free=37648760832, active=3627053056, inactive=441733120, buffers=263172096, cached=801124352, shared=28745728)
0 0
after all:  svmem(total=42195423232, available=38825793536, percent=8.0, used=3104706560, free=38026420224, active=3250180096, inactive=441733120, buffers=263172096, cached=801124352, shared=28745728)
0

Process finished with exit code 0