Bug For SP scheduler

[JinJinGuang]

Describe the bug
Hello, I analyze the sp.py, and find a mismatch for different formats of priority, which may result in some bugs.

The input priorities is a list or a dict, with the following comment:

This can be either a list or a dictionary. If it is a list, it uses the flow_id
as its index to look for the flow's corresponding priority. If it is a dictionary,
it contains (flow_id -> priority) pairs for each possible flow_id.

But self.prio_queue_count has a different map:

if isinstance(priorities, list):
    self.prio_queue_count = [0 for __ in range(len(priorities))] # flow_id -> count
elif isinstance(priorities, dict):
    self.prio_queue_count = {prio: 0 for prio in priorities.values()} # priority -> count
else:
    raise ValueError(
        'Priorities must be either a list or a dictionary.')

Specifically, if priorities is a list, self.prio_queue_count will map flow_id to count. Otherwise, self.prio_queue_count will map the priority of the flow_id to count.

To Reproduce
You can run the following code and see a bug:

import numpy as np
import simpy
from ns.packet.dist_generator import DistPacketGenerator
from ns.packet.sink import PacketSink
from ns.scheduler.sp import SPServer

np.random.seed(42)

def arrival_1():
    """ Packets arrive with a constant interval of 1.5 seconds. """
    return 1.5


def arrival_2():
    """ Packets arrive with a constant interval of 2.0 seconds. """
    return 2.0

def packet_size():
    return int(np.random.exponential(100))

DEBUG = True

env = simpy.Environment()
sp = SPServer(env, 100, [1,100], debug=DEBUG) # line[1]
# sp = SPServer(env, 100, {0:10, 1:1}, debug=DEBUG) # line[2]
# sp = SPServer(env, 100, {0:1, 1:10}, debug=DEBUG) # line[3]
ps = PacketSink(env, rec_flow_ids=False, debug=DEBUG)

pg1 = DistPacketGenerator(env, "flow_1", arrival_1, packet_size, flow_id=0)
pg2 = DistPacketGenerator(env, "flow_2", arrival_2, packet_size, flow_id=1)

pg1.out = sp
pg2.out = sp
sp.out = ps

env.run(until=20)

The error:

Time 1.5, flow_id 0, packet_id 1
Sent out packet 1 of priority 1
Traceback (most recent call last):
  File "D:\anaconda3\lib\site-packages\ns\packet\dist_generator.py", line 80, in run
    self.out.put(packet)
  File "D:\anaconda3\lib\site-packages\ns\scheduler\sp.py", line 179, in put
    self.prio_queue_count[prio] += 1
IndexError: list index out of range

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "e:\Study\2021-summer\ns.py\examples\sp.py", line 36, in <module>
    env.run(until=20)
  File "D:\anaconda3\lib\site-packages\simpy\core.py", line 254, in run
    self.step()
  File "D:\anaconda3\lib\site-packages\simpy\core.py", line 206, in step
    raise exc
IndexError: list index out of range

Confusion
Moreover, I also want to know whether a larger number will result in a higher priority or a lower priority. Because, for the above test code, line[2] and line[3] act as the same: flow_0 has a high priority in both codes, regardless of the priority value.

[baochunli]

In the latest release (0.2.0) that should have fixed this issue, higher priority values imply higher priorities (which is the intuition).