Parallel version of sorting algorithm faster / same as serial version?

Question

I'm trying to implement multithreading and multiprocessing on a sorting algorithm. The way implemented it is by:

• Initialize a list of 10k items

• Assign a random variable between 0-100 for each element

  arr = [1] * 10000
  for x, i in enumerate(arr):
      arr[x] = random.randint(0, 100)
  t_arr = arr
  m_arr = arr
  s_arr = arr
  

• Create 2 sub-lists -- one for values lower than or equal to 50 and one for the rest

I then used bubble sort for both sub-lists in parallel. One using threads and one using processes.

Theoretically both should be faster, but only multiprocessing is. Multithreading produces the same results.

I have already tried different sorting algos, problem is persistent.

# Threading Version
start_time = time.time()
subarr1 = []
subarr2 = []
# Split Array into 2
for i in t_arr:
    if i <= 50:
        subarr1.append(i)
    else:
        subarr2.append(i)
# Sort first array
t1 = threading.Thread(target=bubbleSort, args=(subarr1,))
# Sort second array
t2 = threading.Thread(target=bubbleSort, args=(subarr2,))
t1.start()
t2.start()
t1.join()
t2.join()
end_time = time.time() - start_time
print("--- %s seconds ---" % (end_time))


# Serial Version
start_time = time.time()
subarr1 = []
subarr2 = []
# Split Array into 2
for i in s_arr:
    if i <= 50:
        subarr1.append(i)
    else:
        subarr2.append(i)
# Sort first array
bubbleSort(subarr1)
# Sort second array
bubbleSort(subarr2)
end_time = time.time() - start_time
print("--- %s seconds ---" % (end_time))


# Multiprocessing Version
start_time = time.time()
subarr1 = []
subarr2 = []
# Split Array into 2
for i in s_arr:
    if i <= 50:
        subarr1.append(i)
    else:
        subarr2.append(i)
# Sort first array
p1 = multiprocessing.Process(target=bubbleSort, args=(subarr1,))
# Sort second array
p2 = multiprocessing.Process(target=bubbleSort, args=(subarr2,))
p1.start()
p2.start()
p1.join()
p2.join()
end_time = time.time() - start_time
print("--- %s seconds ---" % (end_time))

• Multithreading: around 6 seconds
• Serial: around 6 seconds (similar to Threading)
• Multprocessing: around 3 seconds

These results are consistent. Any advice?