多线程mandelbrot集不会产生加速

Question

我使用下面的代码来生成Mandelbrot集分形，但是不管我使用多少线程，计算时间都不会改变（大约5秒）。我使用的是英特尔i5-4200M处理器，所以我应该得到不同的结果，至少4线程，但事实并非如此。我错过了什么？我想通过使用1个线程来计算加速比，并将其与使用更多线程进行比较，但我得到的差异不超过几毫秒。生成采用逃逸时间算法。你知道吗

# Multi-threaded Mandelbrot Fractal (Do not run using IDLE!)
# FB - 201104306
import threading
import math
import time
import sys
from PIL import Image

START_TIME = 0
w = 1080 # image width
h = 1080 # image height
image = Image.new("RGB", (w, h))
wh = w * h
maxIt = 256 # max number of iterations allowed
# drawing region (xa < xb & ya < yb)
xa = -2.0
xb = 1.0
ya = -1.5
yb = 1.5
xd = xb - xa
yd = yb - ya
numThr = 1 # number of threads to run
# lock = threading.Lock()

class ManFrThread(threading.Thread): 
    def __init__ (self, k):
          self.k = k
          threading.Thread.__init__(self)
    def run(self):
        # each thread only calculates its own share of pixels
        for i in range(k, wh, numThr):
            kx = i % w
            ky = int(i / w)
            a = xa + xd * kx / (w - 1.0)
            b = ya + yd * ky / (h - 1.0)
            x = a
            y = b
            for kc in range(maxIt):
                x0 = x * x - y * y + a #x0 = x * x - y * y + math.exp(-x) * (a * math.cos(y) + b * math.sin(y)) 
                y = 2.0 * x * y + b #y = 2.0 * x * y + math.exp(-x) * (b * math.cos(y) - a * math.sin(y)) 
                x = x0                
                if x * x + y * y > 4:
                    # various color palettes can be created here
                    red = (kc % 8) * 32
                    green = (16 - kc % 16) * 16
                    blue = (kc % 16) * 16
                    # lock.acquire()
                    global image
                    image.putpixel((kx, ky), (red, green, blue))
                    # lock.release()
                    break

if __name__ == "__main__":
    START_TIME = time.time()
    numThr = sys.argv[1]
    try:
        numThr = int(numThr)
    except:
        print("value must be int")

    tArr = []
    for k in range(numThr): # create all threads
        tArr.append(ManFrThread(k))
    for k in range(numThr): # start all threads
        tArr[k].start()
    for k in range(numThr): # wait until all threads finished
        tArr[k].join()
    image.save("MandelbrotFractal.png", "PNG")
    print("-*-*- %s seconds -*-*-" % (time.time() - START_TIME))