How to control memory usage in multithreading?

I am using multi thread to process image.

It works fine on my computer that has enough memory (increase 2~3 GB when processing many images), but my server only has 1GB memory and the code not work properly.

Sometimes end with Segmentation fault, sometimes:

Exception in thread Thread-13:
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 810, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.7/threading.py", line 763, in run
    self.__target(*self.__args, **self.__kwargs)
  File "passportRecognizeNew.py", line 267, in doSomething
  ...

Code:

import threading

def doSomething(image):
    # picture processing code
    print("processing over")

threads = []

for i in range(20):
    thread = threading.Thread(target=doSomething, args=("image",))
    threads.append(thread)

for t in threads:
    t.setDaemon(True)
    t.start()

t.join()

print("All over")

How to solve this or any way to control memory usage?