Unpreempted behavior with sys.setcheckinterval

[k3xji]

Hi all,

I want unpreempted behavior for some application and do some testing
as below. Well the unpreemption behavior is working fine with
sys.setcheckinterval(sys.maxint). However, when I set the interval to
a lower value, the thread does not being preempted anymore, it runs
until it is finished. The output of the below program is :

Thread 1 is printing out 2000 lines and then Thread 2 prints 2000
lines subsequently.

Here is the code:

import sys
import threading

class B(threading.Thread):
    def __init__(self, tid):
        threading.Thread.__init__(self)
        self.cnt = 0
        self.tid = tid
    def run(self):
        # open file as 'a' to append the line
        f = open('preempt_output', 'a')
        f.write('Thread '+str(self.tid)+'is starting...\n')
        SetUnpreemptable(True)
        while(1):
            self.cnt += 1
            f.write('Thread '+str(self.tid)+':'+str(self.cnt) +'\n')
            if self.cnt == 200:
                SetUnpreemptable(False)
            if self.cnt == 2000:
                break

        f.close()

def SetUnpreemptable(b):
    try:
        if b:
            sys.setcheckinterval(sys.maxint)
        else:
            raise
    except:
        sys.setcheckinterval(dflt_sysinterval)

if __name__ == "__main__":
    dflt_sysinterval = sys.getcheckinterval()


    thrd3 = B(1)
    thrd4 = B(2)
    thrd3.start()
    thrd4.start()