global interpreter lock not working as it should

anton wilson anton.wilson at camotion.com
Tue Aug 6 00:10:41 CEST 2002


> >This is true. Therefore, the only time another thread WILL grab the GIL
> > under Linux is if
> >
> >1) the GIL is released by the currently running thread
> >2) the thread that just released the GIL depletes its timeslice before it
> >   can grab the lock again
> >3) the OS notices the process has depleted it's timeslice and the yanks
> >   it from the CPU (this happens every 100 times per second by default on
> > an i386)
> >4) the waiting thread that recieved the GIL release signal is chosen to
> > run
> >
> >Therefore, we now have a large set of coincidences for CPU-bound python
> >threads. The only reason it works at all is because it happens 100 times
> > per second and the GIL is released frequently by default. So there is a
> > sufficient probability that these 4 cases will happen simultaneously.
>
> Hm. If that's an accurate description (and I am skeptical ;-)

The reason I came to this conclusion because of several things.

1) a thread waiting on the wake-up condition from within the GIL acquire 
function really just calls a sys_sigsuspend system call.
   
   What then happens is that it sets its status to TASK_INTERRUPTIBLE and   	 
  calls the OS schedule function. This in turn notices that the current task  
  is TASK_INTERRUPTIBLE. It checks to see if any signals are pending. If      
  not, the OS removes the task from the run-queue. 

2) Once the thread is removed from the run-queue, only a signal delivery will 
push it back onto the run-queue. Sounds good, but a schedule call is only 
triggered under these circumstances (Linux 2.4.19 + O(1)):

       if (p->prio < rq->curr->prio || rq->curr->policy == SCHED_BATCH)
                        resched_task(rq->curr);

If the current process is a batch or if the current process has a lower 
priority than the woken up process. So, in our case, the priority would need 
to be lower to force a reschedule.

Normal sched_other processes get a boost for sleeping when they are put back 
on the run_queue with this code:

        unsigned long sleep_time = jiffies - p->sleep_timestamp;
        p->sleep_avg += sleep_time;

The timer tick happens at a frequency of 100 Hz and increases jiffies by one. 
Therefore, sleep_avg will only be less than jiffies every 10 millisecs. 
Python tries to give up the interpreter every 10 bytecodes which is at a much 
higher frequency: 7 - 15 microseconds on a regular case from my tests (could 
be higher). Therefore, you see that python will release and grab the GIL in 
worse case 1000 times before a reschedule is called due to a signal delivery, 
allowing the woken up thread to grab the GIL.


Anton




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