David Beazley <d...@dabeaz.com> added the comment: I've also attached a new file schedtest.py that illustrates a subtle difference between having the GIL monitor thread and not having the monitor.
Without the monitor, every thread is responsible for its own scheduling. If you have a lot of threads running, you may have a lot of threads all performing a timed wait and then waking up only to find that the GIL is locked and that they have to go back to waiting. One side effect is that certain threads have a tendency to starve. For example, if you run the schedtest.py with the original GIL, you get a trace where three CPU-bound threads run like this: Thread-3 16632 Thread-2 16517 Thread-1 31669 Thread-2 16610 Thread-1 16256 Thread-2 16445 Thread-1 16643 Thread-2 16331 Thread-1 16494 Thread-3 16399 Thread-1 17090 Thread-1 20860 Thread-3 16306 Thread-1 19684 Thread-3 16258 Thread-1 16669 Thread-3 16515 Thread-1 16381 Thread-3 16600 Thread-1 16477 Thread-3 16507 Thread-1 16740 Thread-3 16626 Thread-1 16564 Thread-3 15954 Thread-2 16727 ... You will observe that Threads 1 and 2 alternate, but Thread 3 starves. Then at some point, Threads 1 and 3 alternate, but Thread 2 starves. By having a separate GIL monitor, threads are no longer responsible for making scheduling decisions concerning timeouts. Instead, the monitor is what times out and yanks threads off the GIL. If you run the same test with the GIL monitor, you get scheduling like this: Thread-1 33278 Thread-2 32278 Thread-3 31981 Thread-1 33760 Thread-2 32385 Thread-3 32019 Thread-1 32700 Thread-2 32085 Thread-3 32248 Thread-1 31630 Thread-2 32200 Thread-3 32054 Thread-1 32721 Thread-2 32659 Thread-3 34150 Threads nicely cycle round-robin. There also appears to be about half as much thread switching (for reasons I don't quite understand). ---------- Added file: http://bugs.python.org/file17095/schedtest.py _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue7946> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
