threading priority

[Joe Wong]

Hi, Is there any way to increase/decrease a thread priority in Python?

Best regards,

-- Wong

 

[Peter Hansen]

Hi, Is there any way to increase/decrease a thread priority in Python?

Not really, and as Python does not use a "free-threading" model,
it wouldn't work as you'd like anyway.

Search Google for "Python GIL" or "Global Interpreter Lock" and you
should learn enough to see why this is so.

-Peter

 

[alecwy]

I googled as suggested, and the answer isn't crystal clear. My
impression is that the problem is that a python thread must acquire the
GIL in order to execute, and the strategy for deciding which thread
should get the GIL when multiple threads are waiting for it is not
based on priority. Is that correct?

-Alec

 

[Peter Hansen]

I googled as suggested, and the answer isn't crystal clear. My
impression is that the problem is that a python thread must acquire the
GIL in order to execute, and the strategy for deciding which thread
should get the GIL when multiple threads are waiting for it is not
based on priority. Is that correct?

That's basically correct. I don't actually know what
the strategy is, though I suspect it's either not
formally documented or explicitly not defined, though
for a given platform there may be some non-arbitrary
pattern...

Think of it this way: you have threads to which you have
given different priorities (somehow, using a platform-dependent
technique). The high priority thread runs first (for the
sake of this example only) in the interpreter, executes
a number of bytecode instructions equal to sys.getcheckinterval()
(miraculously without calling any extension modules or
blocking in any other way, again for the sake of this
example), and then has the GIL taken away from it by
the interpreter, thus blocking the thread. The interpreter
then gives the GIL to the lower priority thread (because
it isn't aware of the priorities assigned to the native
thread) and *it* runs for sys.getcheckinterval()
bytecode instructions, which might be a rather long
time. You have effectively no way to force the higher
priority thread to get the CPU whenever it is ready to
run, so long as the lower priority thread is also ready
to run. In effect, Python doesn't see the priorities at
all, and it is in almost sole control of which thread
runs when.

Or something like that. I'll rapidly be corrected if
I'm fundamentally wrong (and maybe even if I'm not ;-).

-Peter

 

[David Bolen]

That's basically correct. I don't actually know what
the strategy is, though I suspect it's either not
formally documented or explicitly not defined, though
for a given platform there may be some non-arbitrary
pattern...
(...)

I expect the Python interpreter has little to say over thread
prioritization and choice of execution, although it does impose some
granularity on the rate of switching. The GIL itself is implemented
on the lower layer lock implementation, which is taken from the native
threading implementation for the platform.

Therefore, when multiple Python threads are waiting for the GIL, which
one is going to get released will depend on when the underlying OS
satisfies the lock request from the threads, which should be based on
the OS thread scheduling system and have nothing to do with Python
per-se.

I do believe you are correct in that the Python GIL prevents thread
pre-emption by the OS (because all other Python threads are waiting on
the GIL and not in a running state), but the actual act of switching
threads at a switching point (sys.setcheckinterval()) would be an OS
only decision, and subject to whatever standard platform thread
scheduling rules were in place.

So if you were to use a platform specific method to control thread
priority, that method should be honored by the Python threads (subject
to the granularity of the system check interval for context switches).
For example, here's a Windows approach that fiddles with the thread
priority:

- - - - - - - - - - - - - - - - - - - - - - - - -
import threading
import ctypes
import time

w32 = ctypes.windll.kernel32

THREAD_SET_INFORMATION = 0x20
THREAD_PRIORITY_ABOVE_NORMAL = 1

class DummyThread(threading.Thread):

def __init__(self, begin, name, iterations):
threading.Thread.__init__(self)
self.begin = begin
self.tid = None
self.iterations = iterations
self.setName(name)

def setPriority(self, priority):
if not self.isAlive():
print 'Unable to set priority of stopped thread'

handle = w32.OpenThread(THREAD_SET_INFORMATION, False, self.tid)
result = w32.SetThreadPriority(handle, priority)
w32.CloseHandle(handle)
if not result:
print 'Failed to set priority of thread', w32.GetLastError()

def run(self):
self.tid = w32.GetCurrentThreadId()
name = self.getName()

self.begin.wait()
while self.iterations:
print name, 'running'
start = time.time()
while time.time() - start < 1:
pass
self.iterations -= 1


if __name__ == "__main__":

start = threading.Event()

normal = DummyThread(start, 'normal', 10)
high = DummyThread(start, 'high', 10)

normal.start()
high.start()

# XXX - This line adjusts priority - XXX
high.setPriority(THREAD_PRIORITY_ABOVE_NORMAL)

# Trigger thread execution
start.set()
- - - - - - - - - - - - - - - - - - - - - - - - -

And the results of running this with and without the setPriority call:

Without: With:

normal running high running
high running high running
normal running high running
high running high running
normal running normal running
high running high running
normal running high running
high running high running
normal running high running
high running normal running
normal running high running
high running high running
normal running normal running
high running normal running
normal running normal running
high running normal running
normal running normal running
high running normal running
normal running normal running
high running normal running


I'm not entirely positive why the normal thread gets occasionally
executed before the high thread is done. It might be that the
interpreter is actually releasing the GIL in the code I've written for
the thread's run() (maybe during the I/O) which opens up an
opportunity, or it may be that Windows is boosting the other thread
occasionally to avoid starvation. So I expect the normal thread is
getting occasional bursts of bytecode execution (the syscheckinterval).

But clearly the OS level prioritization is largely driving things.

-- David