[Gary Robinson]
> In the application we're writing (http://www.goombah.com) it would be
> helpful for us to give one thread a higher priority than the others. We
> tried the recipe here:
> http://groups-beta.google.com/group/comp.lang.python/msg/6f0e118227a5f5de
> and it didn't seem to work for us.
If you need to fiddle thread priorities, then you'll need to do such
platform-specific things, and fight with the OS documentation to
figure out when and why they don't seem to work.
> We don't need many priority levels. We just need one thread to
> *temporarily* have a higher priority than others.
>
> One thing that occurred to me: There wouldn't by any chance be some way
> a thread could grab the GIL and not let it go until it is ready to do
> so explicitly?
No, unless that thread is running in a C extension and never calls
back into Python until it's willing to yield.
> That would have the potential to solve our problem.
>
> Or maybe there's another way to temporarily let one thread have
> priority over all the others?
No way in Python, although specific platforms may claim to support
relevant gimmicks in their native threads (and Python threads
generally are native platform threads).
A portable approach needs to rely on thread features Python supports
on all platforms. For example, maybe this crude approach is good
enough:
"""
import threading
class Influencer(object):
def __init__(self):
self.mut = threading.Lock()
self.proceed = threading.Event()
self.proceed.set()
self.active = None
def grab(self):
self.mut.acquire()
self.active = threading.currentThread()
self.proceed.clear()
self.mut.release()
def release(self):
self.mut.acquire()
self.active = None
self.proceed.set()
self.mut.release()
def maybe_wait(self):
if (self.proceed.isSet() or
self.active is threading.currentThread()):
pass
else:
self.proceed.wait()
"""
The idea is that all threads see an (shared) instance of Influencer,
and call its .maybe_wait() method from time to time. Normally that
doesn't do anything. If a thread T gets it into its head that it's
more important than other threads, it calls .grab(), which returns at
once. Other threads then block at their next call to .maybe_wait(),
until (if ever) some thread calls .release(). Season to taste; e.g.,
maybe other threads are willing to pause no more than 0.1 second; etc.
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