threading.RLock not subclassible?

Rick Lee rwklee at home.com
Fri Feb 9 02:02:43 CET 2001


Thanks for your explanations and tips.  I found the thread behaviour on the Mac
extremely perplexing.

Regarding your comment on what you think may be the unsafe and unpredictable
code: the code does access a couple of global lists, the only things done with
these lists are len() and append(), which I read somewhere are "atomic".

David Bolen wrote:

> Rick Lee <rwklee at home.com> writes:
>
> > "A multithreaded program executes by dividing its processing time
> > between all active threads.  For example, a program with 10 active
> > threads of execution would allow approximately 1/10 of its CPU time
> > to each thread and cycle between threads in rapid succession."
> >
> > I certainly was not seeing this behaviour on some platforms, even
> > for threads that take a long time to perform computations.
>
> Well, that may be a different issue than your posted point about the
> active thread count.  It is possible that depending on the local
> thread implementation (at the process level, OS kernel level,
> pre-emptive versus cooperative, etc...) that you may get better or
> worse sharing.  Really the only bad case is if the threads are
> cooperative (that is, a thread has to yield before the scheduler moves
> to another thread).  I'm not sure which if any platforms fall into
> that category (perhaps someone else can answer for the typical Unix
> pthreads implementation).
>
> How were you measuring the division of labor amongst the threads?
>
> > - what is the thread switching performance penalty, and how sensitive is the
> > penalty to the number of threads?
>
> There's a hit to the Python interpreter to support threading (I think
> I recall seeing something like 10%) in general, but actually switching
> between the threads should be quite low overhead.  Certainly when
> threads are native to the platform's OS (such as with Windows NT), the
> system scheduler really only thinks in terms of threads anyway, so
> thread context switches are just the normal mode of dispatching.
>
> It should also be fairly insensitive to the number of threads,
> providing overall system resources are not exceeded.
>
> > - is there an upper limit to the number of threads?
>
> That depends on the platform - you'll definitely run out of resources
> eventually, but it's sort of like asking what's the upper limit on the
> number of processes on the system, which can vary.
>
> It also depends on what they are doing, since you can likely support
> far more threads if the majority are blocked, then if you are trying
> to share the CPU among them all as active threads.
>
> Certainly I would expect that hundreds should not be out of the
> question, and probably thousands.  My desktop NT machine just with a
> normal mix of stuff active sits with 42 processes and about 250
> threads, and I expect it could easily handle an order of magnitude
> higher - as long as they weren't all trying to run simultaneously.
>
> But I don't have a hard limit for you, nor do I personally know of
> such values across all platforms.  It's a per-platform resource issue
> and is related to the activity of the code as well, so I'm not sure
> there's a fixed answer.  Certainly if you're thinking in terms of
> thousands you may want to find a way to share them - but if you're
> thinking 10s or hundreds, you're probably ok.
>
> > Regarding what happens on the Mac: I have a multi-threaded program
> > with several threads, each of which blocks without timeout on
> > sockets.accept, sockets.recv, and queues.get respectively.  This
> > program runs perfectly on NT and Linux, but the blocked threads on
> > the Mac don't seem to execute when the blocking condition is removed
> > (I am 99% sure that's what's happening).  If anyone can shed light
> > on this, I will be very grateful.
>
> By blocking condition removed, do you mean that data shows up on the
> socket or queues?  I really can't think why if such data were to
> arrive that the system calls wouldn't be satisfied.  I don't see the
> Mac implementation in the CVS tree I've currently got checked out, so
> I'm not sure how the Mac threading is implemented - perhaps someone
> else could chime in with some info.
>
> > Another weirdness includes a bit of code like this, which is a
> > recursive call on itself to create threads:
>
> Well, my first inclination is that the code itself appears dangerous -
> and unpredictable - because you don't have any protection against
> simultaneous access to the shared objects (e.g., runners).  But it's
> hard to say without seeing more of the surrounding code.  But for
> example, it would appear that by the time you get around to appending
> "me" to your completedRunObj, it's probably been changed by a new
> thread.
>
> Personally, if you're trying to observe behavior of threads, I find
> that a few judicial print statements can help out a lot.  Yes, if your
> problems have a time component, the additional time to do the output
> can interfere, but for basic scheduling (what thread ran when) it can
> be enlightening to just print the names of threads and points of their
> execution all in sequence.  If you keep your output as a single string
> to the "print" statement, it won't get split amongst different threads
> either, which can be a problem sometimes with this approach.
>
> --
> -- David
> --
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