[Python-Dev] "Fixing" the new GIL

Tue Mar 16 21:09:39 CET 2010

The Linux kernel scheduler deals with two types of ratings and has a
heuristic algorithm that rewards and penalizes on a rapid basis.  To
determine the next thread it inspects the priority array to find the highest
priority task that is runnable and then selects the first task in that
priority.  This is a gross simplification of an extremely complex system,
but it is not simply a simple queue.

To duplicate this exact type of scheduling in the interpreter would be a
daunting task, but Dave's example illustrates that some system of reward and
penalty can be beneficial.

Peter has brought up the point, both here and at the Open Space at Pycon,
that before attempting a downsized version of the a priority thread
scheduler that we should look at other programs that have had to deal with
similar problems.

Peter, since you have dealt with the nitty-gritty of concurrency before, can
you suggest some applications that could serve as models for research?

On Tue, Mar 16, 2010 at 3:22 PM, Peter Portante
<peter.a.portante at gmail.com>wrote:

>  Yes, having another thread wait with a timeout is not good. CPU cycles
> waisted doing things that don’t help the thread holding the GIL release it.
> They just note that they are present and then the GIL-holding-thread should
> be responsible for handling the rest. -peter
>
>
>
> On 3/16/10 1:11 PM, "Nir Aides" <nir at winpdb.org> wrote:
>
> Hello Dave,
>
> The following documentation suggests ordering in Linux is not FIFO:
>
> http://www.opengroup.org/onlinepubs/000095399/functions/pthread_cond_timedwait.html#tag_03_518_08_06
> "Threads waiting on mutexes and condition variables are selected to
> proceed in an order dependent upon the scheduling policy rather than in some
> fixed order (for example, FIFO or priority). Thus, the scheduling policy
> determines which thread(s) are awakened and allowed to proceed."
>
> Here is the code:
>
> http://www.google.com/codesearch/p?hl=en#5ge3gHPB4K4/gnu/glibc/glibc-linuxthreads-2.1.1.tar.gz%7CaeB7Uqo7T9g/linuxthreads/queue.h&q=pthread_cond_timedwait&exact_package=http://ftp.gnu.org/gnu/glibc/glibc-linuxthreads-2.1.1.tar.gz
>
> If this is so then it should affect the proposed fixes.
> For example waiting with timeout should be avoided, no?
>
> Nir
>
> 2010/3/16 David Beazley <dave at dabeaz.com>
>
> Python doesn't use a pthreads mutex for the GIL.    It has always used a
> binary semaphore implemented with condition variables (or just a pthreads
> semaphore if available).    The reason the performance is so bad is
> precisely due to the fact that it is using this implementation and the fact
> that there *IS* a FIFO queue of threads (associated with the condition
> variable).   The I/O performance problem with the new GIL is gets much worse
> with many CPU-bound threads precisely because there is a FIFO queue
> involved.   This has been covered in my past GIL presentations.
>
> -Dave
>
>
>
> On Mar 16, 2010, at 5:52 AM, Kristján Valur Jónsson wrote:
>
> > How about attacking the original problem, then?
> >
> > The reason they thrash on pthreads implementation is that a pthreads
> mutex is assumed to be a short-held resource.  Therefore it will be
> optimized in the following ways for multicore machines:
> > 1) There is a certain amount of spinning done, to try to acquire it
> before blocking
> > 2) It will employ un-fair tactics to avoid lock-convoying, meaning that a
> thread coming in to acquire the mutex may get in before others that are
> queued.  This is why "ticking" the GIL works so badly:  The thread that
> releases the lock is usually the one that reaquires it even though others
> may be waiting.  See e.g.
> http://www.bluebytesoftware.com/blog/PermaLink,guid,e40c2675-43a3-410f-8f85-616ef7b031aa.aspxfor a discussion of this (albeit on windows.)
> >
> > On Windows, this isn't a problem.  The reason is, that the GIL on windows
> is implemented using Event objects that don't cut these corners.  The Event
> provides you with a strict FIFO queue of objects waiting for the event.
> >
> > If pthreads doesn't provide a synchronization primitive similar to that,
> someone that doesn't thrash and has a true FIFO queue, it is possible to
> construct such a thing using condition variables and critical sections.
>  Perhaps the posix semaphore api is more appropriate in this case.
> >
> > By the way, this also shows another problem with (old) python.  There is
> only one core locking primitive, the PyThread_type_lock.  It is being used
> both as a critical section in the traditional sense, and also as this
> sort-of-inverse lock that the GIL is.  In the modern world, where the
> intended behaviour of these is quite different, there is no one-size-fits
> all.  On windows in particular, the use of the Event object based lock is
> not ideal for other uses than the GIL.
> >
> >
> > In the new GIL, there appear to be several problems:
> > 1) There is no FIFO queue of threads wanting the queue, thus thread
> scheduling becomes non-deterministic
> > 2) The "ticking" of the GIL is now controled by a condition variable
> timeout.  There appears to be no way to prevent many such timeouts to be in
> progress at the same time, thus you may have an unnecessarily high rate of
> ticking going on.
> > 3) There isn't an immediate gil request made when an IO thread requests
> the gil back, only after an initial timeout.
> >
> > What we are trying to write here is a thread scheduler, and that is
> complex business.
> > K
> >
> >
> >
> >> -----Original Message-----
> >> From: python-dev-bounces+kristjan=ccpgames.com <http://ccpgames.com> @
> python.org <http://python.org>
> >> [mailto:python-dev-bounces+kristjan <python-dev-bounces+kristjan> <
> mailto:python-dev-bounces%2Bkristjan <python-dev-bounces%2Bkristjan>> =
> ccpgames.com <http://ccpgames.com> @python.org <http://python.org> ] On
> Behalf
> >> Of David Beazley
> >> Sent: 15. mars 2010 03:07
> >> To: python-dev at python.org
> >> Subject: Re: [Python-Dev] "Fixing" the new GIL
> >>
> >> happen to be performing CPU intensive work at the same time, it would
> >> be nice if they didn't thrash on multiple cores (the problem with the
> >> old GIL) and if I/O is
> >
>
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