[Cython] Acquisition counted cdef classes

Dag Sverre Seljebotn d.s.seljebotn at astro.uio.no
Wed Oct 26 12:30:11 CEST 2011

On 10/26/2011 12:29 PM, Dag Sverre Seljebotn wrote:
> On 10/26/2011 11:45 AM, mark florisson wrote:
>> On 26 October 2011 08:56, Stefan Behnel<stefan_ml at behnel.de> wrote:
>>> Greg Ewing, 26.10.2011 00:27:
>>>> Dag Sverre Seljebotn wrote:
>>>>> I'd gladly take a factor two (or even four) slowdown of CPython
>>>>> code any
>>>>> day to get rid of the GIL :-). The thing is, sometimes one has 48
>>>>> cores
>>>>> and consider a 10x speedup better than nothing...
>>>> Another thing to consider is that locking around refcount
>>>> changes may not be as expensive in typical Cython code as
>>>> it is in Python.
>>>> The trouble with Python is that you can't so much as scratch
>>>> your nose without touching a big pile of ref counts. But
>>>> if the Cython code is only dealing with a few Python objects
>>>> and doing most of its work at the C level, the relative
>>>> overhead of locking around refcount changes may not be
>>>> significant.
>>>> So it may be worth trying the strategy of just acquiring
>>>> the GIL whenever a refcount needs to be changed in a nogil
>>>> section, and damn the consequences.
>>> Hmm, interesting. That would give new semantics to "nogil" sections,
>>> basically:
>>> """
>>> You can do Python interaction in nogil code, however, this will slow
>>> down
>>> your code. Cython will generate C code to acquire and release the GIL
>>> around
>>> any Python interaction that your code performs, thus serialising any
>>> calls
>>> into the CPython runtime. If you want to avoid this serialisation, use
>>> "cython -a" to find out where Python interaction happens and use static
>>> typing to let Cython generate C code instead.
>>> """
>>> In other words: "with gil" sections hold the GIL by default and give
>>> it away
>>> on explicit request, whereas "nogil" sections have the GIL released by
>>> default and acquire it on implicit need.
>>> The advantage over object level locking is that this does not
>>> increase the
>>> in-memory size of the object structs, and that it works with *any*
>>> Python
>>> object, not just extension types with a compile time known type.
>>> I kind of like that.
>> My problem with that is that if there if any other python thread,
>> you're likely just going to sleep for thousands of CPU cycles as that
>> thread will keep the GIL. Doing this implicitly for operations with
>> such overhead would be unacceptable. I think writing 'with gil:' is
>> fine, it's the performance that's the problem in the first place which
>> prevents you from doing that, not the 9 characters you need to type.
> I'm with Stefan here. We have more or less the exact same problem if you
> inadvertendly do arithmetic with Python floats rather than C doubles.
> The workflow then is to check the HTML for yellow lines. Same with the
> GIL (we could even introduce a new color in the HTML report for where
> you hold the GIL and not).
> The advice to get fast code is

Sorry, I keep hitting post to early... "The advice to get fast code is 
still to 'eliminate the yellow lines'".


> But, we should also introduce directives that emit warnings in both of
> these situations, that you can use while developing to quickly pinpoint
> source code lines ("Type of variable not inferred", "GIL automatically
> acquired").
> DS
>> What I would like is having Cython infer whether the GIL is needed for
>> a function, and mark it "implicitly nogil", so it can be called from
>> nogil contexts without actually having to declare it nogil. This would
>> only work for non-extern things, and you would still need to declare
>> it nogil in your pxd if you want to export it. Apparently many users
>> (even those that have used Cython quite a bit) are confused with what
>> nogil on functions actually does (or they are not even aware it
>> exists).
>>> Stefan
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