[Cython] AddTraceback() slows down generators

[Stefan Behnel]

Stefan Behnel, 26.01.2012 21:57:
> Vitja Makarov, 26.01.2012 21:19:
>> 2012/1/27 Stefan Behnel:
>>> Robert Bradshaw, 21.01.2012 23:09:
>>>> On Sat, Jan 21, 2012 at 10:50 AM, Stefan Behnel wrote:
>>>>> I did some callgrind profiling on Cython's generators and was surprised to
>>>>> find that AddTraceback() represents a serious performance penalty for short
>>>>> running generators.
>>>>>
>>>>> I profiled a compiled Python implementation of itertools.groupby(), which
>>>>> yields (key, group) tuples where the group is an iterator again. I ran this
>>>>> code in Python for benchmarking:
>>>>>
>>>>> """
>>>>> L = sorted(range(1000)*5)
>>>>>
>>>>> all(list(g) for k,g in groupby(L))
>>>>> """
>>>>>
>>>>> Groups tend to be rather short in real code, often just one or a couple of
>>>>> items, so unpacking the group iterator into a list will usually be a quick
>>>>> loop and then the generator raises StopIteration on termination and builds
>>>>> a traceback for it. According to callgrind (which, I should note, tends to
>>>>> overestimate the amount of time spent in memory allocation), the iteration
>>>>> during the group unpacking takes about 30% of the overall runtime of the
>>>>> all() loop, and the AddTraceback() call at the end of each group traversal
>>>>> takes up to 25% (!) on my side. That means that more than 80% of the group
>>>>> unpacking time goes into raising StopIteration from the generators. I
>>>>> attached the call graph with the relative timings.
>>>>>
>>>>> About half of the exception raising time is eaten by PyString_FromFormat()
>>>>> that builds the function-name + line-position string (which, I may note, is
>>>>> basically a convenience feature). This string is a constant for a
>>>>> generator's StopIteration exception, at least for each final return point
>>>>> in a generator, but here it is being recreated over and over again, for
>>>>> each exception that gets raised.
>>>>>
>>>>> Even if we keep creating a new frame instance each time (which should be ok
>>>>> because CPython has a frame instance cache already and we'd only create one
>>>>> during the generator lifetime), the whole code object could actually be
>>>>> cached after the first creation, preferably bound to the lifetime of the
>>>>> generator creator function/method. Or, more generally, one code object per
>>>>> generator termination point, which will be a single point in the majority
>>>>> of cases. For the specific code above, that should shave off almost 20% of
>>>>> the overall runtime of the all() loop.
>>
>> I think that could be easily fixed. CPython doesn't add any traceback
>> info for generator's ending.
>>
>> https://github.com/vitek/cython/commit/63620bc2a29f3064bbdf7a49eefffaae4e3c369d
> 
> Interesting. It doesn't solve the general problem of slow exceptions, but I
> think that's a beautiful way of fixing this particular performance problem
> for generators.

One thing that it doesn't fix is when a generator gets its input from
another iterator and terminates automatically by passing on the
StopIteration that the iterator raises. I.e. any exception *propagation* is
still substantially slower than necessary, and that's a general issue.

Stefan