[Cython] AddTraceback() slows down generators

Robert Bradshaw robertwb at math.washington.edu
Sat Jan 21 23:09:40 CET 2012

On Sat, Jan 21, 2012 at 10:50 AM, Stefan Behnel <stefan_ml at behnel.de> wrote:
> Hi,
> 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's totally worth doing.

Makes sense to me. I did some caching like this for profiling.

- Robert

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