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's totally worth doing. > > Makes sense to me. I did some caching like this for profiling.
Here's a ticket for now: http://trac.cython.org/cython_trac/ticket/760 Stefan _______________________________________________ cython-devel mailing list cython-devel@python.org http://mail.python.org/mailman/listinfo/cython-devel
