Hi, As part of the proposal to enable frame pointers by default in Fedora (https://fedoraproject.org/wiki/Changes/fno-omit-frame-pointer), we did some benchmarking to figure out the expected performance impact. The performance impact was generally minimal, except for the pyperformance benchmark suite where we noticed a more substantial difference between a system built with frame pointers and a system built without frame pointers. The results can be found here: https://github.com/DaanDeMeyer/fpbench (look at the mean difference column for the pyperformance results where the percentage is the slowdown compared to a system built without frame pointers). One of the biggest slowdowns was on the scimark_sparse_mat_mult benchmark which slowed down 9.5% when the system (including python) was built with frame pointers. Note that these benchmarks were run against Python 3.11 on a Fedora 37 x86_64 system (one built with frame pointers, another built without frame pointers). The system used to run the benchmarks was an Amazon EC2 machine.
We did look a bit into the reasons behind this slowdown. I'll quote the investigation by Andrii on the Fesco issue thread here (https://pagure.io/fesco/issue/2817): > So I did look a bit at Python with and without frame pointers trying to > understand pyperformance > regressions. > First, perf data suggests that big chunk of CPU is spent in > _PyEval_EvalFrameDefault, > so I looked specifically into it (also we had to use DWARF mode for perf for > apples-to-apples > comparison, and a bunch of stack traces weren't symbolized properly, which > just again > reminds why having frame pointers is important). > perf annotation of _PyEval_EvalFrameDefault didn't show any obvious hot > spots, the work > seemed to be distributed pretty similarly with or without frame pointers. > Also scrolling through > _PyEval_EvalFrameDefault disassembly also showed that instruction patterns > between fp > and no-fp versions are very similar. > But just a few interesting observations. > The size of _PyEval_EvalFrameDefault function specifically (and all the other > functions didn't > change much in that regard) increased very significantly from 46104 to 53592 > bytes, which is a > considerable 15% increase. Looking deeper, I believe it's all due to more > stack spills and > reloads due to one less register available to keep local variables in > registers instead of on the stack. > Looking at _PyEval_EvalFrameDefault C code, it is a humongous one function > with gigantic switch > statement that implements Python instruction handling logic. So the function > itself is big and it has > a lot of local state in different branches, which to me explained why there > is so much stack spill/load. > Grepping for instruction of the form mov -0xf0(%rbp),%rcx or mov > 0x50(%rsp),%r10 (and their reverse > variants), I see that there is a substantial amount of stack spill/load in > _PyEval_EvalFrameDefault > disassembly already in default no frame pointer variant (1870 out of 11181 > total instructions in that > function, 16.7%), and it just increases further in frame pointer version > (2341 out of 11733 instructions, 20%). > One more interesting observation. With no frame pointers, GCC generates stack > accesses using %rsp > with small positive offsets, which results in pretty compact binary > instruction representation, e.g.: > 0x00000000001cce40 <+44160>: 4c 8b 54 24 50 mov 0x50(%rsp),%r10 > This uses 5 bytes. But if frame pointers are enabled, GCC switches to using > %rbp-relative offsets, > which are all negative. And that seems to result in much bigger instructions, > taking now 7 bytes instead of 5: > 0x00000000001d3969 <+53065>: 48 8b 8d 10 ff ff ff mov -0xf0(%rbp),%rcx > I found it pretty interesting. I'd imagine GCC should be capable to keep > using %rsp addressing just fine > regardless of %rbp and save on instruction sizes, but apparently it doesn't. > Not sure why. But this instruction > increase, coupled with increase of number of spills/reloads, actually > explains huge increase in byte size of > _PyEval_EvalFrameDefault: (2341 - 1870) * 7 + 1870 * 2 = 7037 (2 extra bytes > for existing 1870 instructions > that were switched from %rsp+positive offset to %rbp + negative offset, plus > 7 bytes for each of new 471 instructions). > I'm no compiler expert, but it would be nice for someone from GCC community > to check this as well (please CC > relevant folks, if you know them). > In summary, to put it bluntly, there is just more work to do for CPU > saving/restoring state to/from stack. But I don't > think _PyEval_EvalFrameDefault example is typical of how application code is > written, nor is it, generally speaking, > a good idea to do so much within single gigantic function. So I believe it's > more of an outlier than a typical case. We have a few questions: - Is this slowdown when Python is built with frame pointers to be expected? Has the Python community done any of their own experiments with building Python with and without frame pointers? - Is there anything we can do to fix the slowdown when Python is built with frame pointers? - Should we expect any change in benchmark results if we benchmark against Python 3.12? Supposedly there are changes in Python 3.12 related to frame pointers so we're wondering if those changes might affect these results in any way. Cheers, Daan De Meyer _______________________________________________ Python-Dev mailing list -- python-dev@python.org To unsubscribe send an email to python-dev-le...@python.org https://mail.python.org/mailman3/lists/python-dev.python.org/ Message archived at https://mail.python.org/archives/list/python-dev@python.org/message/LVRUY7KAJ5I532NHMDWJIS5H4HXSGBWD/ Code of Conduct: http://python.org/psf/codeofconduct/
