[issue39521] reversed(mylist) much slower on Python 3.8 32-bit for Windows

[STINNER Victor]

STINNER Victor <vstinner at python.org> added the comment:

I close this issue. It's likely just a hiccup in the PGO compilation. It's not the thing that we can easily control. The good thing is that the common code path iter(list) is efficient ;-)


> The code for listiter_next() and listreviter_next() is almost the same. 

Right. It cannot explain a 2x slowdown.


> python -m timeit -s "a = list(range(1000))" "list(iter(a))"
> 50000 loops, best of 5: 5.73 usec per loop

It means around 5.73 ns per iteration. This is almost "nothing": just a few CPU cycles. For such microbenchmark, you are very close to the bare metal. You have to take in account CPU low-level metrics like usage of the CPU caches.


> Another possible cause is that this is just a random build outcome due to PGO or incidental branch mis-prediction from aliasing (as described in https://stackoverflow.com/a/17906589/1001643 ).

If someone cares about such microbenchmark, I suggest to get access to a profiling tool and measure the CPU cache usage and other metrics like that. On Linux, I know the "perf" command which can be used. I don't know performance tooling on Windows. Maybe search in Intel developer tools.

I expect that list(iter(a)) better uses the CPU (cache? branch predictor?) than list(reversed(a)), because of how listiter_next() and listreviter_next() have been optimized.

Bad code placement has a high cost on performance on such microbenchmarks. See:

* https://llvmdevelopersmeetingbay2016.sched.org/event/8YzY/causes-of-performance-instability-due-to-code-placement-in-x86
* https://vstinner.github.io/analysis-python-performance-issue.html

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resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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<https://bugs.python.org/issue39521>
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