Why does numpy.sin return a different result if the argument size is greater than 8192?

Question

I discovered that numpy.sin behaves differently when the argument size is <= 8192 and when it is > 8192. The difference is in both performance and values ret

Answer 1

As @WarrenWeckesser wrote, "it's almost certainly an Anaconda & Intel MKL issue; cf. https://github.com/numpy/numpy/issues/11448 and https://github.com/ContinuumIO/anaconda-issues/issues/9129".

And unfortunately, the only way to solve the issue under Windows is to uninstall Anaconda and use another distribution with MKL-free numpy. I used python-3.6.6-amd64 from https://www.python.org/ and installed everything else via pip, including numpy 1.14.5. I even managed to make Spyder work (had to downgrade PyQt5 to 5.11.3, it refused to launch on >= 5.12).

Now np.sin(xx) is consistently 0.7071067811865476 (67.1 µs at n = 8192) and np.sqrt(xx) 0.8862269254527579 (16.4 µs). A bit slower, but perfectly reproducible.