Disable OpenMP by default

[astrofrog]

This implements one of my suggestions in #97

Basically this makes OpenMP opt-in. We can still decide to opt in by default for conda packages if the OpenMP environment there is more predictable.

[coveralls]

Coverage remained the same at 96.214% when pulling b78321a on astrofrog:disable-openmp-default into a0b72e7 on astropy:master.

[cdeil]

I would prefer if we remove OpenMP completely. This does not really resolve the concerns I mentioned in #97.

If we offer the option, we'll have packagers and users using OpenMP and have to support it, and it's a portability concern for Astropy core.

Also we have to test it. Note that this PR leaves the option to use OpenMP, but as far as I can see it's completely untested in CI now.

Also: it hasn't really been shown that this multi-threading gives good speed-ups. There have been some benchmarks posted in the PRs adding OpenMP that show a factor 2 speed-up for large arrays (and slow-down for small arrays), but not that our HEALPix C lib is not efficient yet, it can be improved by a factor of a few (see e.g. #34 (comment) and #34 (comment) ) and once it's optimised it's not clear if using many cores helps a lot.

So @astrofrog - If you want to keep the OpenMP, then I agree this is an improvement and we should merge.

[astrofrog]

Removing it is fine by me as long as you are happy with the performance. I only added it originally because there were concerns that without, things were slower than healpy.

[cdeil]

Removing it is fine by me as long as you are happy with the performance. I only added it originally because there were concerns that without, things were slower than healpy.

+10 from me to remove OpenMP.

@lpsinger - Acceptable for you?

@astrofrog - Can you update this PR, or should prepare a new one to do it?

[lpsinger]

@lpsinger - Acceptable for you?

👍

[cdeil]

Ok, then I'm closing this PR. I did open #108 which removes OpenMP.

[jamienoss]

@cdeil Check out astropy/astropy-helpers#410. You might want to use the following pattern in your root setup.py:

...
# The current default is to NOT build with OpenMP support, i.e. ``disable_openmp = True``
disable_openmp = os.environ.get('ASTROPY_HEALPIX_USE_OPENMP', '0') == '0'
generate_openmp_enabled_py(NAME, disable_openmp=disable_openmp)
...

You won't then need the conditional around the add_openmp_flags_if_available(). Additionally, once built & installed you can call package.openmp_enabled.is_openmp_enabled() to query whether the package was built with OpenMP, this can be asserted in feedstock tests etc.