incompatibility with Intel TBB (used within Numba) which is currently the default on Google Colab

[thearia0]

It doesn't happen every time the code is run on Google Colab but often Numba reports an error about not having enough threads

it may be connected to this PR: 0a2e883

[slayoo]

This is a known incompatibility of newer Numba versions with PyMPDATA.
Essentially, Numba has changed its API, and we haven't found a way yet to adapt to the new one.
Passing n_threads=1 to the Stepper constructor constitutes a workaround (but also disables multi-threading entirely)

[slayoo]

@Sfonxu, I'm looking around and cannot find any trace of a record of which is the actual Numba version where the incompatibility appeared, do you have any record of this investigation?

[Sfonxu]

I don't have anything written down really, but looking at some archived info I have from PyMPDATA-MPI development it would be somwhere around Numba >= 0.57.0. Lesser versions were exclusively used for Numba then and only after updating to P3.12 (which meant newer Numba versions) a lot of those threading issues started to become apparent.

EDIT: typos and more precise description

[slayoo]

but on CI we're using newer releases, and our sanity test passes, and there are no timeouts...

PyMPDATA/setup.py

Line 34 in ed8d8c1

13: "==0.61.2",

[slayoo]

however, for Linux, we also only test with OpenMP!

PyMPDATA/.github/workflows/tests.yml

Lines 137 to 138 in ed8d8c1

	- if: startsWith(matrix.platform, 'ubuntu-')
	run: echo NUMBA_THREADING_LAYER=omp >> $GITHUB_ENV

[slayoo]

while Colab's default is Intel TBB:

[slayoo]

and Numba docs quite explicitly mention that TBB might use fewer threads than other backends (https://numba.pydata.org/numba-doc/dev/developer/threading_implementation.html#caveats):

[slayoo]

so it seems, that we should "simply" require that anyone using PyMPDATA picks omp as Numba Threading layer

[slayoo]

well. the warning message that we already print actually does suggest it!

[Sfonxu]

but on CI we're using newer releases, and our sanity test passes, and there are no timeouts...

PyMPDATA/setup.py

Line 34 in ed8d8c1
13: "==0.61.2",

That omp line in tests did come up during investigating if I'm not mistaken. For my laptop w/ Linux if I want to run PyMPDATA I always have to change the threading backend to OpenMP to run any tests or whatnot.

It is possible to change the backend on Colab via adding this before any Numba-dependent code is run:

import numba
numba.config.THREADING_LAYER="omp"

And restarting the session (for instance via the menu under the run all button). This worked for me when testing around this PyMPDATA example on Colab. What I also noticed was that the first run of a session done without changing the backend also kind of worked sometimes, but I can't really confirm why.

[Sfonxu]

One other note is that in the Numba threading docs we read:

The default manner in which Numba searches for and loads a threading layer is tolerant of missing libraries, incompatible runtimes etc.

I wonder if that might also have something to do with this...

EDIT: source

[slayoo]

It is possible to change the backend on Colab via adding this before any Numba-dependent code is run:

import numba
numba.config.THREADING_LAYER="omp"

We should then add it to all notebooks in PySDM and PyMPDATA and change the devops_tests requirement for the header cell accordingly. Perhaps setting an env. var would be simpler? (i.e. faster)

And restarting the session (for instance via the menu under the run all button).

If this is done at the top of the notebook, restart is not needed (as it does not change anything)