UCX 1.14.0 packages make CUDA optional

[jakirkham]

As part of the UCX 1.14.0 upgrade ( conda-forge/ucx-split-feedstock#111 ), CPU & GPU builds were merged making cudatoolkit optional and ucx-proc unneeded. So it should be possible to use UCX more generally

[dalcinl]

I have no idea how these improvements should affect the openmpi package. @leofang ?

[jakirkham]

Think it comes down to changing this...

openmpi-feedstock/recipe/meta.yaml

Lines 47 to 48 in dfc3d61

	- ucx # [enable_cuda]
	- ucx-proc =*=gpu # [enable_cuda]

...to this...

 - ucx                 # [enable_cuda]

---

Though ucx should work on Linux (even without CUDA)

Tried playing with some changes to capture this in PR ( #121 ). Though there may be other valid approaches

Would defer to Leo and you on what makes the most sense here 🙂

[dalcinl]

@jakirkham @minrk Please help me decide what to do in #128.

• From @minrk's comment, looks like I have to remove the requirement ucx-proc =*=gpu.
• What about the cudatoolkit >= {{ cudatoolkit }} requirement? Should I remove it?

[dalcinl]

BTW, I brancked off 4.x. Should we generate fresh builds with updated requirements?

[leofang]

Sorry I completely missed this. Why can't openmpi just depend on ucx unconditionally? ucx can also be used for CPU-only transfers, and based what I learned from @pentschev (IIRC) ucx wouldn't do anything special if the CUDA presence is not detected, meaning we can just do this

# the actual content would be more convolved due to the need of supporting CUDA 11 & 12,
# as they have different package layout. This is for illustration only.
run_constrained:
  - cudatoolkit 

Once we decide how to approach this, I can take care of the CUDA part.

[leofang]

I support "backporting" whatever decision that we land to the v4.x branch.

[dalcinl]

Why can't openmpi just depend on ucx unconditionally?

Maybe we will have to do it anyway. v5.0.x has changed the way they do things by default, components are no longer built as plugins, but rather linked to libmpi.so (see discussion in #128)

[leofang]

Closing. In #128 we use UCX unconditionally.

[jakirkham]

Thanks all! 🙏