Remove deprecated RMM `Buffer` support by pentschev · Pull Request #656 · rapidsai/ucxx

pentschev · 2026-05-19T18:23:14Z

The Buffer support for RMM was deprecated in favor of CCCL. Remove that for UCXX v0.51/RAPIDS 26.08.

bdice · 2026-05-19T18:32:54Z


-  # RMM memory resources for CUDA benchmarks (requires UCXX_ENABLE_RMM and get_rmm.cmake)
+  # RMM memory resources for CUDA benchmarks.
  if(UCXX_BENCHMARKS_ENABLE_CUDA AND UCXX_ENABLE_RMM)


Do we need to remove more RMM references in this block?

We still support RMM benchmarks, they have nothing to do with the Buffer class. Maybe at some point we may want to remove that, but for now I think it's useful for us to have the option to compare between both, unless our plan is to completely remove RMM memory resources and have everyone rely on CCCL directly from 26.08.

No, we're not quite there yet, so let's leave these benchmarks. Most likely we'll never lose all the mrs in rmm, but I am hopeful that we'll eventually lose buffers.

vyasr

The README needs to be updated too, it refers to building the library with rmm support.

vyasr · 2026-05-19T19:21:18Z


-  # RMM memory resources for CUDA benchmarks (requires UCXX_ENABLE_RMM and get_rmm.cmake)
+  # RMM memory resources for CUDA benchmarks.
  if(UCXX_BENCHMARKS_ENABLE_CUDA AND UCXX_ENABLE_RMM)


No, we're not quite there yet, so let's leave these benchmarks. Most likely we'll never lose all the mrs in rmm, but I am hopeful that we'll eventually lose buffers.

vyasr · 2026-05-19T19:43:34Z

 option(BUILD_SHARED_LIBS "Build UCXX shared libraries" ON)
-option(UCXX_ENABLE_RMM "Enable support for CUDA multi-buffer transfer with RMM" OFF)
+option(UCXX_ENABLE_RMM "Enable RMM-backed test and benchmark code paths" OFF)
 # TODO: Flip UCXX_ENABLE_CCCL default to OFF once devcontainer builds pass -DUCXX_ENABLE_CCCL=ON


We need to update the devcontainers to use CCCL here, then we can change this flag here. That way CCCL support stays optional.

Thanks for pointing that, opened rapidsai/devcontainers#705 to address that.

@pentschev Would it be too much to rename UCXX_ENABLE_RMM to UCXX_ENABLE_RMM_TESTING?
I had the same question as @bdice here

pentschev · 2026-05-19T20:08:33Z

The README needs to be updated too, it refers to building the library with rmm support.

Fixed in 5082070

CCCL support for UCXX was added in rapidsai/ucxx#641, and rapidsai/ucxx#656 is removing RMM support. Switching to CCCL here is necessary.

nirandaperera

LGTM. It would be nice if we could rename UCXX_ENABLE_RMM to something more representative now (maybe UCXX_ENABLE_RMM_TESTING).
But I dont want to block the PR for that.

nirandaperera · 2026-05-20T19:07:40Z

 option(BUILD_SHARED_LIBS "Build UCXX shared libraries" ON)
-option(UCXX_ENABLE_RMM "Enable support for CUDA multi-buffer transfer with RMM" OFF)
+option(UCXX_ENABLE_RMM "Enable RMM-backed test and benchmark code paths" OFF)
 # TODO: Flip UCXX_ENABLE_CCCL default to OFF once devcontainer builds pass -DUCXX_ENABLE_CCCL=ON


@pentschev Would it be too much to rename UCXX_ENABLE_RMM to UCXX_ENABLE_RMM_TESTING?
I had the same question as @bdice here

trxcllnt · 2026-05-20T19:50:48Z

Adding this to cpp/tests/CMakeLists.txt should fix the UCXX_TEST linker errors:

  if(UCXX_ENABLE_CCCL)
    target_link_libraries(${CMAKE_TEST_NAME} PRIVATE CUDA::cudart_static)
  endif()

…ve-deprecated-rmm

pentschev · 2026-05-20T20:41:49Z

Adding this to cpp/tests/CMakeLists.txt should fix the UCXX_TEST linker errors:
  if(UCXX_ENABLE_CCCL)
    target_link_libraries(${CMAKE_TEST_NAME} PRIVATE CUDA::cudart_static)
  endif()

Thanks Paul, done in e8ff9b0.

LGTM. It would be nice if we could rename UCXX_ENABLE_RMM to something more representative now (maybe UCXX_ENABLE_RMM_TESTING). But I dont want to block the PR for that.

Thanks Niranda, done in 7e8ca76.

trxcllnt · 2026-05-20T21:40:38Z

Looks like either a similar change needs to be made in python/ucxx, or ucxx needs to specify its CUDA::cudart_static dependency in the public interface when UCXX_ENABLE_CCCL=ON

trxcllnt · 2026-05-20T22:11:15Z

diff --git a/python/ucxx/ucxx/_lib/CMakeLists.txt b/python/ucxx/ucxx/_lib/CMakeLists.txt
index 68922af..d3488bf 100644
--- a/python/ucxx/ucxx/_lib/CMakeLists.txt
+++ b/python/ucxx/ucxx/_lib/CMakeLists.txt
@@ -5,8 +5,10 @@
 # cmake-format: on
 # =================================================================================
 
+find_package(CUDAToolkit REQUIRED)
+
 set(cython_sources arr.pyx libucxx.pyx)
-set(linked_libraries ucxx::ucxx ucxx::python)
+set(linked_libraries ucxx::ucxx ucxx::python CUDA::cudart_static)
 
 rapids_cython_create_modules(
   CXX

pentschev · 2026-05-21T06:55:30Z

/merge

pentschev added 2 commits May 19, 2026 11:16

Remove deprecated RMM Buffer support

5d7728f

Remove unnecessary build dependencies

7ce3713

pentschev self-assigned this May 19, 2026

pentschev requested review from a team as code owners May 19, 2026 18:23

pentschev added the breaking Introduces a breaking change label May 19, 2026

pentschev requested a review from KyleFromNVIDIA May 19, 2026 18:23

pentschev added improvement Improves an existing functionality libucxx ucxx labels May 19, 2026

bdice reviewed May 19, 2026

View reviewed changes

Merge remote-tracking branch 'upstream/main' into remove-deprecated-rmm

683d272

vyasr requested changes May 19, 2026

View reviewed changes

Update README with CCCL info

5082070

pentschev mentioned this pull request May 19, 2026

Switch from building RMM to building CCCL in UCXX rapidsai/devcontainers#705

Merged

vyasr approved these changes May 19, 2026

View reviewed changes

Merge branch 'main' into remove-deprecated-rmm

e6f4e15

pentschev mentioned this pull request May 20, 2026

Use -DUCXX_ENABLE_RMM=ON because it's necessary to configure ucxx::python rapidsai/devcontainers#707

Closed

nirandaperera approved these changes May 20, 2026

View reviewed changes

trxcllnt mentioned this pull request May 20, 2026

Update devcontainer cacheFrom image names to use ucxx version #661

Merged

pentschev added 3 commits May 20, 2026 13:28

Add missing cudart_static linking

e8ff9b0

Rename RMM CMake flags

7e8ca76

Merge remote-tracking branch 'origin/remove-deprecated-rmm' into remo…

2e0c1e0

…ve-deprecated-rmm

More cudart_static linkage

7afe52e

rapids-bot Bot merged commit 7ca8811 into rapidsai:main May 21, 2026
159 of 163 checks passed

pentschev deleted the remove-deprecated-rmm branch May 22, 2026 10:35

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