add new page to index.md

docs/conceptual/setting-cus.rst

@@ -0,0 +1,47 @@
+.. meta::
+    :description: Setting the number of CUs
+    :keywords: AMD, ROCm, cu, number of cus
+
+.. _env-variables-reference:
+
+*************************************************************
+Setting the number of CUs
+*************************************************************
+
+When using GPUs to accelerate compute workloads, it sometimes becomes necessary
+to configure the usage of Compute Units (CU) of the hardware. This is a more advanced
+option, so please read this page before experimentation.
+
+The GPU driver provides two environment variables to set the number of CUs used. The
+first one is ``HSA_CU_MASK`` and the second one is ``ROC_GLOBAL_CU_MASK``. The main
+difference is, that ``ROC_GLOBAL_CU_MASK`` sets the CU mask on queues created by
+the HIP or the OpenCL runtimes. While ``HSA_CU_MASK`` sets the mask on a lower level of
+queue creation in the driver, this mask will also be set for queues being profiled.
+
+The environment variables have the following syntax:
+
+::
+
+    ID = [0-9][0-9]*                         ex. base 10 numbers
+    ID_list = (ID | ID-ID)[, (ID | ID-ID)]*  ex. 0,2-4,7
+    GPU_list = ID_list                       ex. 0,2-4,7
+    CU_list = 0x[0-F]* | ID_list             ex. 0x337F OR 0,2-4,7
+    CU_Set = GPU_list : CU_list              ex. 0,2-4,7:0-15,32-47 OR 0,2-4,7:0x337F
+    HSA_CU_MASK = CU_Set [; CU_Set]*         ex. 0,2-4,7:0-15,32-47; 3-9:0x337F
+
+The GPU indices are taken post ``ROCR_VISIBLE_DEVICES`` reordering. For GPUs listed,
+the listed or masked CUs will be enabled, the rest disabled. Unlisted GPUs will not
+be affected, their CUs will all be enabled.
+
+The parsing of the variable is stopped when a syntax error occurs. The erroneous set
+and the ones following will be ignored. Repeating GPU or CU IDs are a syntax error.
+Specifying a mask with no usable CUs (CU_list is 0x0) is a syntax error. For excluding
+GPU devices use ``ROCR_VISIBLE_DEVICES``.
+
+These environment variables only affect ROCm software, not graphics applications.
+
+It's important to know that not all CU configurations are valid on all devices. For
+instance, on devices where two CUs can be combined into a WGP (for kernels running in
+WGP mode), it is not valid to disable only a single CU in a WGP. `This paper
+<https://www.cs.unc.edu/~otternes/papers/rtsj2022.pdf>`_ can provide more information
+about what to expect, when disabling CUs.

docs/index.md

@@ -110,6 +110,7 @@ Our documentation is organized into the following categories:
   * [MI250](./conceptual/gpu-arch/mi250.md)
   * [MI300](./conceptual/gpu-arch/mi300.md)
 * [GPU memory](./conceptual/gpu-memory.md)
+* [Setting the number of CUs](./conceptual/setting-cus.md)
 * [Compiler disambiguation](./conceptual/compiler-disambiguation.md)
 * [File structure (Linux FHS)](./conceptual/file-reorg.md)
 * [GPU isolation techniques](./conceptual/gpu-isolation.md)

docs/sphinx/_toc.yml.in

@@ -98,6 +98,8 @@ subtrees:
             title: White paper
   - file: conceptual/gpu-memory.md
     title: GPU memory
+  - file: conceptual/setting-cus
+    title: Setting the number of CUs
   - file: conceptual/compiler-disambiguation.md
     title: Compiler disambiguation
   - file: about/compatibility/openmp.md