memtiers: clarify a comment about using subtypes for sorting nodes in tiers

Signed-off-by: Brice Goglin <[email protected]>

Author: bgoglin

hwloc/memattrs.c

@@ -1521,10 +1521,15 @@ hwloc__group_memory_tiers(hwloc_topology_t topology,
     }
   }
 
-  /* Sort nodes.
-   * We could also sort by the existing subtype.
-   * KNL is the only case where subtypes are set in backends, but we set memattrs as well there.
-   * Also HWLOC_MEMTIERS_REFRESH would be a special value to ignore existing subtypes.
+  /* Sort nodes by tier type and bandwidth.
+   *
+   * We could also use the existing subtype but it's not clear it'd be better.
+   * For NVIDIA GPU, "GPUMemory" is set in the Linux backend, and used above to set tier type anyway.
+   * For KNL, the Linux backend sets subtypes and memattrs, sorting by memattrs already works fine.
+   * Existing subtypes could have been imported from XML, usually mostly OK except maybe SPM (fallback for I don't know)?
+   * An envvar (or HWLOC_MEMTIERS_REFRESH special value?) could be passed to ignore existing subtypes,
+   * but "GPUMemory" wouldn't be available anymore, we'd have to use something else like "PCIBusId",
+   * but that one might not always be specific to GPU-backed NUMA nodes?
    */
   hwloc_debug("Sorting memory node infos...\n");
   qsort(nodeinfos, n, sizeof(*nodeinfos), compare_node_infos_by_type_and_bw);