Add sub-RFC for increased availability of NUMA API

rfcs/proposed/numa_support/tbbbind-link-static-hwloc.org

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+# -*- fill-column: 80; -*-
+
+#+title: Link ~tbbbind~ with Static HWLOC for NUMA API predictability
+
+*Note:* This document is a sub-RFC of the [[file:README.md][umbrella RFC about improving NUMA
+support]]. Specifically, the "Increased availability of NUMA support" section.
+
+* Introduction
+oneTBB has a soft dependency on several variants of ~tbbbind~, which the library
+loads during the initialization stage. Each ~tbbbind~, in turn, has a hard
+dependency on a specific version of the HWLOC library [1, 2]. The soft
+dependency means that the library continues the execution even if the system
+loader fails to resolve the hard dependency on HWLOC for ~tbbbind~. In this
+case, oneTBB does not discover the hardware topology. Instead, it defaults to
+viewing all CPU cores as uniform, consistent with TBB behavior when NUMA
+constraints are not used. As a result, the following code returns the irrelevant
+values that do not reflect the actual topology:
+
+#+begin_src C++
+std::vector<oneapi::tbb::numa_node_id> numa_nodes = oneapi::tbb::info::numa_nodes();
+std::vector<oneapi::tbb::core_type_id> core_types = oneapi::tbb::info::core_types();
+#+end_src
+
+This lack of valid HW topology, caused by the absence of a third-party library,
+is the major problem with the current oneTBB behavior. The problem lies in the
+lack of diagnostics making it difficult for developers to detect. As a result,
+the code continues to run but fails to use NUMA as intended.
+
+Dependency on a shared HWLOC library has the following benefits:
+1. Code reuse with all of the positive consequences out of this, including
+   relying on the same code that has been tested and debugged, allowing the OS
+   to share it among different processes, which consequently improves on cache
+   locality and memory footprint. That's the primary purpose of shared
+   libraries.
+2. A drop-in replacement. Users are able to use their own version of HWLOC
+   without recompilation of oneTBB. This specific version of HWLOC could include
+   a hotfix to support a particular and/or new hardware that a customer has, but
+   whose support is not yet upstreamed to HWLOC project. It is also possible
+   that such support won't be upstreamed at all if that hardware is not going to
+   be available for massive users. It could also be a development version of
+   HWLOC that someone wants to test on their systems first. Of course, they can
+   do it with the static version as well, but that's more cumbersome as it
+   requires recompilation of every dependent component.
+
+The only disadvantage from depending on HWLOC library dynamically is that the
+developers that use oneTBB's NUMA support API need to make sure the library is
+available and can be found by oneTBB. Depending on the distribution model of a
+developer's code, this is achieved either by:
+1. Asking the end user to have necessary version of a dependency pre-installed.
+2. Bundling necessary HWLOC version together with other pieces of a product
+   release.
+
+However, the requirement to fulfill one of the above steps for the NUMA API to
+start paying off may be considered as an incovenience and, what is more
+important, it is not always obvious that one of these steps is needed.
+Especially, due to silent behavior in case HWLOC library cannot be found in the
+environment.
+
+The proposal is to reduce the effect of the disadvantage of relying on a dynamic
+HWLOC library. The improvements involve statically linking HWLOC with one of the
+~tbbbind~ libraries distributed together with oneTBB. At the same time, you
+retain the flexibility to specify different version of HWLOC library if needed.
+
+Since HWLOC 1.x is an older version and modern operating systems install HWLOC
+2.x by default, the probability of users being restricted to HWLOC 1.x is
+relatively small. Thus, we can reuse the filename of the ~tbbbind~ library
+linked to HWLOC 1.x for the library linked against a static HWLOC 2.x.
+
+* Proposal
+1. Replace the dynamic link of ~tbbbind~ library currently linked
+   against HWLOC 1.x with a link to a static HWLOC library version 2.x.
+2. Add loading of that ~tbbbind~ variant as the last attempt to resolve the
+   dependency on functionality provided by the ~tbbbind~ layer.
+3. Update the oneTBB documentation, including [[https://oneapi-src.github.io/oneTBB/search.html?q=tbb%3A%3Ainfo][these pages]], to
+   detail the steps for identifying which ~tbbbind~ is being used.
+
+** Advantages
+1. The proposed behavior introduces a fallback mechanism for resolving the HWLOC
+   library dependency when it is not in the environment, while still preferring
+   user-provided versions. As a result, the problematic oneTBB API usage works
+   as expected, returning an enumerated list of actual NUMA nodes and core types
+   on the system the code is running on, provided that the loaded HWLOC library
+   works on that system and that an application properly distributes all
+   binaries of oneTBB, sets the environment so that the necessary variant of
+   ~tbbbind~ library can be found and loaded.
+2. Dropping support for HWLOC 1.x, does not introduce an additional ~tbbbind~
+   variant while maintaining support for widely used versions of HWLOC.
+
+** Disadvantages
+By default, there is still no diagnostics if you fail to correctly setup an
+environment with your version of HWLOC. Although, specifying the ~TBB_VERSION=1~
+environment variable helps identify configuration issues quickly.
+
+* Alternative Handling for Missing System Topology
+The other behavior in case HWLOC library cannot be found is to be more explicit
+about the problem of a missing component and to either issue a warning or to
+refuse working requiring one of the ~tbbbind~ variant to be loaded (e.g., throw
+an exception).
+
+Comparing these alternative approaches to the one proposed.
+** Common Advantages
+- Explicitly indicates that the functionality being used does not work, instead
+  of failing silently.
+- Avoids the need to distribute an additional variant of ~tbbbind~ library.
+
+** Common Disadvantages
+- Requires additional step from the user side to resolve the problem. In other
+  words, it does not provide complete solution to the problem.
+
+*** Disadvantages of Issuing a Warning
+- The warning may be unnoticed, especially if standard streams are closed.
+
+*** Disadvantages of Throwing an Exception
+- May break existing code that does not expect an exception to be thrown.
+- Requires introduction of an additional exception hierarchy.
+
+* References
+1. [[https://www.open-mpi.org/projects/hwloc/][HWLOC project main page]]
+2. [[https://github.com/open-mpi/hwloc][HWLOC project repository on GitHub]]