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 to improve predictability of NUMA support API
+
+*Note:* This is a sub-RFC of the https://github.com/oneapi-src/oneTBB/pull/1535.
+Specifically, its section about "Increased availability of NUMA support".
+
+* Introduction
+oneTBB has a soft dependency on several variants of ~tbbbind~, which are loaded
+by the library as part of its initialization stage. In turn, each ~tbbbind~ has
+a hard dependency on a concrete version of the HWLOC library [1, 2]. The soft
+dependency of oneTBB on ~tbbbind~ allows the library to continue its execution
+even if the system loader is unable to resolve the hard dependency on HWLOC for
+~tbbbind~. In this case, the HW topology is not discovered and the machine is
+seen as if all CPU cores were uniform, which is the default TBB behavior when
+NUMA constraints are not used. Thus, the following code returns the values that
+do not reflect the real topology and do not matter:
+
+#+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 data due to absence of a third party library is
+the major problem with the current oneTBB behavior. There is no diagnostics for
+the issue, which likely makes it unnoticeable by developers, and the code that
+uses oneTBB NUMA support facilities continues running but does not use NUMA as
+intended.
+
+Having a dependency on a shared HWLOC library has advantages:
+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.
+
+This proposal suggests an improvement to reduce the effect of the disadvantage
+being dependent on a dynamic version of HWLOC library by having it linked
+statically with one of the ~tbbbind~ libraries that are distributed together
+with oneTBB, yet leaving possibility to specify another version of HWLOC library
+if users see the need.
+
+Since HWLOC 1.x is an old version of HWLOC and modern versions of operating
+systems install HWLOC 2.x by default, the probability of someone who is
+constrained by using only HWLOC 1.x on their system is relatively small. Thus,
+the filename of the ~tbbbind~ library that is linked against HWLOC 1.x can be
+re-used for the library that is linked against static HWLOC version 2.x.
+
+* Proposal
+1. Replace the dynamic link of ~tbbbind~ library which is currently linked
+   against HWLOC 1.x with the 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 ~tbbbind~ layer.
+3. Update the oneTBB documentation considering [[https://oneapi-src.github.io/oneTBB/search.html?q=tbb%3A%3Ainfo][these documentation pages]] to
+   include steps determining the variant of ~tbbbind~ being used.
+
+** Advantages
+1. The proposed behavior allows having a mechanism for resolving a dependency on
+   HWLOC library in case it cannot be found in the environment, while still
+   preferring user-provided version of HWLOC. As a result, the problematic use of
+   oneTBB API mentioned above should work as expected, returning 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. The drop of support for HWLOC 1.x allows to not introducing additional
+   ~tbbbind~ variant of the library, yet maintaining support for popular
+   versions of HWLOC.
+
+** Disadvantages
+By default still no diagnostics if users failed to setup environment with their
+own version of HWLOC library correctly. Although, specifying ~TBB_VERSION=1~
+envar will help identifying an issue with setup of environment pretty quickly.
+
+* Alternative handling of inability to parse 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 tells that the functionality being used is not going to work
+  instead of just being silent.
+- Does not require additional variant of ~tbbbind~ library to be distributed
+  along with the others.
+
+** 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 still not be visible, especially if standard streams are
+  closed.
+
+** Disadvantages of Throwing an Exception
+- May break existing code as it 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]]