The Portable Hardware Locality (hwloc) software package provides a
portable abstraction (across OS, versions, architectures, ...) of the
hierarchical topology of modern architectures, including NUMA memory
nodes, sockets, shared caches, cores and simultaneous multithreading. It
also gathers various system attributes such as cache and memory
information as well as the locality of I/O devices such as network
interfaces, InfiniBand HCAs or GPUs.

hwloc primarily aims at helping applications with gathering information
about increasingly complex parallel computing platforms so as to exploit
them accordingly and efficiently. For instance, two tasks that tightly
cooperate should probably be placed onto cores sharing a cache. However,
two independent memory-intensive tasks should better be spread out onto
different sockets so as to maximize their memory throughput. As
described in this paper, OpenMP threads have to be placed according to
their affinities and to the hardware characteristics. MPI
implementations apply similar techniques while also adapting their
communication strategies to the network locality as described in this
paper or this one.

hwloc may also help many applications just by providing a portable CPU
and memory binding API and a reliable way to find out how many cores
and/or hardware threads are available.
