This is a truncated and poorly-formatted version of the documentation main page. See https://www.open-mpi.org/projects/hwloc/doc/ for more. Installation hwloc (https://www.open-mpi.org/projects/hwloc/) is available under the BSD license. It is hosted as a sub-project of the overall Open MPI project (https:/ /www.open-mpi.org/). Note that hwloc does not require any functionality from Open MPI -- it is a wholly separate (and much smaller!) project and code base. It just happens to be hosted as part of the overall Open MPI project. Basic Installation Installation is the fairly common GNU-based process: shell$ ./configure --prefix=... shell$ make shell$ make install The hwloc command-line tool "lstopo" produces human-readable topology maps, as mentioned above. Running the "lstopo" tool is a good way to check as a graphical output whether hwloc properly detected the architecture of your node. Optional Dependencies lstopo may also export graphics to the SVG and "fig" file formats. Support for PDF, Postscript, and PNG exporting is provided if the "Cairo" development package (usually cairo-devel or libcairo2-dev) can be found in "lstopo" when hwloc is configured and build. The hwloc core may also benefit from the following development packages: * libpciaccess for full I/O device discovery (libpciaccess-devel or libpciaccess-dev package). On Linux, PCI discovery may still be performed (without vendor/device names) even if libpciaccess cannot be used. * AMD or NVIDIA OpenCL implementations for OpenCL device discovery. * the NVIDIA CUDA Toolkit for CUDA device discovery. See How do I enable CUDA and select which CUDA version to use?. * the NVIDIA Management Library (NVML) for NVML device discovery. It is included in CUDA since version 8.0. Older NVML releases were available within the NVIDIA GPU Deployment Kit from https://developer.nvidia.com/ gpu-deployment-kit . * the NV-CONTROL X extension library (NVCtrl) for NVIDIA display discovery. The relevant development package is usually libXNVCtrl-devel or libxnvctrl-dev. It is also available within nvidia-settings from ftp:// download.nvidia.com/XFree86/nvidia-settings/ and https://github.com/NVIDIA/ nvidia-settings/ . * the AMD ROCm SMI library for RSMI device discovery. The relevant development package is usually rocm-smi-lib64 or librocm-smi-dev. See How do I enable ROCm SMI and select which version to use?. * the oneAPI Level Zero library. The relevant development package is usually level-zero-dev or level-zero-devel. * libxml2 for full XML import/export support (otherwise, the internal minimalistic parser will only be able to import XML files that were exported by the same hwloc release). See Importing and exporting topologies from/to XML files for details. The relevant development package is usually libxml2-devel or libxml2-dev. * libudev on Linux for easier discovery of OS device information (otherwise hwloc will try to manually parse udev raw files). The relevant development package is usually libudev-devel or libudev-dev. * libtool's ltdl library for dynamic plugin loading if the native dlopen cannot be used. The relevant development package is usually libtool-ltdl-devel or libltdl-dev. PCI and XML support may be statically built inside the main hwloc library, or as separate dynamically-loaded plugins (see the Components and plugins section). Also note that if you install supplemental libraries in non-standard locations, hwloc's configure script may not be able to find them without some help. You may need to specify additional CPPFLAGS, LDFLAGS, or PKG_CONFIG_PATH values on the configure command line. For example, if libpciaccess was installed into /opt/pciaccess, hwloc's configure script may not find it by default. Try adding PKG_CONFIG_PATH to the ./configure command line, like this: ./configure PKG_CONFIG_PATH=/opt/pciaccess/lib/pkgconfig ... Note that because of the possibility of GPL taint, the pciutils library libpci will not be used (remember that hwloc is BSD-licensed). Installing from a Git clone Additionally, the code can be directly cloned from Git: shell$ git clone https://github.com/open-mpi/hwloc.git shell$ cd hwloc shell$ ./autogen.sh Note that GNU Autoconf >=2.63, Automake >=1.11 and Libtool >=2.2.6 are required when building from a Git clone. Nightly development snapshots are available on the web site, they can be configured and built without any need for Git or GNU Autotools. Questions and Bugs Bugs should be reported in the tracker (https://github.com/open-mpi/hwloc/ issues). Opening a new issue automatically displays lots of hints about how to debug and report issues. Questions may be sent to the users or developers mailing lists (https:// www.open-mpi.org/community/lists/hwloc.php). There is also a #hwloc IRC channel on Libera Chat (irc.libera.chat). Command-line Examples On a 4-package 2-core machine with hyper-threading, the lstopo tool may show the following graphical output: [dudley] Here's the equivalent output in textual form: Machine NUMANode L#0 (P#0) Package L#0 + L3 L#0 (4096KB) L2 L#0 (1024KB) + L1 L#0 (16KB) + Core L#0 PU L#0 (P#0) PU L#1 (P#8) L2 L#1 (1024KB) + L1 L#1 (16KB) + Core L#1 PU L#2 (P#4) PU L#3 (P#12) Package L#1 + L3 L#1 (4096KB) L2 L#2 (1024KB) + L1 L#2 (16KB) + Core L#2 PU L#4 (P#1) PU L#5 (P#9) L2 L#3 (1024KB) + L1 L#3 (16KB) + Core L#3 PU L#6 (P#5) PU L#7 (P#13) Package L#2 + L3 L#2 (4096KB) L2 L#4 (1024KB) + L1 L#4 (16KB) + Core L#4 PU L#8 (P#2) PU L#9 (P#10) L2 L#5 (1024KB) + L1 L#5 (16KB) + Core L#5 PU L#10 (P#6) PU L#11 (P#14) Package L#3 + L3 L#3 (4096KB) L2 L#6 (1024KB) + L1 L#6 (16KB) + Core L#6 PU L#12 (P#3) PU L#13 (P#11) L2 L#7 (1024KB) + L1 L#7 (16KB) + Core L#7 PU L#14 (P#7) PU L#15 (P#15) Note that there is also an equivalent output in XML that is meant for exporting /importing topologies but it is hardly readable to human-beings (see Importing and exporting topologies from/to XML files for details). On a 4-package 2-core Opteron NUMA machine (with two core cores disallowed by the administrator), the lstopo tool may show the following graphical output (with --disallowed for displaying disallowed objects): [hagrid] Here's the equivalent output in textual form: Machine (32GB total) Package L#0 NUMANode L#0 (P#0 8190MB) L2 L#0 (1024KB) + L1 L#0 (64KB) + Core L#0 + PU L#0 (P#0) L2 L#1 (1024KB) + L1 L#1 (64KB) + Core L#1 + PU L#1 (P#1) Package L#1 NUMANode L#1 (P#1 8192MB) L2 L#2 (1024KB) + L1 L#2 (64KB) + Core L#2 + PU L#2 (P#2) L2 L#3 (1024KB) + L1 L#3 (64KB) + Core L#3 + PU L#3 (P#3) Package L#2 NUMANode L#2 (P#2 8192MB) L2 L#4 (1024KB) + L1 L#4 (64KB) + Core L#4 + PU L#4 (P#4) L2 L#5 (1024KB) + L1 L#5 (64KB) + Core L#5 + PU L#5 (P#5) Package L#3 NUMANode L#3 (P#3 8192MB) L2 L#6 (1024KB) + L1 L#6 (64KB) + Core L#6 + PU L#6 (P#6) L2 L#7 (1024KB) + L1 L#7 (64KB) + Core L#7 + PU L#7 (P#7) On a 2-package quad-core Xeon (pre-Nehalem, with 2 dual-core dies into each package): [emmett] Here's the same output in textual form: Machine (total 16GB) NUMANode L#0 (P#0 16GB) Package L#0 L2 L#0 (4096KB) L1 L#0 (32KB) + Core L#0 + PU L#0 (P#0) L1 L#1 (32KB) + Core L#1 + PU L#1 (P#4) L2 L#1 (4096KB) L1 L#2 (32KB) + Core L#2 + PU L#2 (P#2) L1 L#3 (32KB) + Core L#3 + PU L#3 (P#6) Package L#1 L2 L#2 (4096KB) L1 L#4 (32KB) + Core L#4 + PU L#4 (P#1) L1 L#5 (32KB) + Core L#5 + PU L#5 (P#5) L2 L#3 (4096KB) L1 L#6 (32KB) + Core L#6 + PU L#6 (P#3) L1 L#7 (32KB) + Core L#7 + PU L#7 (P#7) Programming Interface The basic interface is available in hwloc.h. Some higher-level functions are available in hwloc/helper.h to reduce the need to manually manipulate objects and follow links between them. Documentation for all these is provided later in this document. Developers may also want to look at hwloc/inlines.h which contains the actual inline code of some hwloc.h routines, and at this document, which provides good higher-level topology traversal examples. To precisely define the vocabulary used by hwloc, a Terms and Definitions section is available and should probably be read first. Each hwloc object contains a cpuset describing the list of processing units that it contains. These bitmaps may be used for CPU binding and Memory binding. hwloc offers an extensive bitmap manipulation interface in hwloc/bitmap.h. Moreover, hwloc also comes with additional helpers for interoperability with several commonly used environments. See the Interoperability With Other Software section for details. The complete API documentation is available in a full set of HTML pages, man pages, and self-contained PDF files (formatted for both both US letter and A4 formats) in the source tarball in doc/doxygen-doc/. NOTE: If you are building the documentation from a Git clone, you will need to have Doxygen and pdflatex installed -- the documentation will be built during the normal "make" process. The documentation is installed during "make install" to $prefix/share/doc/hwloc/ and your systems default man page tree (under $prefix, of course). Portability Operating System have varying support for CPU and memory binding, e.g. while some Operating Systems provide interfaces for all kinds of CPU and memory bindings, some others provide only interfaces for a limited number of kinds of CPU and memory binding, and some do not provide any binding interface at all. Hwloc's binding functions would then simply return the ENOSYS error (Function not implemented), meaning that the underlying Operating System does not provide any interface for them. CPU binding and Memory binding provide more information on which hwloc binding functions should be preferred because interfaces for them are usually available on the supported Operating Systems. Similarly, the ability of reporting topology information varies from one platform to another. As shown in Command-line Examples, hwloc can obtain information on a wide variety of hardware topologies. However, some platforms and/or operating system versions will only report a subset of this information. For example, on an PPC64-based system with 8 cores (each with 2 hardware threads) running a default 2.6.18-based kernel from RHEL 5.4, hwloc is only able to glean information about NUMA nodes and processor units (PUs). No information about caches, packages, or cores is available. Here's the graphical output from lstopo on this platform when Simultaneous Multi-Threading (SMT) is enabled: [ppc64-with] And here's the graphical output from lstopo on this platform when SMT is disabled: [ppc64-with] Notice that hwloc only sees half the PUs when SMT is disabled. PU L#6, for example, seems to change location from NUMA node #0 to #1. In reality, no PUs "moved" -- they were simply re-numbered when hwloc only saw half as many (see also Logical index in Indexes and Sets). Hence, PU L#6 in the SMT-disabled picture probably corresponds to PU L#12 in the SMT-enabled picture. This same "PUs have disappeared" effect can be seen on other platforms -- even platforms / OSs that provide much more information than the above PPC64 system. This is an unfortunate side-effect of how operating systems report information to hwloc. Note that upgrading the Linux kernel on the same PPC64 system mentioned above to 2.6.34, hwloc is able to discover all the topology information. The following picture shows the entire topology layout when SMT is enabled: [ppc64-full] Developers using the hwloc API or XML output for portable applications should therefore be extremely careful to not make any assumptions about the structure of data that is returned. For example, per the above reported PPC topology, it is not safe to assume that PUs will always be descendants of cores. Additionally, future hardware may insert new topology elements that are not available in this version of hwloc. Long-lived applications that are meant to span multiple different hardware platforms should also be careful about making structure assumptions. For example, a new element may someday exist between a core and a PU. API Example The following small C example (available in the source tree as ``doc/examples/ hwloc-hello.c'') prints the topology of the machine and performs some thread and memory binding. More examples are available in the doc/examples/ directory of the source tree. /* Example hwloc API program. * * See other examples under doc/examples/ in the source tree * for more details. * * Copyright (c) 2009-2016 Inria. All rights reserved. * Copyright (c) 2009-2011 Universit?eacute; Bordeaux * Copyright (c) 2009-2010 Cisco Systems, Inc. All rights reserved. * See COPYING in top-level directory. * * hwloc-hello.c */ #include "hwloc.h" #include <errno.h> #include <stdio.h> #include <string.h> static void print_children(hwloc_topology_t topology, hwloc_obj_t obj, int depth) { char type[32], attr[1024]; unsigned i; hwloc_obj_type_snprintf(type, sizeof(type), obj, 0); printf("%*s%s", 2*depth, "", type); if (obj->os_index != (unsigned) -1) printf("#%u", obj->os_index); hwloc_obj_attr_snprintf(attr, sizeof(attr), obj, " ", 0); if (*attr) printf("(%s)", attr); printf("\n"); for (i = 0; i < obj->arity; i++) { print_children(topology, obj->children[i], depth + 1); } } int main(void) { int depth; unsigned i, n; unsigned long size; int levels; char string[128]; int topodepth; void *m; hwloc_topology_t topology; hwloc_cpuset_t cpuset; hwloc_obj_t obj; /* Allocate and initialize topology object. */ hwloc_topology_init(&topology); /* ... Optionally, put detection configuration here to ignore some objects types, define a synthetic topology, etc.... The default is to detect all the objects of the machine that the caller is allowed to access. See Configure Topology Detection. */ /* Perform the topology detection. */ hwloc_topology_load(topology); /* Optionally, get some additional topology information in case we need the topology depth later. */ topodepth = hwloc_topology_get_depth(topology); /***************************************************************** * First example: * Walk the topology with an array style, from level 0 (always * the system level) to the lowest level (always the proc level). *****************************************************************/ for (depth = 0; depth < topodepth; depth++) { printf("*** Objects at level %d\n", depth); for (i = 0; i < hwloc_get_nbobjs_by_depth(topology, depth); i++) { hwloc_obj_type_snprintf(string, sizeof(string), hwloc_get_obj_by_depth(topology, depth, i), 0); printf("Index %u: %s\n", i, string); } } /***************************************************************** * Second example: * Walk the topology with a tree style. *****************************************************************/ printf("*** Printing overall tree\n"); print_children(topology, hwloc_get_root_obj(topology), 0); /***************************************************************** * Third example: * Print the number of packages. *****************************************************************/ depth = hwloc_get_type_depth(topology, HWLOC_OBJ_PACKAGE); if (depth == HWLOC_TYPE_DEPTH_UNKNOWN) { printf("*** The number of packages is unknown\n"); } else { printf("*** %u package(s)\n", hwloc_get_nbobjs_by_depth(topology, depth)); } /***************************************************************** * Fourth example: * Compute the amount of cache that the first logical processor * has above it. *****************************************************************/ levels = 0; size = 0; for (obj = hwloc_get_obj_by_type(topology, HWLOC_OBJ_PU, 0); obj; obj = obj->parent) if (hwloc_obj_type_is_cache(obj->type)) { levels++; size += obj->attr->cache.size; } printf("*** Logical processor 0 has %d caches totaling %luKB\n", levels, size / 1024); /***************************************************************** * Fifth example: * Bind to only one thread of the last core of the machine. * * First find out where cores are, or else smaller sets of CPUs if * the OS doesn't have the notion of a "core". *****************************************************************/ depth = hwloc_get_type_or_below_depth(topology, HWLOC_OBJ_CORE); /* Get last core. */ obj = hwloc_get_obj_by_depth(topology, depth, hwloc_get_nbobjs_by_depth(topology, depth) - 1); if (obj) { /* Get a copy of its cpuset that we may modify. */ cpuset = hwloc_bitmap_dup(obj->cpuset); /* Get only one logical processor (in case the core is SMT/hyper-threaded). */ hwloc_bitmap_singlify(cpuset); /* And try to bind ourself there. */ if (hwloc_set_cpubind(topology, cpuset, 0)) { char *str; int error = errno; hwloc_bitmap_asprintf(&str, obj->cpuset); printf("Couldn't bind to cpuset %s: %s\n", str, strerror(error)); free(str); } /* Free our cpuset copy */ hwloc_bitmap_free(cpuset); } /***************************************************************** * Sixth example: * Allocate some memory on the last NUMA node, bind some existing * memory to the last NUMA node. *****************************************************************/ /* Get last node. There's always at least one. */ n = hwloc_get_nbobjs_by_type(topology, HWLOC_OBJ_NUMANODE); obj = hwloc_get_obj_by_type(topology, HWLOC_OBJ_NUMANODE, n - 1); size = 1024*1024; m = hwloc_alloc_membind(topology, size, obj->nodeset, HWLOC_MEMBIND_BIND, HWLOC_MEMBIND_BYNODESET); hwloc_free(topology, m, size); m = malloc(size); hwloc_set_area_membind(topology, m, size, obj->nodeset, HWLOC_MEMBIND_BIND, HWLOC_MEMBIND_BYNODESET); free(m); /* Destroy topology object. */ hwloc_topology_destroy(topology); return 0; } hwloc_cpuset_t hwloc_bitmap_t hwloc_cpuset_t A CPU set is a bitmap whose bits are set according to CPU physical OS indexes. Definition: hwloc.h:161 HWLOC_OBJ_NUMANODE @ HWLOC_OBJ_NUMANODE NUMA node. An object that contains memory that is directly and byte-accessible to the host processors... Definition: hwloc.h:257 HWLOC_OBJ_PACKAGE @ HWLOC_OBJ_PACKAGE Physical package. The physical package that usually gets inserted into a socket on the motherboard.... Definition: hwloc.h:212 HWLOC_OBJ_PU @ HWLOC_OBJ_PU Processing Unit, or (Logical) Processor. An execution unit (may share a core with some other logical ... Definition: hwloc.h:222 HWLOC_OBJ_CORE @ HWLOC_OBJ_CORE Core. A computation unit (may be shared by several PUs, aka logical processors). Definition: hwloc.h:218 hwloc_topology_init int hwloc_topology_init(hwloc_topology_t *topologyp) Allocate a topology context. hwloc_topology_t struct hwloc_topology * hwloc_topology_t Topology context. Definition: hwloc.h:712 hwloc_topology_destroy void hwloc_topology_destroy(hwloc_topology_t topology) Terminate and free a topology context. hwloc_topology_load int hwloc_topology_load(hwloc_topology_t topology) Build the actual topology. hwloc_get_nbobjs_by_depth unsigned hwloc_get_nbobjs_by_depth(hwloc_topology_t topology, int depth) Returns the width of level at depth depth. hwloc_get_root_obj static hwloc_obj_t hwloc_get_root_obj(hwloc_topology_t topology) Returns the top-object of the topology-tree. hwloc_get_obj_by_depth hwloc_obj_t hwloc_get_obj_by_depth(hwloc_topology_t topology, int depth, unsigned idx) Returns the topology object at logical index idx from depth depth. hwloc_get_obj_by_type static hwloc_obj_t hwloc_get_obj_by_type(hwloc_topology_t topology, hwloc_obj_type_t type, unsigned idx) Returns the topology object at logical index idx with type type. hwloc_get_nbobjs_by_type static int hwloc_get_nbobjs_by_type(hwloc_topology_t topology, hwloc_obj_type_t type) Returns the width of level type type. hwloc_get_type_or_below_depth static int hwloc_get_type_or_below_depth(hwloc_topology_t topology, hwloc_obj_type_t type) Returns the depth of objects of type type or below. hwloc_get_type_depth int hwloc_get_type_depth(hwloc_topology_t topology, hwloc_obj_type_t type) Returns the depth of objects of type type. hwloc_topology_get_depth int hwloc_topology_get_depth(hwloc_topology_t restrict topology) Get the depth of the hierarchical tree of objects. HWLOC_TYPE_DEPTH_UNKNOWN @ HWLOC_TYPE_DEPTH_UNKNOWN No object of given type exists in the topology. Definition: hwloc.h:849 hwloc_obj_attr_snprintf int hwloc_obj_attr_snprintf(char *restrict string, size_t size, hwloc_obj_t obj, const char *restrict separator, unsigned long flags) Stringify the attributes of a given topology object into a human-readable form. hwloc_obj_type_snprintf int hwloc_obj_type_snprintf(char *restrict string, size_t size, hwloc_obj_t obj, unsigned long flags) Stringify the type of a given topology object into a human-readable form. hwloc_set_cpubind int hwloc_set_cpubind(hwloc_topology_t topology, hwloc_const_cpuset_t set, int flags) Bind current process or thread on CPUs given in physical bitmap set. hwloc_free int hwloc_free(hwloc_topology_t topology, void *addr, size_t len) Free memory that was previously allocated by hwloc_alloc() or hwloc_alloc_membind(). hwloc_alloc_membind void * hwloc_alloc_membind(hwloc_topology_t topology, size_t len, hwloc_const_bitmap_t set, hwloc_membind_policy_t policy, int flags) Allocate some memory on NUMA memory nodes specified by set. hwloc_set_area_membind int hwloc_set_area_membind(hwloc_topology_t topology, const void *addr, size_t len, hwloc_const_bitmap_t set, hwloc_membind_policy_t policy, int flags) Bind the already-allocated memory identified by (addr, len) to the NUMA node(s) specified by set. HWLOC_MEMBIND_BYNODESET @ HWLOC_MEMBIND_BYNODESET Consider the bitmap argument as a nodeset. Definition: hwloc.h:1599 HWLOC_MEMBIND_BIND @ HWLOC_MEMBIND_BIND Allocate memory on the specified nodes. Definition: hwloc.h:1511 hwloc_obj_type_is_cache int hwloc_obj_type_is_cache(hwloc_obj_type_t type) Check whether an object type is a CPU Cache (Data, Unified or Instruction). hwloc_bitmap_asprintf int hwloc_bitmap_asprintf(char **strp, hwloc_const_bitmap_t bitmap) Stringify a bitmap into a newly allocated string. hwloc_bitmap_free void hwloc_bitmap_free(hwloc_bitmap_t bitmap) Free bitmap bitmap. hwloc_bitmap_singlify int hwloc_bitmap_singlify(hwloc_bitmap_t bitmap) Keep a single index among those set in bitmap bitmap. hwloc_bitmap_dup hwloc_bitmap_t hwloc_bitmap_dup(hwloc_const_bitmap_t bitmap) Duplicate bitmap bitmap by allocating a new bitmap and copying bitmap contents. hwloc_obj Structure of a topology object. Definition: hwloc.h:423 hwloc_obj::children struct hwloc_obj ** children Normal children, children[0 .. arity -1]. Definition: hwloc.h:483 hwloc_obj::nodeset hwloc_nodeset_t nodeset NUMA nodes covered by this object or containing this object. Definition: hwloc.h:567 hwloc_obj::os_index unsigned os_index OS-provided physical index number. It is not guaranteed unique across the entire machine,... Definition: hwloc.h:428 hwloc_obj::cpuset hwloc_cpuset_t cpuset CPUs covered by this object. Definition: hwloc.h:539 hwloc_obj::arity unsigned arity Number of normal children. Memory, Misc and I/O children are not listed here but rather in their dedi... Definition: hwloc.h:479 hwloc_obj::type hwloc_obj_type_t type Type of object. Definition: hwloc.h:425 hwloc_obj::attr union hwloc_obj_attr_u * attr Object type-specific Attributes, may be NULL if no attribute value was found. Definition: hwloc.h:442 hwloc_obj::parent struct hwloc_obj * parent Parent, NULL if root (Machine object) Definition: hwloc.h:473 hwloc_obj_attr_u::cache struct hwloc_obj_attr_u::hwloc_cache_attr_s cache hwloc_obj_attr_u::hwloc_cache_attr_s::size hwloc_uint64_t size Size of cache in bytes. Definition: hwloc.h:644 hwloc provides a pkg-config executable to obtain relevant compiler and linker flags. For example, it can be used thusly to compile applications that utilize the hwloc library (assuming GNU Make): CFLAGS += $(shell pkg-config --cflags hwloc) LDLIBS += $(shell pkg-config --libs hwloc) hwloc-hello: hwloc-hello.c $(CC) hwloc-hello.c $(CFLAGS) -o hwloc-hello $(LDLIBS) On a machine 2 processor packages -- each package of which has two processing cores -- the output from running hwloc-hello could be something like the following: shell$ ./hwloc-hello *** Objects at level 0 Index 0: Machine *** Objects at level 1 Index 0: Package#0 Index 1: Package#1 *** Objects at level 2 Index 0: Core#0 Index 1: Core#1 Index 2: Core#3 Index 3: Core#2 *** Objects at level 3 Index 0: PU#0 Index 1: PU#1 Index 2: PU#2 Index 3: PU#3 *** Printing overall tree Machine Package#0 Core#0 PU#0 Core#1 PU#1 Package#1 Core#3 PU#2 Core#2 PU#3 *** 2 package(s) *** Logical processor 0 has 0 caches totaling 0KB shell$ History / Credits hwloc is the evolution and merger of the libtopology project and the Portable Linux Processor Affinity (PLPA) (https://www.open-mpi.org/projects/plpa/) project. Because of functional and ideological overlap, these two code bases and ideas were merged and released under the name "hwloc" as an Open MPI sub-project. libtopology was initially developed by the Inria Runtime Team-Project. PLPA was initially developed by the Open MPI development team as a sub-project. Both are now deprecated in favor of hwloc, which is distributed as an Open MPI sub-project. Further Reading The documentation chapters include * Terms and Definitions * Command-Line Tools * Environment Variables * CPU and Memory Binding Overview * I/O Devices * Miscellaneous objects * Object attributes * Topology Attributes: Distances, Memory Attributes and CPU Kinds * Importing and exporting topologies from/to XML files * Synthetic topologies * Interoperability With Other Software * Thread Safety * Components and plugins * Embedding hwloc in Other Software * Frequently Asked Questions (FAQ) * Upgrading to the hwloc 2.0 API Make sure to have had a look at those too! See https://www.open-mpi.org/projects/hwloc/doc/ for more hwloc documentation, actual links to related pages, images, etc.