Components and plugins

hwloc is organized in components that are responsible for discovering objects. Depending on the topology configuration, some components will be used, some will be ignored. The usual default is to enable the native operating system component, (e.g. `linux` or `solaris`) and the `pci` miscellaneous component. If available, an architecture-specific component (such as `x86`) may also improve the topology detection.

If a XML topology is loaded, the `xml` discovery component will be used instead of all other components. It internally uses a specific class of components for the actual XML import/export routines (`xml_libxml` and `xml_nolibxml`) but these will not be discussed here (see libxml2 and minimalistic XML backends).

# Components enabled by default

The hwloc core contains a list of components sorted by priority. Each one is enabled as long as it does not conflict with the previously enabled ones. This includes native operating system components, architecture-specific ones, and if available, I/O components such as `pci`.

Usually the native operating system component (when it exists, e.g. `linux` or `aix`) is enabled first. Then hwloc looks for an architecture specific component (e.g. `x86`). Finally these also exist a basic component (`no_os`) that just tries to discover the number of PUs in the system.

Each component discovers as much topology information as possible. Most of them, including most native OS components, do nothing unless the topology is still empty. Some others, such as `x86` and `pci`, can complete and annotate what other backends still earlier.

Default priorities ensure that clever components are invoked first. Native operating system components have higher priorities, and are therefore invoked first, because they likely offer very detailed topology information. If needed, it will be later extended by architecture-specific information (e.g. from the `x86` component).

If any configuration function such as hwloc_topology_set_xml() is used before loading the topology, the corresponding component is enabled first. Then, as usual, hwloc enables any other component (based on priorities) that does not conflict.

Certain components that manage a virtual topology, for instance XML topology import, synthetic topology description, or custom building, conflict with all other components. Therefore, one of them may only be loaded (e.g. with `hwloc_topology_set_xml()`) if no other component is enabled.

The environment variable `HWLOC_COMPONENTS_VERBOSE` may be set to get verbose messages about component registration (including their priority) and enabling.

# Selecting which components to use

Once topology configuration functions such as `hwloc_topology_set_custom()` have been taken care of, the priority order of the remaining components may be changed through the `HWLOC_COMPONENTS` environment variable (component names must be separated by commas).

Specifying `x86` in this variable will cause the `x86` component to take precedence over any other component, including the native operating system component. It is therefore loaded first, before hwloc tries to load all remaining non-conflicting components. In this case, `x86` would take care of discovering everything it supports, instead of only completing what the native OS information. This may be useful if the native component is buggy on some platforms.

It is possible to prevent some components from being loaded by prefixing their name with `-` in the list. For instance `x86,-pci` will load the `x86` component, then let hwloc load all the usual components except `pci`.

It is possible to prevent all remaining components from being loaded by placing `stop` in the environment variable. Only the components listed before this keyword will be enabled.

Certain component names (`xml` and `synthetic`) accept an argument (e.g. `xml=file.xml`). These arguments behave exactly as if the corresponding string had been passed to `hwloc_topology_set_xml()` or `hwloc_topology_set_synthetic()`.

# Loading components from plugins

Components may optionally be built as plugins so that the hwloc core library does not directly depend on their dependencies (for instance the `libpci` library). Plugin support may be enabled with the `–enable-plugins` configure option. All components buildable as plugins will then be built as plugins. The configure option may be given a comma-separated list of component names to specify the exact list of components to build as plugins.

Plugins are built as independent dynamic libraries that are installed in `$libdir/hwloc`. All plugins found in this directory are loaded during `topology_init()`. A specific list of directories (colon-separated) to scan may be specified in the `HWLOC_PLUGINS_PATH` environment variable.

Note that loading a plugin just means that the corresponding component is registered to the hwloc core. Components are then only enabled if the topology configuration requests it, as explained in the previous sections.

Also note that plugins should carefully be enabled and used when embedding hwloc in another project, see Embedding hwloc in Other Software for details.

# Adding new discovery components and plugins

The types and functions cited below are declared in the hwloc/plugins.h header. Components are supposed to only use hwloc public headers (hwloc.h and anything under the `include/hwloc` subdirectory) and nothing from the `include/private` subdirectory in the source tree.

## Basics of discovery components

Each discovery component is defined by a `hwloc_disc_component` structure which contains an `instantiate()` callback. This function is invoked when this component is actually used by a topology. It fills a new `hwloc_backend` structure that usually contains `discover()` and/or `notify_new_object()` callbacks taking care of the actual topology discovery.

Note
  If two discovery components have the same name, only the highest priority one is actually made available. This offers a way for third-party plugins to override existing components.

## Registering a new discovery component

Registering components to the hwloc core relies on a `hwloc_component` structure. Its `data` field points to the previously defined `hwloc_disc_component` structure while its `type` should be HWLOC_COMPONENT_TYPE_DISC. This structure should be named `hwloc_<name>_component`.

The configure script should be modified to add `<name>` to its `hwloc_components` shell variable so that the component is actually available.

Note
  The symbol name of the `hwloc_component` structure is independent of the name of the discovery component mentioned in the previous section.

When the component is statically built inside the hwloc library, the symbol `hwloc_<name>_component` is added by configure to the `src/static-components.h`. The core then registers all components listed in this file.

If the new component may be built as a plugin, the configure script should also define the shell variable `hwloc_<name>_component_maybeplugin=1`. When the configure script actually enables the component as a plugin, it will set the variable `hwloc_<name>_component` to `plugin`. The build system may then use this variable to change the way the component is built. It should create a `hwloc_<name>.so` shared object. All these files are loaded in alphabetic order, and the components they contain are registered to the hwloc core.

# Existing components and plugins

All components distributed within hwloc are listed below. The list of actually available components may be listed at running with the `HWLOC_COMPONENTS_VERBOSE` environment variable (see Environment Variables).

aix, darwin, freebsd, hpux, linux, netbsd, osf, solaris, windows
  Each officially supported operating system has its own native component, which is statically built when supported, and which is used by default.
x86
  The x86 architecture (either 32 or 64 bits) has its own component that may complete or replace the previously-found CPU information. It is statically built when supported.
bgq
  This component is specific to IBM BlueGene/Q compute node (running CNK). It is built and enabled by default when `–host=powerpc64-bgq-linux` is passed to configure (see How do I build hwloc for BlueGene/Q?).
no_os
  A basic component that just tries to detect the number of processing units in the system. It mostly serves on operating systems that are not natively supported. It is always statically built.
pci
  PCI object discovery uses the external pciaccess library (aka libpciaccess), or optionally the pciutils library (libpci), see I/O Devices. It may be built as a plugin.
opencl
  The OpenCL component creates co-processor OS device objects such as opencl0d0 (first device of the first OpenCL platform) or opencl1d3 (fourth device of the second platform). Only the AMD OpenCL implementation currently offers locality information. It may be built as a plugin.
cuda
  This component creates co-processor OS device objects such as cuda0 that correspond to NVIDIA GPUs used with CUDA library. It may be built as a plugin.
nvml
  Probing the NVIDIA Management Library creates OS device objects such as nvml0 that are useful for batch schedulers. It also detects the actual PCIe link bandwidth without depending on power management state and without requiring administrator privileges. It may be built as a plugin.
gl
  Probing the NV-CONTROL X extension (NVCtrl library) creates OS device objects such as :0.0 corresponding to NVIDIA displays. They are useful for graphical applications that need to place computation and/or data near a rendering GPU. It may be built as a plugin.
synthetic
  Synthetic topology support (see Synthetic Topologies) is always built statically.
custom
  Custom topology support (see Multi-node Topologies) is always built statically.
xml
  XML topology import (see Importing and exporting topologies from/to XML files) is always built statically. It internally uses one of the XML backends (see libxml2 and minimalistic XML backends).
  - xml_nolibxml is a basic and hwloc-specific XML import/export. It is always statically built.
  - xml_libxml relies on the external libxml2 library for provinding a feature-complete XML import/export. It may be built as a plugin.
fake
  A dummy plugin that does nothing but is used for debugging plugin support.