HWLOC(7) 2.3.0 HWLOC(7)
hwloc hwloc
Sep 28, 2020
NAME
hwloc - General information about hwloc ("hardware locality").
DESCRIPTION
hwloc provides command line tools and a C API to obtain the
hierarchical map of key computing elements, such as: NUMA memory
nodes, shared caches, processor packages, processor cores, and
processor "threads". hwloc also gathers various attributes such as
cache and memory information, and is portable across a variety of
different operating systems and platforms.
Definitions
hwloc has some specific definitions for terms that are used in this
man page and other hwloc documentation.
hwloc CPU set:
A set of processors included in an hwloc object, expressed as a
bitmask indexed by the physical numbers of the CPUs (as announced
by the OS). The hwloc definition of "CPU set" does not carry any
of the same connotations as Linux's "CPU set" (e.g., process
affinity, cgroup, etc.).
hwloc node set:
A set of NUMA memory nodes near an hwloc object, expressed as a
bitmask indexed by the physical numbers of the NUMA nodes (as
announced by the OS).
Linux CPU set:
See http://www.mjmwired.net/kernel/Documentation/cpusets.txt for
a discussion of Linux CPU sets. A super-short-ignoring-many-
details description (taken from that page) is:
"Cpusets provide a mechanism for assigning a set of CPUs and
Memory Nodes to a set of tasks."
Linux Cgroup:
See http://www.mjmwired.net/kernel/Documentation/cgroups.txt for
a discussion of Linux control groups. A super-short-ignoring-
many-details description (taken from that page) is:
"Control Groups provide a mechanism for aggregating/partitioning
sets of tasks, and all their future children, into hierarchical
groups with specialized behaviour."
To be clear, hwloc supports all of the above concepts. It is simply
worth noting that they are different things.
Location Specification
Locations refer to specific regions within a topology. Before reading
- 1 - Formatted: October 24, 2025
HWLOC(7) 2.3.0 HWLOC(7)
hwloc hwloc
Sep 28, 2020
the rest of this man page, it may be useful to read lstopo(1) and/or
run lstopo on your machine to see the reported topology tree. Seeing
and understanding a topology tree will definitely help in
understanding the concepts that are discussed below.
Locations can be specified in multiple ways:
Tuples: Tuples of hwloc "objects" and associated indexes can be
specified in the form object:index. hwloc objects represent
types of mapped items (e.g., packages, cores, etc.) in a
topology tree; indexes are non-negative integers that
specify a unique physical object in a topology tree. Both
concepts are described in detail, below.
Indexes may also be specified as ranges. x-y enumerates
from index x to y. x:y enumerates y objects starting from
index x (wrapping around the end of the index range if
needed). x- enumerates all objects starting from index x.
all, odd, and even are also supported for listing all
objects, or only those with odd or even indexes.
Chaining multiple tuples together in the more general form
object1:index[.object2:index2[...]] is permissable. While
the first tuple's object may appear anywhere in the
topology, the Nth tuple's object must have a shallower
topology depth than the (N+1)th tuple's object. Put simply:
as you move right in a tuple chain, objects must go deeper
in the topology tree. When using logical indexes (which is
the default), indexes specified in chained tuples are
relative to the scope of the parent object. For example,
"package:0.core:1" refers to the second core in the first
package.
When using OS/physical indexes, the first object matching
the given index is used.
PCI and OS devices may also be designed using their
identifier. For example, "pci=02:03.1" is the PCI device
with bus ID "02:03.1". "os=eth0" is the network interface
whose software name is "eth0". PCI devices may also be
filtered based on their vendor and/or device IDs, for
instance "pci[15b3:]:2" for the third Mellanox PCI device
(vendor ID 0x15b3). OS devices may also be filtered based
on their subtype, for instance "os[gpu]:all" for all GPU OS
devices.
Hex: For tools that manipulate object as sets (e.g. hwloc-calc
and hwloc-bind), locations can also be specified as
hexidecimal bitmasks prefixed with "0x". Commas must be
- 2 - Formatted: October 24, 2025
HWLOC(7) 2.3.0 HWLOC(7)
hwloc hwloc
Sep 28, 2020
used to separate the hex digits into blocks of 8, such as
"0xffc0140,0x00020110". Leading zeros in each block do not
need to be specified. For example, "0xffc0140,0x20110" is
equivalent to the prior example, and "0x0000000f" is exactly
equivalent to "0xf". Intermediate blocks of 8 digits that
are all zeoro can be left empty; "0xff0,,0x13" is equivalent
to "0xff0,0x00000000,0x13". If the location is prefixed
with the special string "0xf...f", then all unspecified bits
are set (as if the set were infinite). For example,
"0xf...f,0x1" sets both the first bit and all bits starting
with the 33rd. The string "0xf...f" -- with no other
specified values -- sets all bits.
"all" and "root" are special locations consisting in the root object
in tree. It contains the entire current topology.
Some tools directly operate on these objects (e.g. hwloc-info and
hwloc-annotate). They do not support hexadecimal locations because
each location may correspond to multiple objects. For instance, there
can be exactly one L3 cache per package and NUMA node, which means
it's the same location. If multiple locations are given on the
command-line, these tools will operation on each location individually
and consecutively.
Some other tools internally manipulate objects as sets (e.g. hwloc-
calc and hwloc-bind). They translate each input location into a
hexidecimal location. When I/O or Misc objects are used, they are
translated into the set of processors (or NUMA nodes) that are close
to the given object (because I/O or Misc objects do not contain
processors or NUMA nodes).
If multiple locations are specified on the command-line (delimited by
whitespace), they are combined (the overall location is wider). If
prefixed with "~", the given location will be cleared instead of added
to the current list of locations. If prefixed with "x", the given
location will be and'ed instead of added to the current list. If
prefixed with "^", the given location will be xor'ed.
More complex operations may be performed by using hwloc-calc to
compute intermediate values.
hwloc Objects
Objects in tuples can be any of the following strings (listed from
"biggest" to "smallest"):
machine A set of processors and memory.
numanode A NUMA node; a set of processors around memory which the
processors can directly access. If hbm is used instead of
- 3 - Formatted: October 24, 2025
HWLOC(7) 2.3.0 HWLOC(7)
hwloc hwloc
Sep 28, 2020
numanode in locations, command-line tools only consider
high-bandwidth memory nodes such as Intel Xeon Phi MCDRAM.
package Typically a physical package or chip, that goes into a
package, it is a grouping of one or more processors.
l1cache ... l5cache
A data (or unified) cache.
l1icache ... l3icache
An instruction cache.
core A single, physical processing unit which may still contain
multiple logical processors, such as hardware threads.
pu Short for processor unit (not process!). The smallest
physical execution unit that hwloc recognizes. For example,
there may be multiple PUs on a core (e.g., hardware
threads).
osdev, pcidev, bridge, and misc may also be used to specify special
devices although some of them have dedicated identification ways as
explained in Location Specification.
Finally, note that an object can be denoted by its numeric "depth" in
the topology graph.
hwloc Indexes
Indexes are integer values that uniquely specify a given object of a
specific type. Indexes can be expressed either as logical values or
physical values. Most hwloc utilities accept logical indexes by
default. Passing --physical switches to physical/OS indexes. Both
logical and physical indexes are described on this man page.
Logical indexes are relative to the object order in the output from
the lstopo command. They always start with 0 and increment by 1 for
each successive object.
Physical indexes are how the operating system refers to objects. Note
that while physical indexes are non-negative integer values, the
hardware and/or operating system may choose arbitrary values -- they
may not start with 0, and successive objects may not have consecutive
values.
For example, if the first few lines of lstopo -p output are the
following:
Machine (47GB)
NUMANode P#0 (24GB) + Package P#0 + L3 (12MB)
- 4 - Formatted: October 24, 2025
HWLOC(7) 2.3.0 HWLOC(7)
hwloc hwloc
Sep 28, 2020
L2 (256KB) + L1 (32KB) + Core P#0 + PU P#0
L2 (256KB) + L1 (32KB) + Core P#1 + PU P#0
L2 (256KB) + L1 (32KB) + Core P#2 + PU P#0
L2 (256KB) + L1 (32KB) + Core P#8 + PU P#0
L2 (256KB) + L1 (32KB) + Core P#9 + PU P#0
L2 (256KB) + L1 (32KB) + Core P#10 + PU P#0
NUMANode P#1 (24GB) + Package P#1 + L3 (12MB)
L2 (256KB) + L1 (32KB) + Core P#0 + PU P#0
L2 (256KB) + L1 (32KB) + Core P#1 + PU P#0
L2 (256KB) + L1 (32KB) + Core P#2 + PU P#0
L2 (256KB) + L1 (32KB) + Core P#8 + PU P#0
L2 (256KB) + L1 (32KB) + Core P#9 + PU P#0
L2 (256KB) + L1 (32KB) + Core P#10 + PU P#0
In this example, the first core on the second package is logically
number 6 (i.e., logically the 7th core, starting from 0). Its
physical index is 0, but note that another core also has a physical
index of 0. Hence, physical indexes may only be relevant within the
scope of their parent (or set of ancestors). In this example, to
uniquely identify logical core 6 with physical indexes, you must
specify (at a minimum) both a package and a core: package 1, core 0.
Index values, regardless of whether they are logical or physical, can
be expressed in several different forms (where X, Y, and N are
positive integers):
X The object with index value X.
X-Y All the objects with index values >= X and <= Y.
X- All the objects with index values >= X.
X:N N objects starting with index X, possibly wrapping around
the end of the level.
all A special index value indicating all valid index values.
odd A special index value indicating all valid odd index values.
even A special index value indicating all valid even index
values.
REMEMBER: hwloc's command line tools accept logical indexes for
location values by default. Use --physical and --logical to switch
from one mode to another.
SEE ALSO
hwloc's command line tool documentation: lstopo(1), hwloc-bind(1),
hwloc-calc(1), hwloc-distrib(1), hwloc-ps(1).
- 5 - Formatted: October 24, 2025
HWLOC(7) 2.3.0 HWLOC(7)
hwloc hwloc
Sep 28, 2020
hwloc has many C API functions, each of which have their own man page.
Some top-level man pages are also provided, grouping similar functions
together. A few good places to start might include:
hwlocality_objects(3), hwlocality_types(3), hwlocality_creation(3),
hwlocality_cpuset(3), hwlocality_information(3), and
hwlocality_binding(3).
For a listing of all available hwloc man pages, look at all "hwloc*"
files in the man1 and man3 directories.
- 6 - Formatted: October 24, 2025
