Comparing memory node attributes for finding where to allocate on

## Data Structures

struct | hwloc_location

## Typedefs

typedef unsigned | hwloc_memattr_id_t

## Enumerations

enum | hwloc_memattr_id_e { HWLOC_MEMATTR_ID_CAPACITY = 0 , HWLOC_MEMATTR_ID_LOCALITY = 1 , HWLOC_MEMATTR_ID_BANDWIDTH = 2 , HWLOC_MEMATTR_ID_LATENCY = 3 }
enum | hwloc_location_type_e { HWLOC_LOCATION_TYPE_CPUSET , HWLOC_LOCATION_TYPE_OBJECT }
enum | hwloc_local_numanode_flag_e { HWLOC_LOCAL_NUMANODE_FLAG_LARGER_LOCALITY , HWLOC_LOCAL_NUMANODE_FLAG_SMALLER_LOCALITY , HWLOC_LOCAL_NUMANODE_FLAG_ALL }

## Functions

int | hwloc_memattr_get_by_name (hwloc_topology_t topology, const char *name, hwloc_memattr_id_t *id)
int | hwloc_get_local_numanode_objs (hwloc_topology_t topology, struct hwloc_location *location, unsigned *nr, hwloc_obj_t *nodes, unsigned long flags)
int | hwloc_memattr_get_value (hwloc_topology_t topology, hwloc_memattr_id_t attribute, hwloc_obj_t target_node, struct hwloc_location *initiator, unsigned long flags, hwloc_uint64_t *value)
int | hwloc_memattr_get_best_target (hwloc_topology_t topology, hwloc_memattr_id_t attribute, struct hwloc_location *initiator, unsigned long flags, hwloc_obj_t *best_target, hwloc_uint64_t *value)
int | hwloc_memattr_get_best_initiator (hwloc_topology_t topology, hwloc_memattr_id_t attribute, hwloc_obj_t target, unsigned long flags, struct hwloc_location *best_initiator, hwloc_uint64_t *value)

## Detailed Description

Platforms with heterogeneous memory require ways to decide whether a buffer should be allocated on "fast" memory (such as HBM), "normal" memory (DDR) or even "slow" but large-capacity memory (non-volatile memory). These memory nodes are called "Targets" while the CPU accessing them is called the "Initiator". Access performance depends on their locality (NUMA platforms) as well as the intrinsic performance of the targets (heterogeneous platforms).

The following attributes describe the performance of memory accesses from an Initiator to a memory Target, for instance their latency or bandwidth. Initiators performing these memory accesses are usually some PUs or Cores (described as a CPU set). Hence a Core may choose where to allocate a memory buffer by comparing the attributes of different target memory nodes nearby.

There are also some attributes that are system-wide. Their value does not depend on a specific initiator performing an access. The memory node Capacity is an example of such attribute without initiator.

One way to use this API is to start with a cpuset describing the Cores where a program is bound. The best target NUMA node for allocating memory in this program on these Cores may be obtained by passing this cpuset as an initiator to hwloc_memattr_get_best_target() with the relevant memory attribute. For instance, if the code is latency limited, use the Latency attribute.

A more flexible approach consists in getting the list of local NUMA nodes by passing this cpuset to hwloc_get_local_numanode_objs(). Attribute values for these nodes, if any, may then be obtained with hwloc_memattr_get_value() and manually compared with the desired criteria.

Note
  The API also supports specific objects as initiator, but it is currently not used internally by hwloc. Users may for instance use it to provide custom performance values for host memory accesses performed by GPUs.
  The interface actually also accepts targets that are not NUMA nodes.

## Typedef Documentation

## hwloc_memattr_id_t

typedef unsigned hwloc_memattr_id_t

A memory attribute identifier. May be either one of hwloc_memattr_id_e or a new id returned by hwloc_memattr_register().

## Enumeration Type Documentation

## hwloc_local_numanode_flag_e

enum hwloc_local_numanode_flag_e

Flags for selecting target NUMA nodes.

Enumerator
HWLOC_LOCAL_NUMANODE_FLAG_LARGER_LOCALITY | Select NUMA nodes whose locality is larger than the given cpuset. For instance, if a single PU (or its cpuset) is given in `initiator`, select all nodes close to the package that contains this PU.
HWLOC_LOCAL_NUMANODE_FLAG_SMALLER_LOCALITY | Select NUMA nodes whose locality is smaller than the given cpuset. For instance, if a package (or its cpuset) is given in `initiator`, also select nodes that are attached to only a half of that package.
HWLOC_LOCAL_NUMANODE_FLAG_ALL | Select all NUMA nodes in the topology. The initiator `initiator` is ignored.

## hwloc_location_type_e

enum hwloc_location_type_e

Type of location.

Enumerator
HWLOC_LOCATION_TYPE_CPUSET | Location is given as a cpuset, in the location cpuset union field.
HWLOC_LOCATION_TYPE_OBJECT | Location is given as an object, in the location object union field.

## hwloc_memattr_id_e

enum hwloc_memattr_id_e

Memory node attributes.

Enumerator
HWLOC_MEMATTR_ID_CAPACITY | "Capacity". The capacity is returned in bytes (local_memory attribute in objects). Best capacity nodes are nodes with higher capacity. No initiator is involved when looking at this attribute. The corresponding attribute flags are HWLOC_MEMATTR_FLAG_HIGHER_FIRST.

HWLOC_MEMATTR_ID_LOCALITY

"Locality". The locality is returned as the number of PUs in that locality (e.g. the weight of its cpuset).

Best locality nodes are nodes with smaller locality (nodes that are local to very few PUs). Poor locality nodes are nodes with larger locality (nodes that are local to the entire machine).

No initiator is involved when looking at this attribute. The corresponding attribute flags are HWLOC_MEMATTR_FLAG_HIGHER_FIRST.

HWLOC_MEMATTR_ID_BANDWIDTH | "Bandwidth". The bandwidth is returned in MiB/s, as seen from the given initiator location. Best bandwidth nodes are nodes with higher bandwidth. The corresponding attribute flags are HWLOC_MEMATTR_FLAG_HIGHER_FIRST and HWLOC_MEMATTR_FLAG_NEED_INITIATOR.
HWLOC_MEMATTR_ID_LATENCY | "Latency". The latency is returned as nanoseconds, as seen from the given initiator location. Best latency nodes are nodes with smaller latency. The corresponding attribute flags are HWLOC_MEMATTR_FLAG_LOWER_FIRST and HWLOC_MEMATTR_FLAG_NEED_INITIATOR.

## Function Documentation

## hwloc_get_local_numanode_objs()

int hwloc_get_local_numanode_objs | ( | hwloc_topology_t | topology,
|  | struct hwloc_location * | location,
|  | unsigned * | nr,
|  | hwloc_obj_t * | nodes,
|  | unsigned long | flags
| ) |  |

Return an array of local NUMA nodes.

By default only select the NUMA nodes whose locality is exactly the given `location`. More nodes may be selected if additional flags are given as a OR'ed set of hwloc_local_numanode_flag_e.

If `location` is given as an explicit object, its CPU set is used to find NUMA nodes with the corresponding locality. If the object does not have a CPU set (e.g. I/O object), the CPU parent (where the I/O object is attached) is used.

On input, `nr` points to the number of nodes that may be stored in the `nodes` array. On output, `nr` will be changed to the number of stored nodes, or the number of nodes that would have been stored if there were enough room.

Note
  Some of these NUMA nodes may not have any memory attribute values and hence not be reported as actual targets in other functions.
  The number of NUMA nodes in the topology (obtained by hwloc_bitmap_weight() on the root object nodeset) may be used to allocate the `nodes` array.
  When an object CPU set is given as locality, for instance a Package, and when flags contain both HWLOC_LOCAL_NUMANODE_FLAG_LARGER_LOCALITY and HWLOC_LOCAL_NUMANODE_FLAG_SMALLER_LOCALITY, the returned array corresponds to the nodeset of that object.

## hwloc_memattr_get_best_initiator()

int hwloc_memattr_get_best_initiator | ( | hwloc_topology_t | topology,
|  | hwloc_memattr_id_t | attribute,
|  | hwloc_obj_t | target,
|  | unsigned long | flags,
|  | struct hwloc_location * | best_initiator,
|  | hwloc_uint64_t * | value
| ) |  |

Return the best initiator for the given attribute and target NUMA node.

If the attribute does not relate to a specific initiator (it does not have the flag HWLOC_MEMATTR_FLAG_NEED_INITIATOR), `-1` is returned and `errno` is set to `EINVAL`.

If `value` is non `NULL`, the corresponding value is returned there.

If multiple initiators have the same attribute values, only one is returned (and there is no way to clarify how that one is chosen). Applications that want to detect initiators with identical/similar values, or that want to look at values for multiple attributes, should rather get all values using hwloc_memattr_get_value() and manually select the initiator they consider the best.

The returned initiator should not be modified or freed, it belongs to the topology.

`flags` must be `0` for now.

If there are no matching initiators, `-1` is returned with `errno` set to `ENOENT`;

## hwloc_memattr_get_best_target()

int hwloc_memattr_get_best_target | ( | hwloc_topology_t | topology,
|  | hwloc_memattr_id_t | attribute,
|  | struct hwloc_location * | initiator,
|  | unsigned long | flags,
|  | hwloc_obj_t * | best_target,
|  | hwloc_uint64_t * | value
| ) |  |

Return the best target NUMA node for the given attribute and initiator.

If the attribute does not relate to a specific initiator (it does not have the flag HWLOC_MEMATTR_FLAG_NEED_INITIATOR), location `initiator` is ignored and may be `NULL`.

If `value` is non `NULL`, the corresponding value is returned there.

If multiple targets have the same attribute values, only one is returned (and there is no way to clarify how that one is chosen). Applications that want to detect targets with identical/similar values, or that want to look at values for multiple attributes, should rather get all values using hwloc_memattr_get_value() and manually select the target they consider the best.

`flags` must be `0` for now.

If there are no matching targets, `-1` is returned with `errno` set to `ENOENT`;

Note
  The initiator `initiator` should be of type HWLOC_LOCATION_TYPE_CPUSET when refering to accesses performed by CPU cores. HWLOC_LOCATION_TYPE_OBJECT is currently unused internally by hwloc, but users may for instance use it to provide custom information about host memory accesses performed by GPUs.

## hwloc_memattr_get_by_name()

int hwloc_memattr_get_by_name | ( | hwloc_topology_t | topology,
|  | const char * | name,
|  | hwloc_memattr_id_t * | id
| ) |  |

Return the identifier of the memory attribute with the given name.

## hwloc_memattr_get_value()

int hwloc_memattr_get_value | ( | hwloc_topology_t | topology,
|  | hwloc_memattr_id_t | attribute,
|  | hwloc_obj_t | target_node,
|  | struct hwloc_location * | initiator,
|  | unsigned long | flags,
|  | hwloc_uint64_t * | value
| ) |  |

Return an attribute value for a specific target NUMA node.

If the attribute does not relate to a specific initiator (it does not have the flag HWLOC_MEMATTR_FLAG_NEED_INITIATOR), location `initiator` is ignored and may be `NULL`.

`flags` must be `0` for now.

Note
  The initiator `initiator` should be of type HWLOC_LOCATION_TYPE_CPUSET when refering to accesses performed by CPU cores. HWLOC_LOCATION_TYPE_OBJECT is currently unused internally by hwloc, but users may for instance use it to provide custom information about host memory accesses performed by GPUs.