Hardware Locality (hwloc): CPU binding

Hardware Locality (hwloc) 2.14.0rc1

Enumerations | Functions
CPU binding

## Enumerations
enum | hwloc_cpubind_flags_t { HWLOC_CPUBIND_PROCESS , HWLOC_CPUBIND_THREAD , HWLOC_CPUBIND_STRICT , HWLOC_CPUBIND_NOMEMBIND }

## Functions
int | hwloc_set_cpubind (hwloc_topology_t topology, hwloc_const_cpuset_t set, int flags)
int | hwloc_get_cpubind (hwloc_topology_t topology, hwloc_cpuset_t set, int flags)
int | hwloc_set_proc_cpubind (hwloc_topology_t topology, hwloc_pid_t pid, hwloc_const_cpuset_t set, int flags)
int | hwloc_get_proc_cpubind (hwloc_topology_t topology, hwloc_pid_t pid, hwloc_cpuset_t set, int flags)
int | hwloc_set_thread_cpubind (hwloc_topology_t topology, hwloc_thread_t thread, hwloc_const_cpuset_t set, int flags)
int | hwloc_get_thread_cpubind (hwloc_topology_t topology, hwloc_thread_t thread, hwloc_cpuset_t set, int flags)
int | hwloc_get_last_cpu_location (hwloc_topology_t topology, hwloc_cpuset_t set, int flags)
int | hwloc_get_proc_last_cpu_location (hwloc_topology_t topology, hwloc_pid_t pid, hwloc_cpuset_t set, int flags)

## Detailed Description

Some operating systems only support binding threads or processes to a single PU. Others allow binding to larger sets such as entire Cores or Packages or even random sets of individual PUs. In such operating system, the scheduler is free to run the task on one of these PU, then migrate it to another PU, etc. It is often useful to call hwloc_bitmap_singlify() on the target CPU set before passing it to the binding function to avoid these expensive migrations. See the documentation of hwloc_bitmap_singlify() for details.

Some operating systems do not provide all hwloc-supported mechanisms to bind processes, threads, etc. hwloc_topology_get_support() may be used to query about the actual CPU binding support in the currently used operating system.

When the requested binding operation is not available and the HWLOC_CPUBIND_STRICT flag was passed, the function returns -1. `errno` is set to `ENOSYS` when it is not possible to bind the requested kind of object processes/threads. errno is set to `EXDEV` when the requested cpuset can not be enforced (e.g. some systems only allow one CPU, and some other systems only allow one NUMA node).

If HWLOC_CPUBIND_STRICT was not passed, the function may fail as well, or the operating system may use a slightly different operation (with side-effects, smaller binding set, etc.) when the requested operation is not exactly supported.

The most portable version that should be preferred over the others, whenever possible, is the following one which just binds the current program, assuming it is single-threaded:

hwloc_set_cpubind(topology, set, 0),
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.

If the program may be multithreaded, the following one should be preferred to only bind the current thread:

hwloc_set_cpubind(topology, set, HWLOC_CPUBIND_THREAD),
HWLOC_CPUBIND_THREAD
@ HWLOC_CPUBIND_THREAD
Bind current thread of current process.
Definition hwloc.h:1262

See also
  Some example codes are available under doc/examples/ in the source tree.

Note
  To unbind, just call the binding function with either a full cpuset or a cpuset equal to the system cpuset.
  On some operating systems, CPU binding may have effects on memory binding, see HWLOC_CPUBIND_NOMEMBIND
  Running lstopo --top or hwloc-ps can be a very convenient tool to check how binding actually happened.

## Enumeration Type Documentation

## ◆ hwloc_cpubind_flags_t

enum hwloc_cpubind_flags_t

Process/Thread binding flags.

These bit flags can be used to refine the binding policy.

The default (0) is to bind the current process, assumed to be single-threaded, in a non-strict way. This is the most portable way to bind as all operating systems usually provide it.

Note
  Not all systems support all kinds of binding. See the "Detailed Description" section of CPU binding for a description of errors that can occur.

Enumerator
HWLOC_CPUBIND_PROCESS | Bind all threads of the current (possibly) multithreaded process.
HWLOC_CPUBIND_THREAD | Bind current thread of current process.
HWLOC_CPUBIND_STRICT | Request for strict binding from the OS. By default, when the designated CPUs are all busy while other CPUs are idle, operating systems may execute the thread/process on those other CPUs instead of the designated CPUs, to let them progress anyway. Strict binding means that the thread/process will _never_ execute on other CPUs than the designated CPUs, even when those are busy with other tasks and other CPUs are idle. Note Depending on the operating system, strict binding may not be possible (e.g., the OS does not implement it) or not allowed (e.g., for an administrative reasons), and the function will fail in that case. When retrieving the binding of a process, this flag checks whether all its threads actually have the same binding. If the flag is not given, the binding of each thread will be accumulated. Note This flag is meaningless when retrieving the binding of a thread.
HWLOC_CPUBIND_NOMEMBIND | Avoid any effect on memory binding. On some operating systems, some CPU binding function would also bind the memory on the corresponding NUMA node. It is often not a problem for the application, but if it is, setting this flag will make hwloc avoid using OS functions that would also bind memory. This will however reduce the support of CPU bindings, i.e. potentially return -1 with errno set to `ENOSYS` in some cases. This flag is only meaningful when used with functions that set the CPU binding. It is ignored when used with functions that get CPU binding information.

## Function Documentation

## ◆ hwloc_get_cpubind()

int hwloc_get_cpubind | ( | hwloc_topology_t | topology,
|  | hwloc_cpuset_t | set,
|  | int | flags )

Get current process or thread binding.

The CPU-set `set` (previously allocated by the caller) is filled with the list of PUs which the process or thread (according to flags) was last bound to.

Returns
  0 on success, -1 on error.

## ◆ hwloc_get_last_cpu_location()

int hwloc_get_last_cpu_location | ( | hwloc_topology_t | topology,
|  | hwloc_cpuset_t | set,
|  | int | flags )

Get the last physical CPU where the current process or thread ran.

The CPU-set `set` (previously allocated by the caller) is filled with the list of PUs which the process or thread (according to flags) last ran on.

The operating system may move some tasks from one processor to another at any time according to their binding, so this function may return something that is already outdated.

`flags` can include either HWLOC_CPUBIND_PROCESS or HWLOC_CPUBIND_THREAD to specify whether the query should be for the whole process (union of all CPUs on which all threads are running), or only the current thread. If the process is single-threaded, flags can be set to zero to let hwloc use whichever method is available on the underlying OS.

Returns
  0 on success, -1 on error.

## ◆ hwloc_get_proc_cpubind()

int hwloc_get_proc_cpubind | ( | hwloc_topology_t | topology,
|  | hwloc_pid_t | pid,
|  | hwloc_cpuset_t | set,
|  | int | flags )

Get the current physical binding of process `pid`.

The CPU-set `set` (previously allocated by the caller) is filled with the list of PUs which the process was last bound to.

Returns
  0 on success, -1 on error.

Note
  `hwloc_pid_t` is `pid_t` on Unix platforms, and `HANDLE` on native Windows platforms.
  As a special case on Linux, if a tid (thread ID) is supplied instead of a pid (process ID) and HWLOC_CPUBIND_THREAD is passed in flags, the binding for that specific thread is returned.
  On non-Linux systems, HWLOC_CPUBIND_THREAD can not be used in `flags`.

## ◆ hwloc_get_proc_last_cpu_location()

int hwloc_get_proc_last_cpu_location | ( | hwloc_topology_t | topology,
|  | hwloc_pid_t | pid,
|  | hwloc_cpuset_t | set,
|  | int | flags )

Get the last physical CPU where a process ran.

The CPU-set `set` (previously allocated by the caller) is filled with the list of PUs which the process last ran on.

The operating system may move some tasks from one processor to another at any time according to their binding, so this function may return something that is already outdated.

Returns
  0 on success, -1 on error.

Note
  `hwloc_pid_t` is `pid_t` on Unix platforms, and `HANDLE` on native Windows platforms.
  As a special case on Linux, if a tid (thread ID) is supplied instead of a pid (process ID) and HWLOC_CPUBIND_THREAD is passed in flags, the last CPU location of that specific thread is returned.
  On non-Linux systems, HWLOC_CPUBIND_THREAD can not be used in `flags`.

## ◆ hwloc_get_thread_cpubind()

int hwloc_get_thread_cpubind | ( | hwloc_topology_t | topology,
|  | hwloc_thread_t | thread,
|  | hwloc_cpuset_t | set,
|  | int | flags )

Get the current physical binding of thread `tid`.

The CPU-set `set` (previously allocated by the caller) is filled with the list of PUs which the thread was last bound to.

Returns
  0 on success, -1 on error.

Note
  `hwloc_thread_t` is `pthread_t` on Unix platforms, and `HANDLE` on native Windows platforms.
  HWLOC_CPUBIND_PROCESS can not be used in `flags`.

## ◆ 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`.

Returns
  0 on success.
  -1 with errno set to `ENOSYS` if the action is not supported.
  -1 with errno set to `EXDEV` if the binding cannot be enforced.

## ◆ hwloc_set_proc_cpubind()

int hwloc_set_proc_cpubind | ( | hwloc_topology_t | topology,
|  | hwloc_pid_t | pid,
|  | hwloc_const_cpuset_t | set,
|  | int | flags )

Bind a process `pid` on CPUs given in physical bitmap `set`.

Returns
  0 on success, -1 on error.

Note
  `hwloc_pid_t` is `pid_t` on Unix platforms, and `HANDLE` on native Windows platforms.
  As a special case on Linux, if a tid (thread ID) is supplied instead of a pid (process ID) and HWLOC_CPUBIND_THREAD is passed in flags, the binding is applied to that specific thread.
  On non-Linux systems, HWLOC_CPUBIND_THREAD can not be used in `flags`.

## ◆ hwloc_set_thread_cpubind()

int hwloc_set_thread_cpubind | ( | hwloc_topology_t | topology,
|  | hwloc_thread_t | thread,
|  | hwloc_const_cpuset_t | set,
|  | int | flags )

Bind a thread `thread` on CPUs given in physical bitmap `set`.

Returns
  0 on success, -1 on error.

Note
  `hwloc_thread_t` is `pthread_t` on Unix platforms, and `HANDLE` on native Windows platforms.
  HWLOC_CPUBIND_PROCESS can not be used in `flags`.