# 8.1. Overview

PRRTE provides a set of three controls for assigning process locations and ranks:

1. Mapping: Assigns a default location to each process

2. Ranking: Assigns a unique integer rank value to each process

3. Binding: Constrains each process to run on specific processors

This section provides an overview of these three controls. Unless otherwise this behavior is shared by `prun(1)` (working with a PRRTE DVM), and `prterun(1)`. More detail about PRRTE process placement is available in the following sections (using `--help placement-<section>`):

- `examples`: some examples of the interactions between mapping, ranking, and binding options.

- `fundamentals`: provides deeper insight into PRRTE’s mapping, ranking, and binding options.

- `limits`: explains the difference between overloading and oversubscribing resources.

- `diagnostics`: describes options for obtaining various diagnostic reports that aid the user in verifying and tuning the placement for a specific job.

- `rankfiles`: explains the format and use of the rankfile mapper for specifying arbitrary process placements.

- `deprecated`: a list of deprecated options and their new equivalents.

- `all`: outputs all the placement help except for the `deprecated` section.

## 8.1.1. Quick Summary

The two binaries that most influence process layout are `prte(1)` and `prun(1)`. The `prte(1)` process discovers the allocation, establishes a Distributed Virtual Machine by starting a `prted(1)` daemon on each node of the allocation, and defines the efault mapping/ranking/binding policies for all jobs. The `prun(1)` process defines the specific mapping/ranking/binding for a specific job. Most of the command line controls are targeted to `prun(1)` since each job has its own unique requirements.

`prterun(1)` is just a wrapper around `prte(1)` for a single job PRRTE DVM. It is doing the job of both `prte(1)` and `prun(1)`, and, as such, accepts the sum all of their command line arguments. Any example that uses `prun(1)` can substitute the use of `prterun(1)` except where otherwise noted.

The `prte(1)` process attempts to automatically discover the nodes in the allocation by querying supported resource managers. If a supported resource manager is not present then `prte(1)` relies on a hostfile provided by the user. In the absence of such a hostfile it will run all processes on the localhost.

If running under a supported resource manager, the `prte(1)` process will start the daemon processes (`prted(1)`) on the remote nodes using the corresponding resource manager process starter. If no such starter is available then `ssh` (or `rsh`) is used.

Minus user direction, PRRTE will automatically map processes in a round-robin fashion by CPU, binding each process to its own CPU. The type of CPU used (core vs hwthread) is determined by (in priority order):

- user directive on the command line via the HWTCPUS qualifier to the `--map-by` directive

- setting the `rmaps_default_mapping_policy` MCA parameter to include the `HWTCPUS` qualifier. This parameter sets the default value for a PRRTE DVM — qualifiers are carried across to DVM jobs started via `prun` unless overridden by the user’s command line

- defaulting to `CORE` in topologies where core CPUs are defined, and to `hwthreads` otherwise.

By default, the ranks are assigned in accordance with the mapping directive — e.g., jobs that are mapped by-node will have the process ranks assigned round-robin on a per-node basis.

PRRTE automatically binds processes unless directed not to do so by the user. Minus direction, PRRTE will bind individual processes to their own CPU within the object to which they were mapped. Should a node become oversubscribed during the mapping process, and if oversubscription is allowed, all subsequent processes assigned to that node will not be bound.

# 8.2. Definition of ‘slot’

The term “slot” is used extensively in the rest of this documentation. A slot is an allocation unit for a process. The number of slots on a node indicate how many processes can potentially execute on that node. By default, PRRTE will allow one process per slot.

If PRRTE is not explicitly told how many slots are available on a node (e.g., if a hostfile is used and the number of slots is not specified for a given node), it will determine a maximum number of slots for that node in one of two ways:

1. Default behavior: By default, PRRTE will attempt to discover the number of processor cores on the node, and use that as the number of slots available.

2. When `--use-hwthread-cpus` is used: If `--use-hwthread-cpus` is specified on the command line, then PRRTE will attempt to discover the number of hardware threads on the node, and use that as the number of slots available.

This default behavior also occurs when specifying the `--host` option with a single host. Thus, the command:

    shell$ prun --host node1 ./a.out

launches a number of processes equal to the number of cores on node `node1`, whereas:

    shell$ prun --host node1 --use-hwthread-cpus ./a.out

launches a number of processes equal to the number of hardware threads on `node1`.

When PRRTE applications are invoked in an environment managed by a resource manager (e.g., inside of a Slurm job), and PRRTE was built with appropriate support for that resource manager, then PRRTE will be informed of the number of slots for each node by the resource manager. For example:

    shell$ prun ./a.out

launches one process for every slot (on every node) as dictated by the resource manager job specification.

Also note that the one-process-per-slot restriction can be overridden in unmanaged environments (e.g., when using hostfiles without a resource manager) if oversubscription is enabled (by default, it is disabled). Most parallel applications and HPC environments do not oversubscribe; for simplicity, the majority of this documentation assumes that oversubscription is not enabled.

## 8.2.1. Slots are not hardware resources

Slots are frequently incorrectly conflated with hardware resources. It is important to realize that slots are an entirely different metric than the number (and type) of hardware resources available.

Here are some examples that may help illustrate the difference:

1. More processor cores than slots: Consider a resource manager job environment that tells PRRTE that there is a single node with 20 processor cores and 2 slots available. By default, PRRTE will only let you run up to 2 processes.

   Meaning: you run out of slots long before you run out of processor cores.

2. More slots than processor cores: Consider a hostfile with a single node listed with a `slots=50` qualification. The node has 20 processor cores. By default, PRRTE will let you run up to 50 processes.

   Meaning: you can run many more processes than you have processor cores.

# 8.3. Definition of “processor element”

By default, PRRTE defines that a “processing element” is a processor core. However, if `--use-hwthread-cpus` is specified on the command line, then a “processing element” is a hardware thread.