# Job Launch Design Guide

## Overview

This guide describes at a high level the processes which occur in order to initiate a job including the daemons and plugins involved in the process. It describes the process of job allocation, step allocation, task launch and job termination. The functionality of tens of thousands of lines of code has been distilled here to a couple of pages of text, so much detail is missing.

## Job Allocation

The first step of the process is to create a job allocation, which is a claim on compute resources. A job allocation can be created using the salloc, sbatch or srun command. The salloc and sbatch commands create resource allocations while the srun command will create a resource allocation (if not already running within one) plus launch tasks. Each of these commands will fill in a data structure identifying the specifications of the job allocation requirement (e.g. node count, task count, etc.) based upon command line options and environment variables and send the RPC to the slurmctld daemon. The UID and GID of the user launching the job will be included in a credential which will be used later to restrict access to the job, so further steps run in the allocation will need to be launched using the same UID and GID as the one used to create the allocation. If the new job request is the highest priority, the slurmctld daemon will attempt to select resources for it immediately, otherwise it will validate that the job request can be satisfied at some time and queue the request. In either case the request will receive a response almost immediately containing one of the following:

- A job ID and the resource allocation specification (nodes, cpus, etc.)
- A job ID and notification of the job being in a queued state OR
- An error code

The process of selecting resources for a job request involves multiple steps, some of which involve plugins. The process is as follows:

1. Call job_submit plugins to modify the request as appropriate
2. Validate that the options are valid for this user (e.g. valid partition name, valid limits, etc.)
3. Determine if this job is the highest priority runnable job, if so then really try to allocate resources for it now, otherwise only validate that it could run if no other jobs existed
4. Determine which nodes could be used for the job. If the feature specification uses an exclusive OR option, then multiple iterations of the selection process below will be required with disjoint sets of nodes
5. Call the select plugin to select the best resources for the request
6. The select plugin will consider network topology and the topology within a node (e.g. sockets, cores, and threads) to select the best resources for the job
7. If the job can not be initiated using available resources and preemption support is configured, the select plugin will also determine if the job can be initiated after preempting lower priority jobs. If so then initiate preemption as needed to start the job

## Step Allocation

The srun command is always used for job step creation. It fills in a job step request RPC using information from the command line and environment variables then sends that request to the slurmctld daemon. It is important to note that many of the srun options are intended for job allocation and are not supported by the job step request RPC (for example the socket, core and thread information is not supported). If a job step uses all of the resources allocated to the job then the lack of support for some options is not important. If one wants to execute multiple job steps using various subsets of resources allocated to the job, this shortcoming could prove problematic. It is also worth noting that the logic used to select resources for a job step is relatively simple and entirely contained within the slurmctld daemon code (the select plugin is not used for job steps). If the request can not be immediately satisfied due to a request for exclusive access to resources, the appropriate error message will be sent and the srun command will retry the request on a periodic basis. (NOTE: It would be desirable to queue the job step requests to support job step dependencies and better performance in the initiation of job steps, but that is not currently supported.) If the request can be satisfied, the response contains a digitally signed credential (by the cred plugin) identifying the resources to be used.

## Task Launch

The srun command builds a task launch request data structure including the credential, executable name, file names, etc. and sends it to the slurmd daemon on node zero of the job step allocation. The slurmd daemon validates the signature and forwards the request to the slurmd daemons on other nodes to launch tasks for that job step. The degree of fanout in this message forwarding is configurable using the TreeWidth parameter. Each slurmd daemon tests that the job has not been cancelled since the credential was issued (due to a possible race condition) and spawns a slurmstepd program to manage the job step. Note that the slurmctld daemon is not directly involved in task launch in order to minimize the overhead on this critical resource.

Each slurmstepd program executes a single job step. Besides the functions listed below, the slurmstepd program also executes several SPANK plugin functions at various times.

1. Performs MPI setup (using the appropriate plugin)
2. Calls the switch plugin to perform any needed network configuration
3. Creates a container for the job step using a proctrack plugin
4. Change user ID to that of the user
5. Configures I/O for the tasks (either using files or a socket connection back to the srun command
6. Sets up environment variables for the tasks including many task-specific environment variables
7. Fork/exec the tasks

## Job Step Termination

There are several ways in which a job step or job can terminate, each with slight variation in the logic executed. The simplest case is if the tasks run to completion. The srun will note the termination of output from the tasks and notify the slurmctld daemon that the job step has completed. slurmctld will simply log the job step termination. The job step can also be explicitly cancelled by a user, reach the end of its time limit, etc. and those follow a sequence of steps very similar to that for job termination, which is described below.

## Job Termination

Job termination can either be user initiated (e.g. scancel command) or system initiated (e.g. time limit reached). The termination ultimately requires the slurmctld daemon to notify the slurmd daemons on allocated nodes that the job is to be ended. The slurmd daemon does the following:

1. Send a SIGCONT and SIGTERM signal to any user tasks
2. Wait KillWait seconds if there are any user tasks
3. Send a SIGKILL signal to any user tasks
4. Wait for all tasks to complete
5. Execute any Epilog program
6. Send an epilog_complete RPC to the slurmctld daemon

Last modified 9 June 2021