# Overview

The Simple Linux Utility for Resource Management (SLURM) is an open source, fault-tolerant, and highly scalable cluster management and job scheduling system for large and small Linux clusters. SLURM requires no kernel modifications for its operation and is relatively self-contained. As a cluster resource manager, SLURM has three key functions. First, it allocates exclusive and/or non-exclusive access to resources (compute nodes) to users for some duration of time so they can perform work. Second, it provides a framework for starting, executing, and monitoring work (normally a parallel job) on the set of allocated nodes. Finally, it arbitrates contention for resources by managing a queue of pending work. Optional plugins can be used for accounting, advanced reservation, gang scheduling (time sharing for parallel jobs), backfill scheduling, resource limits by user or bank account, and sophisticated multifactor job prioritization algorithms.

SLURM has been developed through the collaborative efforts of Lawrence Livermore National Laboratory (LLNL), Hewlett-Packard, Bull, Linux NetworX and many other contributors.

## Architecture

SLURM has a centralized manager, slurmctld, to monitor resources and work. There may also be a backup manager to assume those responsibilities in the event of failure. Each compute server (node) has a slurmd daemon, which can be compared to a remote shell: it waits for work, executes that work, returns status, and waits for more work. The slurmd daemons provide fault-tolerant hierarchical communications. There is an optional slurmdbd (Slurm DataBase Daemon) which can be used to record accounting information for multiple Slurm-managed clusters in a single database. User tools include srun to initiate jobs, scancel to terminate queued or running jobs, sinfo to report system status, squeue to report the status of jobs, and sacct to get information about jobs and job steps that are running or have completed. The smap and sview commands graphically reports system and job status including network topology. There is an administrative tool scontrol available to monitor and/or modify configuration and state information on the cluster. The administrative tool used to manage the database is sacctmgr. It can be used to identify the clusters, valid users, valid bank accounts, etc. APIs are available for all functions.

Figure 1. SLURM components

SLURM has a general-purpose plugin mechanism available to easily support various infrastructures. This permits a wide variety of SLURM configurations using a building block approach. These plugins presently include:

- Accounting Storage: text file (default if jobacct_gather != none), MySQL, PGSQL, SlurmDBD (Slurm Database Daemon) or none
- Authentication of communications: authd, munge, or none (default).
- Checkpoint: AIX, OpenMPI, XLCH, or none.
- Cryptography (Digital Signature Generation): munge (default) or OpenSSL.
- Job Accounting Gather: AIX, Linux, or none(default)
- Job Completion Logging: text file, arbitrary script, MySQL, PGSQL, SlurmDBD, or none (default).
- MPI: LAM, MPICH1-P4, MPICH1-shmem, MPICH-GM, MPICH-MX, MVAPICH, OpenMPI and none (default, for most other versions of MPI including MPICH2 and MVAPICH2).
- Priority: Assigns priorities to jobs as they arrive. Options include multifactor job prioritization (assigns job priority based upon fair-share allocate, size, age, QoS, and/or partition) and basic (assigns job priority based upon age for First In First Out ordering, default).
- Process tracking (for signaling): AIX (using a kernel extension), Linux process tree hierarchy, process group ID, RMS (Quadrics Linux kernel patch), and SGI's Process Aggregates (PAGG).
- Node selection: Bluegene (a 3-D torus interconnect BGL or BGP), consumable resources (to allocate individual processors and memory) or linear (to dedicate entire nodes).
- Scheduler: builtin (First In First Out, default), backfill (starts jobs early if doing so does not delay the expected initiation time of any higher priority job), gang (time-slicing for parallel jobs), The Maui Scheduler, and Moab Cluster Suite. There is also a multifactor job prioritization plugin available for use with the basic, backfill and gang schedulers only. Jobs can be prioritized by age, size, fair-share allocation, etc. Many resource limits are also configurable by user or bank account.
- Switch or interconnect: Quadrics (Elan3 or Elan4), Federation Federation (IBM High Performance Switch), or none (actually means nothing requiring special handling, such as Ethernet or Myrinet, default).
- Task Affinity: Affinity (bind tasks to processors or CPU sets) or none (no binding, the default).
- Network Topology: 3d_torus (optimize resource selection based upon a 3d_torus interconnect, default for Cray XT, Sun Constellation and IBM BlueGene), tree (optimize resource selection based upon switch connections) or none (the default).

The entities managed by these SLURM daemons, shown in Figure 2, include nodes, the compute resource in SLURM, partitions, which group nodes into logical sets, jobs, or allocations of resources assigned to a user for a specified amount of time, and job steps, which are sets of (possibly parallel) tasks within a job. The partitions can be considered job queues, each of which has an assortment of constraints such as job size limit, job time limit, users permitted to use it, etc. Priority-ordered jobs are allocated nodes within a partition until the resources (nodes, processors, memory, etc.) within that partition are exhausted. Once a job is assigned a set of nodes, the user is able to initiate parallel work in the form of job steps in any configuration within the allocation. For instance, a single job step may be started that utilizes all nodes allocated to the job, or several job steps may independently use a portion of the allocation. SLURM provides resource management for the processors allocated to a job, so that multiple job steps can be simultaneously submitted and queued until there are available resources within the job's allocation.

Figure 2. SLURM entities

## Configurability

Node state monitored include: count of processors, size of real memory, size of temporary disk space, and state (UP, DOWN, etc.). Additional node information includes weight (preference in being allocated work) and features (arbitrary information such as processor speed or type). Nodes are grouped into partitions, which may contain overlapping nodes so they are best thought of as job queues. Partition information includes: name, list of associated nodes, state (UP or DOWN), maximum job time limit, maximum node count per job, group access list, priority (important if nodes are in multiple partitions) and shared node access policy with optional over-subscription level for gang scheduling (e.g. YES, NO or FORCE:2). Bit maps are used to represent nodes and scheduling decisions can be made by performing a small number of comparisons and a series of fast bit map manipulations. A sample (partial) SLURM configuration file follows.

    #
    # Sample /etc/slurm.conf
    #
    ControlMachine=linux0001
    BackupController=linux0002
    #
    AuthType=auth/munge
    Epilog=/usr/local/slurm/sbin/epilog
    PluginDir=/usr/local/slurm/lib
    Prolog=/usr/local/slurm/sbin/prolog
    SlurmctldPort=7002
    SlurmctldTimeout=120
    SlurmdPort=7003
    SlurmdSpoolDir=/var/tmp/slurmd.spool
    SlurmdTimeout=120
    StateSaveLocation=/usr/local/slurm/slurm.state
    TmpFS=/tmp
    #
    # Node Configurations
    #
    NodeName=DEFAULT Procs=4 TmpDisk=16384 State=IDLE
    NodeName=lx[0001-0002] State=DRAINED
    NodeName=lx[0003-8000] RealMemory=2048 Weight=2
    NodeName=lx[8001-9999] RealMemory=4096 Weight=6 Feature=video
    #
    # Partition Configurations
    #
    PartitionName=DEFAULT MaxTime=30 MaxNodes=2
    PartitionName=login Nodes=lx[0001-0002] State=DOWN
    PartitionName=debug Nodes=lx[0003-0030] State=UP Default=YES
    PartitionName=class Nodes=lx[0031-0040] AllowGroups=students
    PartitionName=DEFAULT MaxTime=UNLIMITED MaxNodes=4096
    PartitionName=batch Nodes=lx[0041-9999]

Last modified 31 March 2009