[slurm-users] How to limit # of execution slots for a given node

Paul Edmon pedmon at cfa.harvard.edu
Fri Jan 7 14:21:13 UTC 2022

Also I recommend setting:

    Number of cores reserved for system use. These cores will not be
    available for allocation to user jobs. Depending upon the
    *TaskPluginParam* option of *SlurmdOffSpec*, Slurm daemons (i.e.
    slurmd and slurmstepd) may either be confined to these resources
    (the default) or prevented from using these resources. Isolation of
    the Slurm daemons from user jobs may improve application
    performance. If this option and *CpuSpecList* are both designated
    for a node, an error is generated. For information on the algorithm
    used by Slurm to select the cores refer to the core specialization
    documentation ( https://slurm.schedmd.com/core_spec.html ). 


    Amount of memory, in megabytes, reserved for system use and not
    available for user allocations. If the task/cgroup plugin is
    configured and that plugin constrains memory allocations (i.e.
    *TaskPlugin=task/cgroup* in slurm.conf, plus *ConstrainRAMSpace=yes*
    in cgroup.conf), then Slurm compute node daemons (slurmd plus
    slurmstepd) will be allocated the specified memory limit. Note that
    having the Memory set in *SelectTypeParameters* as any of the
    options that has it as a consumable resource is needed for this
    option to work. The daemons will not be killed if they exhaust the
    memory allocation (ie. the Out-Of-Memory Killer is disabled for the
    daemon's memory cgroup). If the task/cgroup plugin is not
    configured, the specified memory will only be unavailable for user

These will restrict specific memory and cores for system use. This is 
probably the best way to go rather than spoofing your config.

-Paul Edmon-

On 1/7/2022 2:36 AM, Rémi Palancher wrote:
> Le jeudi 6 janvier 2022 à 22:39, David Henkemeyer<david.henkemeyer at gmail.com>  a écrit :
>> All,
>> When my team used PBS, we had several nodes that had a TON of CPUs, so many, in fact, that we ended up setting np to a smaller value, in order to not starve the system of memory.
>> What is the best way to do this with Slurm? I tried modifying # of CPUs in the slurm.conf file, but I noticed that Slurm enforces that "CPUs" is equal to Boards * SocketsPerBoard * CoresPerSocket * ThreadsPerCore. This left me with having to "fool" Slurm into thinking there were either fewer ThreadsPerCore, fewer CoresPerSocket, or fewer SocketsPerBoard. This is a less than ideal solution, it seems to me. At least, it left me feeling like there has to be a better way.
> I'm not sure you can lie to Slurm about the real number of CPUs on the nodes.
> If you want to prevent Slurm from allocating more than n CPUs below the total number of CPUs of these nodes, I guess one solution is to use MaxCPUsPerNode=n at the partition level.
> You can also mask "system" CPUs with CpuSpecList at node level.
> The later is better if you need fine grain control over the exact list of reserved CPUs regarding NUMA topology or whatever.
> --
> Rémi Palancher
> Rackslab: Open Source Solutions for HPC Operations
> https://rackslab.io
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