Xaver, It is my understanding that if we want to have stable systems that don't run out of memory, we do need to manage the amount of memory needed for everything not running within a slurm job, yes.
In our cluster, we are using `CR_Core_Memory` (so we do constrain our job memory) and we set the `RealMemory` to the actual full amount of memory available on the machine - I believe these really are given in megabytes (MB), not mebibytes (MiB). I think their example of (e.g. "2048") is intended to convey this because 2000 MiB is 2048 MB. We set the `MemSpecLimit` for each node to set memory aside for everything in the system that is not running within a slurm job. This include the slurm daemon itself, the kernel, filesystem drivers, metrics collection agents, etc -- anything else we are running outside the control of slurm jobs. The `MemSpecLimit` just sets aside the specified amount and the result will be that the maximum memory jobs can use on the node is (RealMemory - MemSpecLimit). When using cgroups to limit memory, slurmd will also be allocated the specified limit so that the slurm daemon cannot encroach on job memory. However, note that `MemSpecLimit` is documented to not work unless your `SelectTypeParameters` includes Memory as a consumable resource. Since you are using `CR_Core` (which does not configure Memory as a consumable resource) then I believe your system will not be constraining job memory at all. Jobs can oversubscribe memory as many times over as there are cores, and any job would be able to run the machine out of memory by using more than is available. With this setting, I guess you could say you don't have to manage reserving memory for the OS and slurmd, but only in the sense that any job could consume all the memory and cause the system OOM killer to kill a random process (including slurmd or something else system critical). Cheers, Josh. -- Dr. Joshua C. Randall Director of Software Engineering, HPC Altos Labs email: jrand...@altoslabs.com On Mon, May 12, 2025 at 10:27 AM Xaver Stiensmeier via slurm-users < slurm-users@lists.schedmd.com> wrote: > Dear Slurm-User List, > > currently, in our slurm.conf, we are setting: > > SelectType: select/cons_tres > SelectTypeParameters: CR_Core > > and in our node configuration *RealMemory *was basically reduced by an > amount to make sure the node always had enough RAM to run the OS. However, > this is apparently now how it is supposed to be done: > > Lowering RealMemory with the goal of setting aside some amount for the OS > and not available for job allocations will not work as intended if Memory > is not set as a consumable resource in *SelectTypeParameters*. So one of > the *_Memory options need to be enabled for that goal to be accomplished. ( > https://slurm.schedmd.com/slurm.conf.html#OPT_RealMemory) > > This leads to four questions regarding holding back RAM for worker nodes. > Answers/help with any of those questions would be appreciated. > > *1.* Is reserving enough RAM for the worker node's OS and slurmd actually > a thing you have to manage? > *2.* If so how can we reserve enough RAM for the worker node's OS and > slurmd when using CR_Core? > *3.* Is that maybe a strong argument against using CR_Core that we > overlooked? > > And semi-related: https://slurm.schedmd.com/slurm.conf.html#OPT_RealMemory > talks about taking a value in megabytes. > > *4.* Is RealMemory really expecting megabytes or is it mebibytes? > > Best regards, > Xaver > > -- > slurm-users mailing list -- slurm-users@lists.schedmd.com > To unsubscribe send an email to slurm-users-le...@lists.schedmd.com > -- Altos Labs UK Limited | England | Company reg 13484917 Registered address: 3rd Floor 1 Ashley Road, Altrincham, Cheshire, United Kingdom, WA14 2DT
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