* Added a Q&A note on how to examine a SLURM job memory utilization.

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Wirawan Purwanto 7 months ago
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Examining CPU & Memory Utilization of SLURM Jobs
================================================
<!-- Original Q&A spurred by Kevin Garner's question, 2023-12-04 -->
## Question: How to see memory usage of a SLURM job?
## Answer:
It depends on whether the job is still running or has been completed / terminated.
If the job is still running, one can check the "instantaneous" (current) memory usage by ssh-ing to the node used by the job and examine the memory usage by invoking the `top` command.
If the SLURM job has been completed, SLURM holds some statistics concerning completed job including the memory usage. It can be found by querying SLURM using the `sacct` command. Please see here:
https://slurm.schedmd.com/sacct.html
The `--format` and `--units` flags are what you want to explore. The `--format` flag can be used to specify which fields to display. The following fields are most relevant":
```
AveRSS
MaxRSS
MaxRSSNode
MaxRSSTask
AveVMSize
MaxVMSize
MaxVMSizeNode
MaxVMSizeTask
```
They define the average, maximum, and the node that has the maximum memory usage. `VMSize` refers the amount of (virtual) memory allocated, but generally not all the allocated memory will be used in practice. `RSS` is the amount of memory *actually* occupied and used by the program. We typically want to pay attention to the RSS. Other SLURM accounting fields can be seen in `sacct`'s mannual page, subheading [JOB ACCOUNTING FIELDS](https://slurm.schedmd.com/sacct.html#lbAF).
Below I provide an example (by way of a bash script) of the invocation of sacct to return several fields of interest:
```
#!/bin/bash
FIELDS_PRESET1=JobID,JobIDRaw,JobName%-20,User,Partition,Elapsed%12,NCPUS%5,CPUTime%12,TotalCPU%12,AveRSS,MaxRSS.Submit,Start,End,State,NodeList%20
filter_sacct_only_main_jobs () {
awk '($2 ~ /^[0-9]+$/) || (FNR <= 2) { print }'
}
sacct -o $FIELDS_PRESET1 "$@" \
| filter_sacct_only_main_jobs
```
A small auxiliary AWK filtering script is used above to select only the primary SLURM record. You can bypass that filtering to return all the subrecords, which will expose the resource utilization for each programs / steps launched in the job script. The subrecords vary depending on the jobs; but an MPI parallel job will usually has at minimum a subrecord corresponding to the invocation of that MPI program, which is what is useful.
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