2025-08-05 - Improved memory reporting on Mjolnir – helping you book jobs more efficiently

Dear Mjolnir users,

I want to let you know about an important change I’ve made to how Mjolnir reports memory usage.

Until now, Mjolnir used a measurement called RSS (Resident Set Size). RSS counts all memory pages a process uses in full, even if they are shared between multiple processes. For jobs that use many processes (for example Bowtie2 or similar tools), this could lead to double‑counting, making it look like your jobs used much more memory than they really did.

This has now been changed to PSS (Proportional Set Size). PSS divides the cost of shared memory pages between the processes that use them, giving a much more realistic picture of how much unique memory your job actually uses.

What does this mean for you?

• The memory usage you see in sacct (see example command below) and other Slurm reports is now a more accurate reflection of what your job actually consumed.
• In many cases, you may find that your jobs were reported as using far more memory than they truly needed. With this new reporting, you can confidently adjust your --mem requests to better match your real usage, which will help improve scheduling efficiency for everyone.
• If you had previously increased your memory requests because of unexpectedly high reported usage, you may now want to review and lower those values where appropriate.
  
  

sacct -u <username> --units=G --format="User,JobID%20,JobName,Partition,NodeList,Elapsed,CPUTime,ReqMem,MaxRSS,State%15,AllocTRES%32,Start”

I want to give a special thanks to Luke, who brought this to my attention. Observations like his are incredibly valuable to me and help keep Mjolnir efficient, fair, and reliable for all users.

If you have any questions or notice anything unusual, please don’t hesitate to reach out.

Best regards,

Bent