add note on lmod cache when loading an EESSI module

[smoors]

$ module unuse $MODULEPATH
$ module use /cvmfs/software.eessi.io/init/modules
$ module load EESSI/2025.06                                                                                                                                       
Lmod has detected the following error:  Unable to load module because of error when evaluating modulefile:
     /cvmfs/software.eessi.io/init/modules/EESSI/2025.06.lua: /usr/share/lmod/lmod/libexec/ModuleA.lua:676: Did not find mpath in mA

     Please check the modulefile and especially if there is a line number specified in the above message
If you don't understand the warning or error, contact the helpdesk at hpc@vub.be 
While processing the following module(s):
    Module fullname  Module Filename
    ---------------  ---------------
    EESSI/2025.06    /cvmfs/software.eessi.io/init/modules/EESSI/2025.06.lua

$ LMOD_IGNORE_CACHE=1 module load EESSI/2025.06
Module for EESSI/2025.06 loaded successfully

$ module --version                                                                                                                                                                                 

Modules based on Lua: Version 8.7.65 2025-08-05 10:24 -06:00
    by Robert McLay mclay@tacc.utexas.edu

[boegel]

@ocaisa Is that the best we can do here?

[casparvl]

Ha, I just pinged Alan as well. His words: disabling the cache will be very expensive.

@smoors I'm wondering why you are running into this usse. I have the feeling that rather then just disabling the cache, there is an actual issue that should be fixed instead.

[ocaisa]

I think we need to understand what is going wrong here first, ignoring the cache is very expensive for things like module avail.

My suspicion would be that there is a user spider cache there for /cvmfs/software.eessi.io/init/modules that was generated with the Lmod from EESSI, that Lmod version is too new to be compatible with the system Lmod. If you delete the local cache, the old Lmod will create a new one and things should work again.

[smoors]

My suspicion would be that there is a user spider cache there for /cvmfs/software.eessi.io/init/modules that was generated with the Lmod from EESSI, that Lmod version is too new to be compatible with the system Lmod. If you delete the local cache, the old Lmod will create a new one and things should work again.

there is no local cache in my account, which is only generated if there is no system cache, and we do have a system cache. our Lmod is more recent than the one from EESSI.

[ocaisa]

Oh, that's worse, now I would really like to know what it is going on.

[smoors]

note that i only have to disable the cache when loading an EESSI module, not when subsequently loading any modules in the EESSI stack. this works perfectly:

$ LMOD_IGNORE_CACHE=1 ml EESSI/2025.06                                                                                                                                                                 
Module for EESSI/2025.06 loaded successfully
$ ml GROMACS/2025.2-foss-2025a

[ocaisa]

This looks relevant: TACC/Lmod#780 , and specifically TACC/Lmod#780 (comment)

[ocaisa]

I think the less impactful suggestion here is export LMOD_CACHED_LOADS=no, does that work for you?

[smoors]

I think the less impactful suggestion here is export LMOD_CACHED_LOADS=no, does that work for you?

that doesn't work for me.

i really don't think setting LMOD_IGNORE_CACHE=1 only for loading an EESSI module is "impactful", as there are only 2 modules available at that point.

[casparvl]

That may be true, the risk is that people misinterpret and think "you know what, I'll just export LMOD_IGNORE_CACHE=1 in my .bashrc so I don't need to remember every time". Plus, if there's a fundamental bug, we should fix the bug rather than tell people to work around it :)

Also, understanding the issue better will also help us to give more targetted advice. I.e. if it was the issue that Alan linked to, we could give much more concrete advice "if you see XXX and are running with LMOD version YYY or older, either update to version ZZZ or load the EESSI module with ... WARNING: do NOT export LMOD_IGNORE_CACHE=1 as a global setting, as this would make interaction with the modules provided by EESSI extremely slow."

[smoors]

That may be true, the risk is that people misinterpret and think "you know what, I'll just export LMOD_IGNORE_CACHE=1 in my .bashrc so I don't need to remember every time". Plus, if there's a fundamental bug, we should fix the bug rather than tell people to work around it :)

Also, understanding the issue better will also help us to give more targetted advice. I.e. if it was the issue that Alan linked to, we could give much more concrete advice "if you see XXX and are running with LMOD version YYY or older, either update to version ZZZ or load the EESSI module with ... WARNING: do NOT export LMOD_IGNORE_CACHE=1 as a global setting, as this would make interaction with the modules provided by EESSI extremely slow."

of course, i fully agree! i wasn't suggesting we shouldn't try to fix/understand it better, i was merely responding to Alan's "less impactful" suggestion.

[ocaisa]

I do think it is pretty likely that someone would export LMOD_IGNORE_CACHE=1 and then things will slow down for them. If it had worked, export LMOD_CACHED_LOADS=no would just affect loads and would be harder to notice.