Fastsim Eras and Modifiers

[vlimant]

While looking into #49098 and seeing #48153, I am contemplating the fastSim proc modifier it's use case and the interference between
--era Run3_2024_Fastsim --fast and --era Run3_2024 --fast --procModifier fastSim in cmsDriver.
I fear that we have an inconsistency in how to set this up that would lead to mistakes.
Can we use this GH issue to cleanup the fastSim procModifier, or fix all of the "non era" modifier (Run3_2024_Fastsim is not per say an era) to use the fastSim procmodifer.

[cmsbuild]

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[cmsbuild]

A new Issue was created by @vlimant.

@Dr15Jones, @ftenchini, @makortel, @mandrenguyen, @sextonkennedy, @smuzaffar can you please review it and eventually sign/assign? Thanks.

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[makortel]

assign fastsim

[makortel]

assign Configuration/ProcessModifiers,Configuration/Eras

[kpedro88]

1. --fast includes --procModifier fastSim (as well as other changes to the sequences in cmsDriver.py)
2. The custom top-level Era Run3_2024_FastSim (which is in fact an Era as the term is typically used, i.e. a ModifierChain) is necessary because GEM is not yet included in FastSim (work is ongoing, but aimed primarily at Run 4). The only options I see to avoid this would be:
   a. Change run3_GEM to (run3_GEM & ~fastSim) everywhere in CMSSW
   b. Define a concrete run3_GEM_actual = (run3_GEM & ~fastSim) and use this in the definition of all Run3 top-level Eras

The latter option is obviously less disruptive, but still not a trivial modification, and it would require careful backporting to many releases if the goal is to eliminate the Run3 FastSim-specific Eras for central production.

I would rather recommend that central production simply follow the validated recipes in runTheMatrix instead of composing flags by hand, which is always likely to lead to mistakes.

[makortel]

ping @cmsbuild

[makortel]

assign pdmv

[vlimant]

• --fast includes --procModifier fastSim (as well as other changes to the sequences in cmsDriver.py)

I was not able to spot where this is happening ; hence my worries.

[cmsbuild]

New categories assigned: fastsim,operations,pdmv

@AdrianoDee,@antoniovagnerini,@civanch,@davidlange6,@DickyChant,@fabiocos,@ftenchini,@kpedro88,@mandrenguyen,@mdhildreth,@miquork,@ssekmen you have been requested to review this Pull request/Issue and eventually sign? Thanks