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CMSSW Multi microarch releases

CMSSW_14_0_X and above releases are build for multiple micro-architectures support for x86_64. CMSSW_14_0, CMSSW_14_1 and CMSSW_14_2 use x86-64-v2 as default micro-architecture while x86-64-v3 is provided as an additional micro-architectures. In order to save time and resources, by default SCRAM is configured to used only x86-64-v2. But if you need to use x86-64-v3 set of librairies/plugins of cmssw then you can run scram build enable-multi-targets in your developer area. This will allow scram to build your checked out packages for both micro-archs and automatiically select the best set of libraries at runtime.

Starting with CMSSW_15_0_X, we have the follow changes

  • The default micro-architecture is x86-64-v3
  • Additional micro-architecture is x86-64-v2
  • Setting runtime environment in CMSSW release (e.g. running cmsenv directly from the release path) will automatically select the best micro-architecture e.g.
    • Nodes with x86-64-v2 only support: Runtime env will be set to use x86-64-v2 set of libraries/plugins
    • Nodes with x86-64-v3 and above support: Runtime env will be set to use x86-64-v3 set of libraries/plugisn
  • Creating developer area will also automatically enable/disable multi-microarchitecture support e.g.
    • Nodes with x86-64-v2 only support: SCRAM will create developer area with multi-microarchitecture support enabled. Any checked out packages will be build for both micro-architectures. At runtime SCRAM will selected the best matching micro-architecture
    • Nodes with x86-64-v3 and above support: SCRAM will create developer area with multi-microarchitecture support disabled. Any checked out packages will only be built for default (x86-64-v3) micro-architecture. At runtime SCRAM will only use default (x86-64-v3) micro-architecture libraries.
  • SCRAM will also set following environment variables for CMSSW_15_0 and above
    • SCRAM_DEFAULT_MICROARCH: Default micro-arch used for cmssw release e.g. for CMSSW_15_0_X it will be x86-64-v3
    • SCRAM_MIN_SUPPORTED_MICROARCH: Least required micro-arch to run cmssw e.g , for CMSSW_15_0, is developer area has multi-microarchitecture support enabled then SCRAM_MIN_SUPPORTED_MICROARCH will be set to x86-64-v2 otherwise it will be x86-64-v3