[petsc-dev] PETSc optimization

[Barry Smith]

  The space of "best" choices for optimization flags for PETSc is huge; each OS, each compiler, and each particular piece of hardware may have different "best" choices, in addition, the previous choice may not even allow compiling with any small change in any of these things. Finally particular choices may not be compatible with choices may be other packages being linked with PETSc. Hence PETSc's configure punts on trying to select a good choice (and just uses -O).  Surely this is an issue with most portable open source packages not being able to select "good" optimization flags in a portable way?

  Does the Spack system contain any infrastructure to help solve this common problem? That ensures all the packages it builds have good compatible optimization flags for the particular system?

  Barry

Perhaps the configure message you mention printed by PETSc is inappropriate for Spack builds of PETSc and we should print something different to help guide the user to the correct Spack solution to the issue?

   

  



> On Aug 8, 2023, at 12:35 PM, Liu Wei AWE via petsc-dev <petsc-dev at mcs.anl.gov> wrote:
> 
> Hi all
>  
> I am currently building a large software stack using Spack with PETSc 3.19 as part of the dependency library.
>  
> Spotted the following message during the build process
>  
> Using default optimization C flags “-g -O”. You might consider manually setting optimal optimization flags for your system with
> COPTFLAGS="optimization flags" see config/examples/arch-*-opt.py for examples
>  
> Previously when we install PETSc manually, optimisation flags are enforced via configure script e.g.
> COPTFLAGS=”-g -O3 -march=native”
> CXXOPTFLAGS=”-g -O3 -march=native”    
> …
> (or “-g -O3 -xhost” for intel compiler)
>  
> Whilst spack spec syntax allows compiler parameters via cflags/cxxflags/fflags  
> https://github.com/spack/spack/blob/develop/var/spack/repos/builtin/packages/petsc/package.py <https://github.com/spack/spack/blob/develop/var/spack/repos/builtin/packages/petsc/package.py#L400>
> the requirement is not obvious, especially in a long build process.
>  
> The question is should optimisation be amended at spack recipe stage or should PETSc configure enforce some compiler optimisation (if –with-debugging=0 is set) ?
>  
> Regards
>  
> Dr Wei Liu
> High Performance Computing
> To chat with me on Teams click here <https://teams.microsoft.com/l/chat/0/0?users=wei.liu@awe.co.uk>
> T: +44 01189 856209
> M: wei.liu at awe.co.uk <mailto:wei.liu at awe.co.uk>
> AWE Aldermaston,
> Reading, Berkshire, RG7 4PR
>  
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