[petsc-dev] de-global variablelizing PETSc

[Jed Brown]

Barry Smith <bsmith at mcs.anl.gov> writes:
>   1) Hmm, the check for “initialized” of something is done often and
>   through out the run of the code. Will each thread through out the
>   run of the code need to check the lock each time?

No, the "initialized" variable only ever changes in one direction.  The
user should not have to call XXFinalizePackage() and if they really
must, _all_ variables would have to be thread local (or make the PETSc
library instance an object and pass it explicitly to every constructor
[1]).

>   2) Can the lock be made 100% portable and independent of the
>   threading model etc that user is using?

Independent of the threading model, but somewhat dependent on the
compiler and/or architecture.  For example, C11 stdatomic.h is not
widely available yet, and third-party implementations (like
http://concurrencykit.org/) require inline assembly, which Cray's
compiler suite does not support.  GCC has supported atomics for a long
time and many vendors offer that syntax.

http://gcc.gnu.org/onlinedocs/gcc-4.8.2/gcc/C-Extensions.html#C-Extensions

>   3) Philosophy - why use a lock when it isn’t truly needed?

MPI will be using a lock regardless if you use threads.  And the kernel
uses locks all over the place (and more sophisticated techniques like
RCU).  Locks are an important tool and one that I think is important.


[1] This is the real way to support arbitrary threading models and
    usage.  It would cause a huge (mostly-cosmetic) impact on user code,
    but the idea would be that PetscInitialize() would return a "Petsc"
    library context and _every_ constructor or function that currently
    has global state would take this thing as its first argument.  Users
    that want separate instances of the library in separate threads
    would MPI_Init_thread() and pass their communicator to
    PetscInitialize() (could call it PetscInitializeThread, but it would
    take an explicit MPI_Comm parameter, though you could pass
    MPI_COMM_NULL; the static variable PETSC_COMM_WORLD would have to
    go, perhaps replaced by PetscCommWorld(petsclib)).
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