On Wed, 26 Nov, 2014 at 8:32 AM, Johan Hake <[email protected]> wrote:
On Wed, Nov 26, 2014 at 9:22 AM, Garth N. Wells <[email protected]>
wrote:
On Wed, 26 Nov, 2014 at 7:50 AM, Johan Hake <[email protected]>
wrote:
On Wed, Nov 26, 2014 at 8:34 AM, Garth N. Wells <[email protected]>
wrote:
On Tue, 25 Nov, 2014 at 9:48 PM, Johan Hake <[email protected]>
wrote:
Hello!
I just pushed some fixes to the jit interface of DOLFIN. Now one
can jit on different mpi groups.
Nice.
Previously jiting was only done on rank 1 of the mpi_comm_world.
Now it is done on rank 1 of any passed group communicator.
Do you mean rank 0?
Yes, of course.
There is no demo atm showing this but a test has been added:
test/unit/python/jit/test_jit_with_mpi_groups.py
Here an expression, a subdomain, and a form is constructed on
different ranks using group. It is somewhat tedious as one need
to initialize PETSc with the same group, otherwise PETSc will
deadlock during initialization (the moment a PETSc la object is
constructed).
This is ok. It's arguably a design flaw that we don't make the
user handle MPI initialisation manually.
Sure, it is just somewhat tedious. You cannot start your typical
script with importing dolfin.
The procedure in Python for this is:
1) Construct mpi groups using mpi4py
2) Initalize petscy4py using the groups
3) Wrap groups to petsc4py comm (dolfin only support petsc4py not
mpi4py)
4) import dolfin
5) Do group specific stuff:
a) Function and forms no change needed as communicator
is passed via mesh
b) domain = CompiledSubDomain("...", mpi_comm=group_comm)
c) e = Expression("...", mpi_comm=group_comm)
It's not so clear whether passing the communicator means that the
Expression is only defined/available on group_comm, or if
group_comm is simply to control who does the JIT. Could you
clarify this?
My knowledge is not that good in MPI. I have only tried to access
(and construct) the Expression on ranks included in that group.
Also when I tried construct one using a group communicator on a
rank that is not included in the group, I got an when calling
MPI_size on it. There is probably a perfectly reasonable
explaination to this.
Could you clarify what goes on behind-the-scenes with the
communicator? Is it only used in a call to get the process rank?
What do the ranks other than zero do?
Not sure what you want to know. Instead of using mpi_comm_world to
construct meshes you use the group communicator. This communicator
has its own local group of ranks. JITing is still done on rank 0
of the local group, which might and most often is different from rank
0 process of the mpi_comm_word.
I just want to be clear (and have in the docstring) that
e = Expression("...", mpi_comm=group_comm)
is valid only on group_comm (if this is the case), or make clear that
the communicator only determines the process that does the JIT.
If we required all Expressions to have a domain/mesh, as Martin
advocates, things would be clearer.
The group communicator works exactly like the world communicator but
now on just a subset of the processes. There were some sharp edges
with deadlocks as a consequence, when barriers were taken on the
world communicator. This is done by default when dolfin is imported
and petcs gets initialized with the world communicator. So we need to
initialized petsc using the group communicator. Other than that there
are not real differences.
That doesn't sound right. PETSc initialisation does not take a
communicator. It is collective on MPI_COMM_WORLD, but each PETSc object
takes a communicator at construction, which can be something other than
MPI_COMM_WORLD or MPI_COMM_SELF.
Garth
Johan
Garth
Please try it out and report any sharp edges. A demo would also
be fun to include :)
We could run tests on different communicators to speed them up on
machines with high core counts!
True!
Johan
Garth
Johan
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