On Mon, Aug 09, 2010 at 04:04:24PM +0100, Garth N. Wells wrote: > On Mon, 2010-08-09 at 16:54 +0200, Anders Logg wrote: > > On Mon, Aug 09, 2010 at 03:47:17PM +0100, Garth N. Wells wrote: > > > On Mon, 2010-08-09 at 16:41 +0200, Anders Logg wrote: > > > > On Mon, Aug 09, 2010 at 03:30:03PM +0100, Garth N. Wells wrote: > > > > > On Mon, 2010-08-09 at 16:24 +0200, Anders Logg wrote: > > > > > > On Mon, Aug 09, 2010 at 03:21:59PM +0100, Garth N. Wells wrote: > > > > > > > On Mon, 2010-08-09 at 16:14 +0200, Anders Logg wrote: > > > > > > > > On Mon, Aug 09, 2010 at 02:34:45PM +0100, Garth N. Wells wrote: > > > > > > > > > On Mon, 2010-08-09 at 15:20 +0200, Anders Logg wrote: > > > > > > > > > > On Mon, Aug 09, 2010 at 02:12:49PM +0100, Garth N. Wells > > > > > > > > > > wrote: > > > > > > > > > > > On Mon, 2010-08-09 at 15:05 +0200, Anders Logg wrote: > > > > > > > > > > > > On Mon, Aug 09, 2010 at 01:54:04PM +0100, Garth N. > > > > > > > > > > > > Wells wrote: > > > > > > > > > > > > > On Mon, 2010-08-09 at 14:46 +0200, Anders Logg wrote: > > > > > > > > > > > > > > On Mon, Aug 09, 2010 at 01:09:47PM +0100, Garth N. > > > > > > > > > > > > > > Wells wrote: > > > > > > > > > > > > > > > On Mon, 2010-08-09 at 13:53 +0200, Anders Logg > > > > > > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > On Mon, Aug 09, 2010 at 12:47:10PM +0100, Garth > > > > > > > > > > > > > > > > N. Wells wrote: > > > > > > > > > > > > > > > > > On Mon, 2010-08-09 at 13:37 +0200, Anders > > > > > > > > > > > > > > > > > Logg wrote: > > > > > > > > > > > > > > > > > > On Sat, Aug 07, 2010 at 01:24:44PM +0100, > > > > > > > > > > > > > > > > > > Garth N. Wells wrote: > > > > > > > > > > > > > > > > > > > On Fri, 2010-08-06 at 19:55 +0100, Garth > > > > > > > > > > > > > > > > > > > N. Wells wrote: > > > > > > > > > > > > > > > > > > > > On Fri, 2010-08-06 at 20:53 +0200, > > > > > > > > > > > > > > > > > > > > Anders Logg wrote: > > > > > > > > > > > > > > > > > > > > > On Fri, Aug 06, 2010 at 07:51:18PM > > > > > > > > > > > > > > > > > > > > > +0100, Garth N. Wells wrote: > > > > > > > > > > > > > > > > > > > > > > On Fri, 2010-08-06 at 20:36 +0200, > > > > > > > > > > > > > > > > > > > > > > Anders Logg wrote: > > > > > > > > > > > > > > > > > > > > > > > On Fri, Aug 06, 2010 at > > > > > > > > > > > > > > > > > > > > > > > 04:55:44PM +0100, Garth N. Wells > > > > > > > > > > > > > > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > > > > On Fri, 2010-08-06 at 08:42 > > > > > > > > > > > > > > > > > > > > > > > > -0700, Johan Hake wrote: > > > > > > > > > > > > > > > > > > > > > > > > > On Friday August 6 2010 > > > > > > > > > > > > > > > > > > > > > > > > > 08:16:26 you wrote: > > > > > > > > > > > > > > > > > > > > > > > > > > ------------------------------------------------------------ > > > > > > > > > > > > > > > > > > > > > > > > > > revno: 4896 > > > > > > > > > > > > > > > > > > > > > > > > > > committer: Garth N. Wells > > > > > > > > > > > > > > > > > > > > > > > > > > <gn...@cam.ac.uk> > > > > > > > > > > > > > > > > > > > > > > > > > > branch nick: dolfin-all > > > > > > > > > > > > > > > > > > > > > > > > > > timestamp: Fri 2010-08-06 > > > > > > > > > > > > > > > > > > > > > > > > > > 16:13:29 +0100 > > > > > > > > > > > > > > > > > > > > > > > > > > message: > > > > > > > > > > > > > > > > > > > > > > > > > > Add simple Stokes solver > > > > > > > > > > > > > > > > > > > > > > > > > > for parallel testing. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Other Stokes demos don't > > > > > > > > > > > > > > > > > > > > > > > > > > run in parallel because > > > > > > > > > > > > > > > > > > > > > > > > > > MeshFunction io is not > > > > > > > > > > > > > > > > > > > > > > > > > > supported in parallel. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Does anyone have an overview > > > > > > > > > > > > > > > > > > > > > > > > > of what is needed for this to > > > > > > > > > > > > > > > > > > > > > > > > > be fixed. I > > > > > > > > > > > > > > > > > > > > > > > > > couldn't find a blueprint on > > > > > > > > > > > > > > > > > > > > > > > > > it. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Here it is: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > https://blueprints.launchpad.net/dolfin/+spec/parallel-io > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > I am interested in getting > > > > > > > > > > > > > > > > > > > > > > > > > this fixed :) > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Me too! We need to look at all > > > > > > > > > > > > > > > > > > > > > > > > the io since much of it is > > > > > > > > > > > > > > > > > > > > > > > > broken in > > > > > > > > > > > > > > > > > > > > > > > > parallel. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > We need to settle on how to > > > > > > > > > > > > > > > > > > > > > > > > handle XML data. I favour (and > > > > > > > > > > > > > > > > > > > > > > > > I know Niclas > > > > > > > > > > > > > > > > > > > > > > > > Janson does too) the VTK > > > > > > > > > > > > > > > > > > > > > > > > approach in which we have a > > > > > > > > > > > > > > > > > > > > > > > > 'master file' that > > > > > > > > > > > > > > > > > > > > > > > > points to other XML files which > > > > > > > > > > > > > > > > > > > > > > > > contain portions of the > > > > > > > > > > > > > > > > > > > > > > > > vector/mesh, > > > > > > > > > > > > > > > > > > > > > > > > etc. Process zero can read the > > > > > > > > > > > > > > > > > > > > > > > > 'master file' and then instruct > > > > > > > > > > > > > > > > > > > > > > > > the other > > > > > > > > > > > > > > > > > > > > > > > > processes on which file(s) they > > > > > > > > > > > > > > > > > > > > > > > > should read in. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > This only works if the data is > > > > > > > > > > > > > > > > > > > > > > > already partitioned. Most of our > > > > > > > > > > > > > > > > > > > > > > > demos > > > > > > > > > > > > > > > > > > > > > > > assume that we have the mesh in > > > > > > > > > > > > > > > > > > > > > > > one single file which is then > > > > > > > > > > > > > > > > > > > > > > > partitioned on the fly. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > The approach does work for data > > > > > > > > > > > > > > > > > > > > > > which is not partitioned. Just like > > > > > > > > > > > > > > > > > > > > > > with > > > > > > > > > > > > > > > > > > > > > > VTK, one can read the 'master file' > > > > > > > > > > > > > > > > > > > > > > or the individual files. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > The initial plan was to support > > > > > > > > > > > > > > > > > > > > > > > two different ways of reading > > > > > > > > > > > > > > > > > > > > > > > data in parallel: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > 1. One file and automatic > > > > > > > > > > > > > > > > > > > > > > > partitioning > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > DOLFIN gets one file "mesh.xml", > > > > > > > > > > > > > > > > > > > > > > > each process reads one part of it > > > > > > > > > > > > > > > > > > > > > > > (just > > > > > > > > > > > > > > > > > > > > > > > skipping other parts of the > > > > > > > > > > > > > > > > > > > > > > > file), then the mesh is > > > > > > > > > > > > > > > > > > > > > > > partitioned and > > > > > > > > > > > > > > > > > > > > > > > redistributed. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > 2. Several files and no > > > > > > > > > > > > > > > > > > > > > > > partitioning > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > DOLFIN get multiple files and > > > > > > > > > > > > > > > > > > > > > > > each process reads one part. In > > > > > > > > > > > > > > > > > > > > > > > this > > > > > > > > > > > > > > > > > > > > > > > case, the mesh and all associated > > > > > > > > > > > > > > > > > > > > > > > data is already partitioned. This > > > > > > > > > > > > > > > > > > > > > > > should be very easy to fix since > > > > > > > > > > > > > > > > > > > > > > > everything that is needed is > > > > > > > > > > > > > > > > > > > > > > > already > > > > > > > > > > > > > > > > > > > > > > > in place; we just need to fix the > > > > > > > > > > > > > > > > > > > > > > > logic. In particular, the data > > > > > > > > > > > > > > > > > > > > > > > section of each local mesh > > > > > > > > > > > > > > > > > > > > > > > contains all auxilliary parallel > > > > > > > > > > > > > > > > > > > > > > > data. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > This can be handled in two > > > > > > > > > > > > > > > > > > > > > > > different ways. Either a user > > > > > > > > > > > > > > > > > > > > > > > specifies the > > > > > > > > > > > > > > > > > > > > > > > name of the file as "mesh*.xml", > > > > > > > > > > > > > > > > > > > > > > > in which case DOLFIN appends say > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > "_%d" % MPI::process_number() > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > on each local process. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > The other way is to have a master > > > > > > > > > > > > > > > > > > > > > > > file which lists all the other > > > > > > > > > > > > > > > > > > > > > > > files. In this case, I don't see > > > > > > > > > > > > > > > > > > > > > > > a need for process 0 to take any > > > > > > > > > > > > > > > > > > > > > > > kind > > > > > > > > > > > > > > > > > > > > > > > of responsibility for > > > > > > > > > > > > > > > > > > > > > > > communicating file names. It > > > > > > > > > > > > > > > > > > > > > > > would work fine for > > > > > > > > > > > > > > > > > > > > > > > each process to read the master > > > > > > > > > > > > > > > > > > > > > > > file and then check which file it > > > > > > > > > > > > > > > > > > > > > > > should use. Each process could > > > > > > > > > > > > > > > > > > > > > > > also check that the total number > > > > > > > > > > > > > > > > > > > > > > > of > > > > > > > > > > > > > > > > > > > > > > > processes matches the number of > > > > > > > > > > > > > > > > > > > > > > > partitions in the file. We could > > > > > > > > > > > > > > > > > > > > > > > let > > > > > > > > > > > > > > > > > > > > > > > process 0 handle the parsing of > > > > > > > > > > > > > > > > > > > > > > > the master file and then > > > > > > > > > > > > > > > > > > > > > > > communicate > > > > > > > > > > > > > > > > > > > > > > > the file names but maybe that is > > > > > > > > > > > > > > > > > > > > > > > an extra complication. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > This fails when the number of files > > > > > > > > > > > > > > > > > > > > > > differs from the number of > > > > > > > > > > > > > > > > > > > > > > processes. It's very important to > > > > > > > > > > > > > > > > > > > > > > support m files on n processes. > > > > > > > > > > > > > > > > > > > > > > We've > > > > > > > > > > > > > > > > > > > > > > discussed this at length before. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > I don't remember. Can you remind me > > > > > > > > > > > > > > > > > > > > > of what the reasons are? > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > I perform a simulation using m > > > > > > > > > > > > > > > > > > > > processes, and write the result to m > > > > > > > > > > > > > > > > > > > > files. Later I want to use the result > > > > > > > > > > > > > > > > > > > > later in another computation using > > > > > > > > > > > > > > > > > > > > n processors. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > I've looked a little into parallel io, > > > > > > > > > > > > > > > > > > > and looked at what Trilinos and > > > > > > > > > > > > > > > > > > > PETSc do. Both support HDF5, and HDF5 has > > > > > > > > > > > > > > > > > > > been developed to work > > > > > > > > > > > > > > > > > > > parallel. HDF5 does not advocate the > > > > > > > > > > > > > > > > > > > one-file per processes (too awkward > > > > > > > > > > > > > > > > > > > and complicated they say), but advocates > > > > > > > > > > > > > > > > > > > a one file approach. It has > > > > > > > > > > > > > > > > > > > tools that allow different processes to > > > > > > > > > > > > > > > > > > > write to different parts of the > > > > > > > > > > > > > > > > > > > same file in parallel. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > >From reading this, what I propose (for > > > > > > > > > > > > > > > > > > > >now) is: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > 1. We only ever write one XML file for a > > > > > > > > > > > > > > > > > > > given object. This file can be > > > > > > > > > > > > > > > > > > > read by different processes, with each > > > > > > > > > > > > > > > > > > > reading in only a chunk. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > 2. We should add an XML format for > > > > > > > > > > > > > > > > > > > partitioning data (Trilinos calls > > > > > > > > > > > > > > > > > > > this a 'map'). If a map file is present, > > > > > > > > > > > > > > > > > > > it is used to define the > > > > > > > > > > > > > > > > > > > partitions. It may make sense to have a > > > > > > > > > > > > > > > > > > > map file for each process (but > > > > > > > > > > > > > > > > > > > no need for a 'master file'). > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > I suggest something slightly different. I'm > > > > > > > > > > > > > > > > > > ok with the one file > > > > > > > > > > > > > > > > > > approach, but it would be good to store > > > > > > > > > > > > > > > > > > that data in a partitioned > > > > > > > > > > > > > > > > > > way. Our current model for parallel > > > > > > > > > > > > > > > > > > computing is that each process has > > > > > > > > > > > > > > > > > > a Mesh and each process has the > > > > > > > > > > > > > > > > > > partitioning data it needs stored in > > > > > > > > > > > > > > > > > > the data section of the Mesh. So each > > > > > > > > > > > > > > > > > > process has just a regular mesh > > > > > > > > > > > > > > > > > > with some auxilliary data attached to it. > > > > > > > > > > > > > > > > > > That makes it easy to read > > > > > > > > > > > > > > > > > > and write using already existing code. (No > > > > > > > > > > > > > > > > > > need for a special parallel > > > > > > > > > > > > > > > > > > format.) > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > But we could easily throw all that data > > > > > > > > > > > > > > > > > > into one big file, something > > > > > > > > > > > > > > > > > > like this: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > <distributed_mesh num_parts="16"> > > > > > > > > > > > > > > > > > > <mesh ...> > > > > > > > > > > > > > > > > > > ... > > > > > > > > > > > > > > > > > > </mesh> > > > > > > > > > > > > > > > > > > <mesh ...> > > > > > > > > > > > > > > > > > > ... > > > > > > > > > > > > > > > > > > </mesh> > > > > > > > > > > > > > > > > > > ... > > > > > > > > > > > > > > > > > > </distributed_mesh> > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > I would like to separate mesh and > > > > > > > > > > > > > > > > > partitioning data. A partitioning of a > > > > > > > > > > > > > > > > > given mesh is not unique to a mesh, so it > > > > > > > > > > > > > > > > > should be separated. A > > > > > > > > > > > > > > > > > partition could still go in the same XML file > > > > > > > > > > > > > > > > > though. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > It is separate from the mesh, but it is stored > > > > > > > > > > > > > > > > locally as part of the > > > > > > > > > > > > > > > > Mesh in MeshData. I think this has proved to be > > > > > > > > > > > > > > > > a very good way > > > > > > > > > > > > > > > > (efficient and simple) to store the data. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > This breaks the concept of using a given Mesh on > > > > > > > > > > > > > > > a different number of > > > > > > > > > > > > > > > processes. For a given mesh, I may want to store > > > > > > > > > > > > > > > and use different > > > > > > > > > > > > > > > partitions. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > The same applied to Vectors. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Or do you suggest that we store it differently > > > > > > > > > > > > > > > > on file than we do as > > > > > > > > > > > > > > > > part of the DOLFIN data structures? > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > I suggest that we decouple the Mesh and the > > > > > > > > > > > > > > > partition data for file > > > > > > > > > > > > > > > output. It comes back to permitting a variable > > > > > > > > > > > > > > > number of processes. > > > > > > > > > > > > > > > > > > > > > > > > > > > > I thought that the storage output from m processes > > > > > > > > > > > > > > would be the mesh > > > > > > > > > > > > > > partitioned into m pieces with auxilliary > > > > > > > > > > > > > > partitioning > > > > > > > > > > > > > > information. This would then be read by n processes > > > > > > > > > > > > > > and repartitioned. > > > > > > > > > > > > > > > > > > > > > > > > > > > > If you don't want the partitioning into a specific > > > > > > > > > > > > > > number of > > > > > > > > > > > > > > partitions (like m partitions above), then what > > > > > > > > > > > > > > extra partitioning > > > > > > > > > > > > > > data would be required? It would just be a regular > > > > > > > > > > > > > > mesh file. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > It would map entities to processes. It could also be > > > > > > > > > > > > > used for Vectors. > > > > > > > > > > > > > > > > > > > > > > > > Then how is that not coupled to a specific number of > > > > > > > > > > > > processes? > > > > > > > > > > > > > > > > > > > > > > > > And why would it be better than what we have today, > > > > > > > > > > > > which stores all > > > > > > > > > > > > the parallel data we need (not just that mapping but > > > > > > > > > > > > everything else > > > > > > > > > > > > that can and must be generated from it) as part of each > > > > > > > > > > > > mesh? > > > > > > > > > > > > > > > > > > > > > > > > > > Partitioning data is always coupled to a specific > > > > > > > > > > > > > > number of processes > > > > > > > > > > > > > > as far as I can imagine. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Naturally, and so it should be. But the mesh output > > > > > > > > > > > > > doesn't have to be, > > > > > > > > > > > > > which why I suggest having a Mesh and partitioning > > > > > > > > > > > > > data, e.g. > > > > > > > > > > > > > > > > > > > > > > > > > > // Let DOLFIN partition and distribute the mesh > > > > > > > > > > > > > Mesh meshA("mesh.xml") > > > > > > > > > > > > > > > > > > > > > > > > Is this just one regular mesh file? > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Yes, and ParMETIS would partition the mesh. > > > > > > > > > > > > > > > > > > > > > > > > // Partition the mesh according to my_partition.xml > > > > > > > > > > > > > mesh. Throw an > > > > > > > > > > > > > // error if there is a mis-match between > > > > > > > > > > > > > my_partition and the number > > > > > > > > > > > > > // of processes > > > > > > > > > > > > > Mesh meshB("mesh.xml", "my_partition.xml") > > > > > > > > > > > > > > > > > > > > > > > > I don't understand this use-case. The first line above > > > > > > > > > > > > would already > > > > > > > > > > > > partition the mesh (using ParMETIS), and then you want > > > > > > > > > > > > to repartition > > > > > > > > > > > > it. Is it the case that my_partition.xml contains a > > > > > > > > > > > > better > > > > > > > > > > > > partitioning than what ParMETIS can compute? > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > In the case "meshB("mesh.xml", "my_partition.xml")", > > > > > > > > > > > ParMETIS would > > > > > > > > > > > never be called. The mesh would be distributed according > > > > > > > > > > > to > > > > > > > > > > > "my_partition.xml". > > > > > > > > > > > > > > > > > > > > I missed that you created two different meshes. ok, that > > > > > > > > > > sounds like a > > > > > > > > > > good thing to have. > > > > > > > > > > > > > > > > > > > > But how would the mesh.xml be stored on file? Just one big > > > > > > > > > > file? > > > > > > > > > > > > > > > > > > > > > > > > > > > > Yes, just as we have now. > > > > > > > > > > > > > > > > ok. > > > > > > > > > > > > > > > > > > And how is the above example related to the storing of > > > > > > > > > > *partitioned* > > > > > > > > > > meshes? > > > > > > > > > > > > > > > > > > > > > > > > > > > > >From reading around, I don't think that we should store > > > > > > > > > >partitioned > > > > > > > > > meshes in the sense that each partition is a separate file. > > > > > > > > > We should > > > > > > > > > store an entire mesh in one file, and store the partitioning > > > > > > > > > data if > > > > > > > > > desired. > > > > > > > > > > > > > > > > I don't understand why not. It's a good feature to have to be > > > > > > > > able to > > > > > > > > restart a simulation (on the same number of processors) and > > > > > > > > completely > > > > > > > > avoid the partitioning step. Moreover, everything we need is > > > > > > > > already > > > > > > > > implemented since we can read/write meshes including mesh data. > > > > > > > > > > > > > > > > > > > > > > It is a good feature. To do it, save the mesh and the partition, > > > > > > > and > > > > > > > then restart using > > > > > > > > > > > > > > Mesh mesh("mesh.xml", "partition.xml"); > > > > > > > > > > > > > > This will not involve any partitioning. > > > > > > > > > > > > It's true that it wouldn't involve any partitioning (as in calling > > > > > > ParMETIS) but it would involve redistributing the mesh between all > > > > > > processes and computing all the auxilliary data that is needed, like > > > > > > the global vertex numbering and mappings to neighboring processes. > > > > > > > > > > No, because the process would only read the bits which it needs. The > > > > > partition data would contains the other data. > > > > > > > > So it would not just be a simple list of markers for which cells > > > > belong where? It would be all that other stuff that we stored in > > > > MeshData today? > > > > > > > > > > By storing one file per process (which is almost already supported), > > > > > > that would also be avoided. Each process would read a regular mesh > > > > > > file and be ready to go. No communication and no computation > > > > > > involved. > > > > > > > > > > > > > > In summary, if I understand correctly that you want things > > > > > > > > stored in > > > > > > > > one file, we would have the following: > > > > > > > > > > > > > > > > 1. Reading from one file and automatic > > > > > > > > partitioning/redistribution > > > > > > > > > > > > > > > > This is already supported today, and it already supports the > > > > > > > > use case > > > > > > > > of first running on m processors and then n processors since in > > > > > > > > both > > > > > > > > cases the mesh will be read from one single file and > > > > > > > > partitioned. > > > > > > > > > > > > > > > > > > > > > > Yes. > > > > > > > > > > > > > > > 2. Reading from multiple files where the mesh has already been > > > > > > > > partitioned and m = n. This is almost supported today. We just > > > > > > > > need to > > > > > > > > decide on the logic for reading from multiple files. > > > > > > > > > > > > > > > > > > > > > > I advocate not supporting multiple files. Keeping to one > > > > > > > file-file model > > > > > > > makes things simple. > > > > > > > > > > > > Multiple files would not be much of a complication since it is > > > > > > almost > > > > > > already in place. Each process just reads its corresponding file > > > > > > without any communication. > > > > > > > > > > > > > > > > Having dug around in the XML files over the past few days, I wouldn't > > > > > agree with this. It may seem outwardly simple, but it means files > > > > > having > > > > > different states and it gets messy. > > > > > > > > What do you mean by different states? Each process just reads a plain > > > > mesh XML file which is no different from a regular mesh file except > > > > that the mesh data would contain a few more things. > > > > > > > > > > In one case. But we also support reading/writing from/to one file or > > > multiple files. We need to decide who should open a file (is object > > > local or distributed), should DOLFIN or the user add a process suffix, > > > how should the object be created (which depends on how it's read in). > > > It's simpler (for the user and the developer) if there is just one way > > > to do something. > > > > How about the following: > > > > 1. Input/output is always just one file > > > > 2. Always assume parallel when MPI::num_processes() > 1 > > > > 3. When reading a regular mesh file, it is always partitioned > > > > This round back-to-front. The majority of meshes read in won't already > be partitioned. > > > 4. The partitioning is either handled by ParMETIS, or alternatively, > > according to a file or MeshFunction<uint> specified by the user. In > > particular, the file containing the partition can just be a regular > > MeshFunction<uint> stored in XML. > > > > 5. When specifying the partition as above, the only difference from > > now is that the call to ParMETIS is bypassed. The rest remains the > > same. > > > > 6. When writing a mesh in parallel, we write something like this: > > > > <distributed_mesh> > > <mesh> > > > > </mesh> > > <mesh> > > > > </mesh> > > ... > > </distributed_mesh> > > > > 7. When reading in parallel, DOLFIN will check if it gets a regular > > mesh file (in which case it partitions it like in 3) or if it gets a > > distributed mesh. In that case, it will check that the number of > > processors matches (m = n) and then just continue to let each process > > read the data it needs by just skipping to the appropriate location in > > the file and reading the portion it needs. > > > > That way, it would be very clear what happens. Clear to the user (by > > just inspecting the XML file, is it a mesh or a distributed mesh) and > > to DOLFIN. > > > > It doesn't seem that I'm making my point very clearly ;).
Perhaps not... ;-) Keep trying. At some point, I might see the light.
> A mesh is a mesh. When I open the mesh file I want to be able to just
> read it, no matter how many processes I'm using or how it was previously
> partitioned. This is why I object to partitioning data being stored in
> the mesh. We shouldn't have <distributed_mesh>.
So you advocate just the same old mesh format we have today? (And
possibly some additional HDF5 format later if/when we need it.)
> If we have a MeshFuction which defines a partition, with this the mesh
> XML reader knows which parts of the mesh file it is responsible for.
Yes, and I agree that's a good feature. My point is just this, if you
want to rerun a simulation on m = n processors, you have three options:
1. Just read from mesh.xml and let ParMETIS recompute the partitioning
and redistribute the mesh.
2. Read from mesh.xml and supply partition.xml (storing a
MeshFunction). DOLFIN can then use this to accomplish two things:
(a) Each process reads exactly the data it needs (no redistribution necessary)
(b) No need to call ParMETIS
We would still need to recompute all auxilliary parallel data (like
the mapping to neighboring processes).
3. Read from either one file containing <distribute_mesh> or from
multiple files with plain meshes. This would by-pass all the above
steps and be the most efficient step.
The points I want to make is that case (2) does not handle anything
that case (3) does not handle; case (3) is the most efficient; and
case (3) is (almost) already implemented so there are no complications
involved in handling it.
My suggestion for handling case (3) is (as before) the following:
Mesh mesh("mesh*.xml")
The * would indicate that we should read multiple files.
I'll be offline for a few hours so we can continue this lengthy
discussion later tonight... :-)
--
Anders
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