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DOI

Device AWare Gather Scatter (DAWGS)

dawgs is the testbed development of a GPU-enabled gather scatter library and experimentation of GPU-aware MPI.

The problem setup is a uniform NX x NY x NZ mesh of hexahehral elements, one which a global node ordering is constructed and a gather-scatter operation is run and benchmarked for several MPI communication algorithms.

How to compile dawgs

There are a couple of prerequisites for building dawgs;

  • An MPI stack. Any will work;

Installing MPI can be done using whatever package manager your operating system provides.

To build and run dawgs:

$ git clone --recursive <dawgs repo>
$ cd /path/to/dawgs
$ make -j `nproc`

To build and run dawgs with GPU-aware MPI support:

$ make -j `nproc` gpu-aware-mpi=true

How to run dawgs

Here is an example problem size that you can run on one GPU:

$ mpirun -np 1 ./dawgsMain -m HIP -nx 24 -ny 24 -nz 24 -p 3

Here is the meaning of each of the command line options

  • nx: the number of spectral elements in the x-direction per MPI rank
  • ny: the number of spectral elements in the y-direction per MPI rank
  • nz: the number of spectral elements in the z-direction per MPI rank
  • p: the order of the polynomial (the number of nodes per hex is (p+1)^3)
  • m: the mode to run OCCA in, HIP is for AMD GPUs but CUDA and Serial are also supported

Running on multiple GPUs can by done by passing a larger argument to np and specifying the number of MPI ranks in each coordinate direction:

$ mpirun -np 2 ./dawgsMain -m HIP -nx 24 -ny 24 -nz 24 -p 3

Verifying correctness

To verify that the computation is correct, an optional -cc TRUE argument can be passed. Upen doing so, the result from a gather-scatter is compared with a legacy implementation using gslib:

$ mpirun -np 2 ./dawgsMain -m CUDA -nx 10 -ny 10 -nz 10 -p 3 -cc TRUE
...
Device GatherScatter:  Error = 0
Host   GatherScatter:  Error = 0
Device GatherScatter:  Error = 0
Host   GatherScatter:  Error = 0
Device Gather+Scatter: Error = 0
Host   Gather+Scatter: Error = 0

All Errors should report zero, or near zero.

Scaling Studies

An optional -sw argument can be passed to run a collection of problem sizes and polynomial degrees for quickly collecting scaling data. Other input parameters are typically not required for this mode:

$ ./dawgsMain -m CUDA -sw TRUE
...
Host GatherScatter:   Ranks = 1, Global DOFS = 42875, Max Local DOFS = 314432, Degree = 1, Time taken = 0.0017945 s, DOFS/s = 2.38924e+07, DOFS/(s*rank) = 2.38924e+07
Device GatherScatter: Ranks = 1, Global DOFS = 42875, Max Local DOFS = 314432, Degree = 1, Time taken = 1.94e-05 s, DOFS/s = 2.21005e+09, DOFS/(s*rank) = 2.21005e+09
Host Gather:          Ranks = 1, Global DOFS = 42875, Max Local DOFS = 314432, Degree = 1, Time taken = 0.0011335 s, DOFS/s = 3.78253e+07, DOFS/(s*rank) = 3.78253e+07
Device Gather:        Ranks = 1, Global DOFS = 42875, Max Local DOFS = 314432, Degree = 1, Time taken = 1.45e-05 s, DOFS/s = 2.9569e+09, DOFS/(s*rank) = 2.9569e+09
Host Scatter:         Ranks = 1, Global DOFS = 42875, Max Local DOFS = 314432, Degree = 1, Time taken = 0.0013037 s, DOFS/s = 3.28872e+07, DOFS/(s*rank) = 3.28872e+07
Device Scatter:       Ranks = 1, Global DOFS = 42875, Max Local DOFS = 314432, Degree = 1, Time taken = 1.52e-05 s, DOFS/s = 2.82072e+09, DOFS/(s*rank) = 2.82072e+09
...

How to clean build objects

To clean the dawgs build objects:

$ cd /path/to/dawgs/repo
$ make realclean

Please invoke make help for more supported options.