Add basic information about HPG-MxP benchmark

hpg-mxp.org/index.md

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-# HPG-MxP
+# High Performance Generalized Minimal Residual - Mixed-Precision (HPG-MxP) Benchmark #
 
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+Ichitaro Yamazaki, Jennifer Loe, Christian Glusa, Sivasankaran Rajamanickam
+Piotr Luszczek, and Jack Dongarra
+
+## Introduction ##
+
+HPG-MxP is a software package that performs a fixed number of multigrid preconditioned
+(using a Gauss-Seidel smoother) Generalized minimal residual (PGMRES) iterations.
+
+The HPG-MxP rating is a weighted GFLOP/s (billion floating-point operations per
+second) value that is composed of the operations performed in the PGMRES
+iteration phase over the time taken to finish the solve.  The overheads such as
+the time of problem construction and any structural modifications to improve
+the final performance are amortized over several iterations (the amortization
+weight) and added to the runtime.
+
+Integer arrays describing the original problem structure have global and local
+scope (the global indices are unique across the entire distributed memory
+system, while the local indices are unique only within a single memory image of
+a single MPI rank).  Integer data for global/local indices have three modes:
+
+* 32/32 - global and local integers are 32-bit entities
+* 64/32 - global integers are 64-bit, local are 32-bit
+* 64/64 - global and local integers are 64-bit entities
+
+These various modes are required in order to address sufficiently big problems
+if the range of indexing exceeds value of 2 to power 31 (roughly 2.1B), or to
+conserve storage costs if the range of indexing is less than 2 to power 31.
+
+The HPG-MxP software package requires the availibility on your system of an
+implementation of the  Message Passing Interface (MPI) if  MPI build of HPG-MxP
+was enabled at configuration time, and a compiler that supports OpenMP pragma
+syntax.  An implementation compliant with MPI version 1.1 is sufficient.
+
+## Valid Runs ##
+
+HPG-MxP can be run in just a few minutes from start to finish.  However, official
+runs must be at least 1800 seconds (30 minutes) as reported in the output file.
+The Quick Path option is an exception for machines that are in production mode.
+In this situation, which should be confirmed by sending a note to the HPG-MxP
+Benchmark authors, the Quick Path option can be invoked by setting the run time
+parameter equal to 0 (zero).
+
+A valid run must also execute a problem size that is large enough so that data
+arrays accessed in the GMRES iteration loop do not fit in the cache of the device
+in a way that would be unrealistic in a real application setting.  At present, this
+restriction means that the problem size should be large enough to occupy a
+significant fraction of the *main memory*, or at least 1/4 of the total.
+
+Future memory system architectures may require restatement of the specific memory
+size requirements.  But the guiding principle will always be that the problem
+size should reflect what would be reasonable for a real sparse iterative solver.
+
+## Documentation ##
+
+The source code documentation can be generated with a Doxygen (version 1.8 or
+newer).