lammps/lib/gpu

/***************************************************************************
                                   README
                             -------------------
                               W. Michael Brown

  README for building LAMMPS GPU Library

 __________________________________________________________________________
    This file is part of the LAMMPS GPU Library
 __________________________________________________________________________

    begin                : Thu Jun 25 2009
    copyright            : (C) 2009 by W. Michael Brown
    email                : wmbrown@sandia.gov
 ***************************************************************************/

/* -----------------------------------------------------------------------
   Copyright (2009) Sandia Corporation.  Under the terms of Contract
   DE-AC04-94AL85000 with Sandia Corporation, the U.S. Government retains
   certain rights in this software.  This software is distributed under 
   the GNU General Public License.
   ----------------------------------------------------------------------- */


                          GENERAL NOTES
                          
This library, pair_gpu_lib.a, provides routines for GPGPU acceleration of
LAMMPS pair styles. Currently, only CUDA enabled GPUs are supported. 
Compilation of this library requires installing the CUDA GPU driver and
CUDA toolkit for your operating system. In addition to the LAMMPS library,
the binary nvc_get_devices will also be built. This can be used to query
the names and properties of GPU devices on your system.

NOTE: Installation of the CUDA SDK is not required.

Current pair styles supporting GPU Accelartion:

  1. lj/cut/gpu
  2. gayberne/gpu



                     MULTIPLE LAMMPS PROCESSES
                     
When using GPGPU acceleration, you are restricted to one physical GPU per
LAMMPS process. This can be multiple GPUs on a single node or across multiple
nodes. Intructions on GPU assignment can be found in the LAMMPS documentation.
          


                            SPEEDUPS

The speedups that can be obtained using this library are highly
dependent on the GPU architecture and the computational expense of
the pair potential. When comparing a single precision Tesla C1060 run to a 
serial Intel Xeon 5140 2.33 GHz serial run, the speedup is ~4.42x for lj/cut
with a cutoff of 2.5. For gayberne with a cutoff of 7, the speedup is >103x
for 8000 particles. The speedup will improve with an increase in the
number of particles or an increase in the cutoff.


                         
                    BUILDING AND PRECISION MODES

To build, edit the CUDA_CPP, CUDA_ARCH, CUDA_PREC, and CUDA_LINK files for
your machine. Type make. Additionally, the GPU package must be installed and
compiled for LAMMPS. The library supports 3 precision modes as determined by 
the CUDA_PREC variable:

  CUDA_PREC = -D_SINGLE_SINGLE  # Single precision for all calculations
  CUDA_PREC = -D_DOUBLE_DOUBLE  # Double precision for all calculations
  CUDA_PREC = -D_SINGLE_DOUBLE  # Accumulation of forces, etc. in double

NOTE: Double precision is only supported on certain GPUS (with
      compute capability>=1.3).
      
NOTE: The gayberne/gpu pair style will only be installed if the ASPHERE
      package has been installed before installing the GPU package in LAMMPS.



                           GPU MEMORY

Upon initialization of the pair style, the library will reserve memory for
64K atoms per GPU or 70% of each cards GPU memory, whichever value is limiting. 
The value of 70% can be changed by editing the PERCENT_GPU_MEMORY definition 
in the source file. The value of 64K cannot be increased and is the maximum 
number of atoms allowed per GPU. Using the 'neigh_modify one' modifier in
your LAMMPS input script can help to increase maximum number of atoms per
GPU for cards with limited memory.