162 lines
6.4 KiB
Plaintext
162 lines
6.4 KiB
Plaintext
"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
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:link(lws,http://lammps.sandia.gov)
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:link(ld,Manual.html)
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:link(lc,Section_commands.html#comm)
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:line
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package command :h3
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[Syntax:]
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package style args :pre
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style = {gpu} or {cuda} or {omp} :ulb,l
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args = arguments specific to the style :l
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{gpu} args = mode first last split
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mode = force or force/neigh
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first = ID of first GPU to be used on each node
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last = ID of last GPU to be used on each node
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split = fraction of particles assigned to the GPU
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{cuda} args = to be determined
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{omp} args = Nthreads mode
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Nthreads = # of OpenMP threads to associate with each MPI process
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mode = force or force/neigh (optional) :pre
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:ule
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[Examples:]
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package gpu force 0 0 1.0
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package gpu force 0 0 0.75
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package gpu force/neigh 0 0 1.0
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package gpu force/neigh 0 1 -1.0
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package cuda blah
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package omp * force/neigh
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package omp 4 force :pre
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[Description:]
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This command invokes package-specific settings. Currently the
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following packages use it: GPU, USER-CUDA, and USER-OMP.
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See "this section"_Section_accelerate.html of the manual for more
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details about using these various packages for accelerating
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a LAMMPS calculation.
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:line
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The {gpu} style invokes options associated with the use of the GPU
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package. It allows you to select and initialize GPUs to be used for
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acceleration via this package and configure how the GPU acceleration
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is performed. These settings are required in order to use any style
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with GPU acceleration.
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The {mode} setting specifies where neighbor list calculations will be
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performed. If {mode} is force, neighbor list calculation is performed
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on the CPU. If {mode} is force/neigh, neighbor list calculation is
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performed on the GPU. GPU neighbor list calculation currently cannot
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be used with a triclinic box. GPU neighbor list calculation currently
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cannot be used with "hybrid"_pair_hybrid.html pair styles. GPU
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neighbor lists are not compatible with styles that are not
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GPU-enabled. When a non-GPU enabled style requires a neighbor list,
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it will also be built using CPU routines. In these cases, it will
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typically be more efficient to only use CPU neighbor list builds.
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The {first} and {last} settings specify the GPUs that will be used for
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simulation. On each node, the GPU IDs in the inclusive range from
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{first} to {last} will be used.
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The {split} setting can be used for load balancing force calculation
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work between CPU and GPU cores in GPU-enabled pair styles. If 0 <
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{split} < 1.0, a fixed fraction of particles is offloaded to the GPU
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while force calculation for the other particles occurs simulataneously
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on the CPU. If {split}<0, the optimal fraction (based on CPU and GPU
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timings) is calculated every 25 timesteps. If {split} = 1.0, all force
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calculations for GPU accelerated pair styles are performed on the
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GPU. In this case, "hybrid"_pair_hybrid.html, "bond"_bond_style.html,
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"angle"_angle_style.html, "dihedral"_dihedral_style.html,
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"improper"_improper_style.html, and "long-range"_kspace_style.html
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calculations can be performed on the CPU while the GPU is performing
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force calculations for the GPU-enabled pair style. If all CPU force
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computations complete before the GPU, LAMMPS will block until the GPU
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has finished before continuing the timestep.
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As an example, if you have two GPUs per node and 8 CPU cores per node,
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and would like to run on 4 nodes (32 cores) with dynamic balancing of
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force calculation across CPU and GPU cores, you could specify
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package gpu force/neigh 0 1 -1 :pre
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In this case, all CPU cores and GPU devices on the nodes would be
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utilized. Each GPU device would be shared by 4 CPU cores. The CPU
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cores would perform force calculations for some fraction of the
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particles at the same time the GPUs performed force calculation for
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the other particles.
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:line
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The {cuda} style invokes options associated with the use of the
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USER-CUDA package. These still need to be documented.
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:line
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The {omp} style invokes options associated with the use of the
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USER-OMP package.
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The first argument allows to explicitly set the number of OpenMP threads
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to be allocated for each MPI process. For example, if your system has nodes
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with dual quad-core processors, it has a total of 8 cores per node.
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You could run MPI on 2 cores on each node (e.g. using options for the
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mpirun command), and set the {Nthreads} setting to 4. This would
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effectively use all 8 cores on each node. Since each MPI process
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would spawn 4 threads (one of which runs as part of the MPI process
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itself).
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For performance reasons, you should not set {Nthreads} to more threads
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than there are physical cores (per MPI task), but LAMMPS cannot check
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for this.
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An {Nthreads} value of '*' instructs LAMMPS to use whatever is the
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default for the given OpenMP environment. This is usually determined
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via the {OMP_NUM_THREADS} environment variable or the compiler runtime.
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Please note that in most cases the default for OpenMP capable compilers
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is to use one thread for each available CPU core when {OMP_NUM_THREADS}
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is not set, which can lead to extremely bad performance.
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Which combination of threads and MPI tasks gives the best performance
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is difficult to predict and can depend on many components of your input.
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Not all features of LAMMPS support OpenMP and the parallel efficiency
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can be very different, too.
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The {mode} setting specifies where neighbor list calculations will be
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multi-threaded as well. If {mode} is force, neighbor list calculation
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is performed in serial. If {mode} is force/neigh, a multi-threaded
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neighbor list build is used. Using the force/neigh setting is almost
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always faster and should produce idential neighbor lists at the expense
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of using some more memory (neighbor list pages are always allocated
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for all threads at the same time and each thread works on its own pages).
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:line
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[Restrictions:]
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This command cannot be used after the simulation box is defined by a
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"read_data"_read_data.html or "create_box"_create_box.html command.
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The cuda style of this command can only be invoked if LAMMPS was built
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with the USER-CUDA package. See the "Making
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LAMMPS"_Section_start.html#start_3 section for more info.
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The gpu style of this command can only be invoked if LAMMPS was built
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with the GPU package. See the "Making
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LAMMPS"_Section_start.html#start_3 section for more info.
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The omp style of this command can only be invoked if LAMMPS was built
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with the USER-OMP package. See the "Making
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LAMMPS"_Section_start.html#start_3 section for more info.
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[Related commands:] none
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[Default:] none
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