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284 lines
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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
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<HR>
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<H3>package command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>package style args
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</PRE>
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<UL><LI>style = <I>gpu</I> or <I>cuda</I> or <I>omp</I>
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<LI>args = arguments specific to the style
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<PRE> <I>gpu</I> args = mode first last split keyword value ...
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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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zero or more keyword/value pairs may be appended
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keywords = <I>threads_per_atom</I> or <I>cellsize</I>
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<I>threads_per_atom</I> value = Nthreads
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Nthreads = # of GPU threads used per atom
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<I>cellsize</I> value = dist
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dist = length (distance units) in each dimension for neighbor bins
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<I>cuda</I> args = keyword value ...
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one or more keyword/value pairs may be appended
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keywords = <I>gpu/node</I> or <I>gpu/node/special</I> or <I>timing</I> or <I>test</I> or <I>override/bpa</I>
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<I>gpu/node</I> value = N
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N = number of GPUs to be used per node
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<I>gpu/node/special</I> values = N gpu1 .. gpuN
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N = number of GPUs to be used per node
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gpu1 .. gpuN = N IDs of the GPUs to use
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<I>timing</I> values = none
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<I>test</I> values = id
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id = atom-ID of a test particle
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<I>override/bpa</I> values = flag
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flag = 0 for TpA algorithm, 1 for BpA algorithm
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<I>omp</I> 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)
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</PRE>
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>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 gpu/node/special 2 0 2
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package cuda test 3948
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package omp * force/neigh
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package omp 4 force
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>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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</P>
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<P>To use the accelerated GPU and USER-OMP styles, the use of the package
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command is required. However, as described in the "Defaults" section
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below, if you use the "-sf gpu" or "-sf omp" <A HREF = "Section_start.html#start_7">command-line
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options</A> to enable use of these styles,
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then default package settings are enabled. In that case you only need
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to use the package command if you want to change the defaults.
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</P>
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<P>To use the accelerate USER-CUDA styles, the package command is not
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required as defaults are assigned internally. You only need to use
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the package command if you want to change the defaults.
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</P>
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<P>See <A HREF = "Section_accelerate.html">Section_accelerate</A> of the manual for
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more details about using these various packages for accelerating
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LAMMPS calculations.
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</P>
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<HR>
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<P>The <I>gpu</I> style invokes options associated with the use of the GPU
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package.
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</P>
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<P>The <I>mode</I> setting specifies where neighbor list calculations will be
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performed. If <I>mode</I> is force, neighbor list calculation is performed
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on the CPU. If <I>mode</I> 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 <A HREF = "pair_hybrid.html">hybrid</A> 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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</P>
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<P>The <I>first</I> and <I>last</I> 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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<I>first</I> to <I>last</I> will be used.
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</P>
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<P>The <I>split</I> 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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<I>split</I> < 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 <I>split</I><0, the optimal fraction (based on CPU and GPU
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timings) is calculated every 25 timesteps. If <I>split</I> = 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, <A HREF = "pair_hybrid.html">hybrid</A>, <A HREF = "bond_style.html">bond</A>,
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<A HREF = "angle_style.html">angle</A>, <A HREF = "dihedral_style.html">dihedral</A>,
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<A HREF = "improper_style.html">improper</A>, and <A HREF = "kspace_style.html">long-range</A>
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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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</P>
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<P>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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</P>
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<PRE>package gpu force/neigh 0 1 -1
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</PRE>
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<P>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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</P>
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<P>The <I>threads_per_atom</I> keyword allows control of the number of GPU
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threads used per-atom to perform the short range force calculation.
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By default, the value will be chosen based on the pair style, however,
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the value can be set with this keyword to fine-tune performance. For
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large cutoffs or with a small number of particles per GPU, increasing
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the value can improve performance. The number of threads per atom must
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be a power of 2 and currently cannot be greater than 32.
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</P>
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<P>The <I>cellsize</I> keyword can be used to control the size of the cells used
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for binning atoms in neighbor list calculations. Setting this value is
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normally not needed; the optimal value is close to the default
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(equal to the cutoff distance for the short range interactions
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plus the neighbor skin). GPUs can perform efficiently with much larger cutoffs
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than CPUs and this can be used to reduce the time required for long-range
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calculations or in some cases to eliminate them with models such as
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<A HREF = "pair_coul.html">coul/wolf</A> or <A HREF = "pair_coul.html">coul/dsf</A>. For very large cutoffs,
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it can be more efficient to use smaller values for cellsize in parallel
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simulations. For example, with a cutoff of 20*sigma and a neighbor skin of
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sigma, a cellsize of 5.25*sigma can be efficient for parallel simulations.
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</P>
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<HR>
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<P>The <I>cuda</I> style invokes options associated with the use of the
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USER-CUDA package.
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</P>
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<P>The <I>gpu/node</I> keyword specifies the number <I>N</I> of GPUs to be used on
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each node. An MPI process with rank <I>K</I> will use the GPU (K mod N).
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This implies that processes should be assigned with successive ranks
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on each node, which is the default with most (or even all) MPI
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implementations. The default value for <I>N</I> is 2.
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</P>
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<P>The <I>gpu/node/special</I> keyword also specifies the number (N) of GPUs
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to be used on each node, but allows more control over their
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specification. An MPI process with rank <I>K</I> will use the GPU <I>gpuI</I>
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with l = (K mod N) + 1. This implies that processes should be assigned
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with successive ranks on each node, which is the default with most (or
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even all) MPI implementations. For example if you have three GPUs on
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a machine, one of which is used for the X-Server (the GPU with the ID
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1) while the others (with IDs 0 and 2) are used for computations you
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would specify:
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</P>
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<PRE>package cuda gpu/node/special 2 0 2
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</PRE>
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<P>A main purpose of the <I>gpu/node/special</I> optoin is to allow two (or
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more) simulations to be run on one workstation. In that case one
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would set the first simulation to use GPU 0 and the second to use GPU
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1. This is not necessary though, if the GPUs are in what is called
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<I>compute exclusive</I> mode. Using that setting, every process will get
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its own GPU automatically. This <I>compute exclusive</I> mode can be set
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as root using the <I>nvidia-smi</I> tool which is part of the CUDA
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installation.
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</P>
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<P>Note that if the <I>gpu/node/special</I> keyword is not used, the USER-CUDA
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package sorts existing GPUs on each node according to their number of
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multiprocessors. This way, compute GPUs will be priorized over
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X-Server GPUs.
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</P>
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<P>Use of the <I>timing</I> keyword will output detailed timing information
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for various subroutines.
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</P>
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<P>The <I>test</I> keyword will output info for the the specified atom at
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several points during each time step. This is mainly usefull for
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debugging purposes. Note that the simulation will be severly slowed
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down if this option is used.
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</P>
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<P>The <I>override/bpa</I> keyword can be used to specify which mode is used
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for pair-force evaluation. TpA = one thread per atom; BpA = one block
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per atom. If this keyword is not used, a short test at the begin of
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each run will determine which method is more effective (the result of
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this test is part of the LAMMPS output). Therefore it is usually not
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necessary to use this keyword.
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</P>
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<HR>
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<P>The <I>omp</I> style invokes options associated with the use of the
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USER-OMP package.
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</P>
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<P>The first argument allows to explicitly set the number of OpenMP
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threads to be allocated for each MPI process. For example, if your
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system has nodes with dual quad-core processors, it has a total of 8
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cores per node. You could run MPI on 2 cores on each node (e.g. using
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options for the mpirun command), and set the <I>Nthreads</I> setting to 4.
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This would effectively use all 8 cores on each node. Since each MPI
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process would spawn 4 threads (one of which runs as part of the MPI
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process itself).
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</P>
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<P>For performance reasons, you should not set <I>Nthreads</I> 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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</P>
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<P>An <I>Nthreads</I> 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 <I>OMP_NUM_THREADS</I> environment variable or the compiler
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runtime. Please note that in most cases the default for OpenMP
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capable compilers is to use one thread for each available CPU core
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when <I>OMP_NUM_THREADS</I> is not set, which can lead to extremely bad
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performance.
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</P>
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<P>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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</P>
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<P>The <I>mode</I> setting specifies where neighbor list calculations will be
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multi-threaded as well. If <I>mode</I> is force, neighbor list calculation
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is performed in serial. If <I>mode</I> 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
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expense of using some more memory (neighbor list pages are always
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allocated for all threads at the same time and each thread works on
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its own pages).
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</P>
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<HR>
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<P><B>Restrictions:</B>
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</P>
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<P>This command cannot be used after the simulation box is defined by a
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<A HREF = "read_data.html">read_data</A> or <A HREF = "create_box.html">create_box</A> command.
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</P>
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<P>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 <A HREF = "Section_start.html#start_3">Making
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LAMMPS</A> section for more info.
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</P>
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<P>The gpu style of this command can only be invoked if LAMMPS was built
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with the GPU package. See the <A HREF = "Section_start.html#start_3">Making
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LAMMPS</A> section for more info.
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</P>
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<P>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 <A HREF = "Section_start.html#start_3">Making
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LAMMPS</A> section for more info.
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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "suffix.html">suffix</A>
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</P>
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<P><B>Default:</B>
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</P>
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<P>If the "-sf gpu" <A HREF = "Section_start.html#start_7">command-line switch</A> is
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used then it is as if the command "package gpu force/neigh 0 0 1" were
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invoked, to specify default settings for the GPU package. If the
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command-line switch is not used, then no defaults are set, and you
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must specify the appropriate package command in your input script.
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</P>
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<P>The default settings for the USER CUDA package are "package cuda gpu
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2". This is the case whether the "-sf cuda" <A HREF = "Section_start.html#start_7">command-line
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switch</A> is used or not.
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</P>
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<P>If the "-sf omp" <A HREF = "Section_start.html#start_7">command-line switch</A> is
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used then it is as if the command "package omp *" were invoked, to
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specify default settings for the USER-OMP package. If the
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command-line switch is not used, then no defaults are set, and you
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must specify the appropriate package command in your input script.
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</P>
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</HTML>
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