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5.3.3 KOKKOS package :h5
The KOKKOS package was developed primarily by Christian Trott (Sandia)
with contributions of various styles by others, including Sikandar
Mashayak (UIUC), Stan Moore (Sandia), and Ray Shan (Sandia). The
underlying Kokkos library was written primarily by Carter Edwards,
Kokkos is a templated C++ library that provides abstractions to allow
a single implementation of an application kernel (e.g. a pair style) to run efficiently on
different kinds of hardware, such as GPUs, Intel Xeon Phis, or many-core
CPUs. Kokkos maps the C++ kernel onto different backend languages such as CUDA, OpenMP, or Pthreads.
The Kokkos library also provides data abstractions to adjust (at
compile time) the memory layout of data structures like 2d and
3d arrays to optimize performance on different hardware. For more information on Kokkos, see
"Github"_https://github.com/kokkos/kokkos. Kokkos is part of
"Trilinos"_http://trilinos.sandia.gov/packages/kokkos. The Kokkos library was written primarily by Carter Edwards,
Christian Trott, and Dan Sunderland (all Sandia).
The KOKKOS package contains versions of pair, fix, and atom styles
The LAMMPS KOKKOS package contains versions of pair, fix, and atom styles
that use data structures and macros provided by the Kokkos library,
which is included with LAMMPS in lib/kokkos.
The Kokkos library is part of
"Trilinos"_http://trilinos.sandia.gov/packages/kokkos and can also be
downloaded from "Github"_https://github.com/kokkos/kokkos. Kokkos is a
templated C++ library that provides two key abstractions for an
application like LAMMPS. First, it allows a single implementation of
an application kernel (e.g. a pair style) to run efficiently on
different kinds of hardware, such as a GPU, Intel Phi, or many-core
CPU.
The Kokkos library also provides data abstractions to adjust (at
compile time) the memory layout of basic data structures like 2d and
3d arrays and allow the transparent utilization of special hardware
load and store operations. Such data structures are used in LAMMPS to
store atom coordinates or forces or neighbor lists. The layout is
chosen to optimize performance on different platforms. Again this
functionality is hidden from the developer, and does not affect how
the kernel is coded.
These abstractions are set at build time, when LAMMPS is compiled with
the KOKKOS package installed. All Kokkos operations occur within the
context of an individual MPI task running on a single node of the
machine. The total number of MPI tasks used by LAMMPS (one or
multiple per compute node) is set in the usual manner via the mpirun
or mpiexec commands, and is independent of Kokkos.
which is included with LAMMPS in /lib/kokkos. The KOKKOS package was developed primarily by Christian Trott (Sandia)
and Stan Moore (Sandia) with contributions of various styles by others, including Sikandar
Mashayak (UIUC), Ray Shan (Sandia), and Dan Ibanez (Sandia). For more information on developing using Kokkos abstractions
see the Kokkos programmers' guide at /lib/kokkos/doc/Kokkos_PG.pdf.
Kokkos currently provides support for 3 modes of execution (per MPI
task). These are OpenMP (for many-core CPUs), Cuda (for NVIDIA GPUs),
and OpenMP (for Intel Phi). Note that the KOKKOS package supports
running on the Phi in native mode, not offload mode like the
USER-INTEL package supports. You choose the mode at build time to
task). These are Serial (MPI-only for CPUs and Intel Phi), OpenMP (threading
for many-core CPUs and Intel Phi), and CUDA (for NVIDIA GPUs). You choose the mode at build time to
produce an executable compatible with specific hardware.
Here is a quick overview of how to use the KOKKOS package
for CPU acceleration, assuming one or more 16-core nodes.
More details follow.
[Building LAMMPS with the KOKKOS package:]
use a C++11 compatible compiler
NOTE: Kokkos support within LAMMPS must be built with a C++11 compatible
compiler. This means GCC version 4.7.2 or later, Intel 14.0.4 or later, or
Clang 3.5.2 or later is required.
The recommended method of building the KOKKOS package is to start with the provided Kokkos
Makefiles in /src/MAKE/OPTIONS/. You may need to modify the KOKKOS_ARCH variable in the Makefile
to match your specific hardware. For example:
for Sandy Bridge CPUs, set KOKKOS_ARCH=SNB
for Broadwell CPUs, set KOKKOS_ARCH=BWD
for K80 GPUs, set KOKKOS_ARCH=Kepler37
for P100 GPUs and Power8 CPUs, set KOKKOS_ARCH=Pascal60,Power8 :ul
See the [Advanced Kokkos Options] section below for a listing of all KOKKOS_ARCH options.
[Compile for CPU-only (MPI only, no threading):]
use a C++11 compatible compiler and
set KOKKOS_ARCH variable in /src/MAKE/OPTIONS/Makefile.kokkos_mpi_only
as described above
cd lammps/src
make yes-kokkos
make mpi KOKKOS_DEVICES=OpenMP # build with the KOKKOS package
make kokkos_omp # or Makefile.kokkos_omp already has variable set :pre
make kokkos_mpi_only :pre
mpirun -np 16 lmp_mpi -k on -sf kk -in in.lj # 1 node, 16 MPI tasks/node, no threads
mpirun -np 2 -ppn 1 lmp_mpi -k on t 16 -sf kk -in in.lj # 2 nodes, 1 MPI task/node, 16 threads/task
mpirun -np 2 lmp_mpi -k on t 8 -sf kk -in in.lj # 1 node, 2 MPI tasks/node, 8 threads/task
mpirun -np 32 -ppn 4 lmp_mpi -k on t 4 -sf kk -in in.lj # 8 nodes, 4 MPI tasks/node, 4 threads/task :pre
[Compile for CPU-only (MPI plus OpenMP threading):]
specify variables and settings in your Makefile.machine that enable OpenMP, GPU, or Phi support
include the KOKKOS package and build LAMMPS
enable the KOKKOS package and its hardware options via the "-k on" command-line switch use KOKKOS styles in your input script :ul
Here is a quick overview of how to use the KOKKOS package for GPUs,
assuming one or more nodes, each with 16 cores and a GPU. More
details follow.
discuss use of NVCC, which Makefiles to examine
use a C++11 compatible compiler
KOKKOS_DEVICES = Cuda, OpenMP
KOKKOS_ARCH = Kepler35
make yes-kokkos
make machine :pre
mpirun -np 1 lmp_cuda -k on t 6 -sf kk -in in.lj # one MPI task, 6 threads on CPU
mpirun -np 4 -ppn 1 lmp_cuda -k on t 6 -sf kk -in in.lj # ditto on 4 nodes :pre
mpirun -np 2 lmp_cuda -k on t 8 g 2 -sf kk -in in.lj # two MPI tasks, 8 threads per CPU
mpirun -np 32 -ppn 2 lmp_cuda -k on t 8 g 2 -sf kk -in in.lj # ditto on 16 nodes :pre
Here is a quick overview of how to use the KOKKOS package
for the Intel Phi:
use a C++11 compatible compiler
KOKKOS_DEVICES = OpenMP
KOKKOS_ARCH = KNC
make yes-kokkos
make machine :pre
host=MIC, Intel Phi with 61 cores (240 threads/phi via 4x hardware threading):
mpirun -np 1 lmp_g++ -k on t 240 -sf kk -in in.lj # 1 MPI task on 1 Phi, 1*240 = 240
mpirun -np 30 lmp_g++ -k on t 8 -sf kk -in in.lj # 30 MPI tasks on 1 Phi, 30*8 = 240
mpirun -np 12 lmp_g++ -k on t 20 -sf kk -in in.lj # 12 MPI tasks on 1 Phi, 12*20 = 240
mpirun -np 96 -ppn 12 lmp_g++ -k on t 20 -sf kk -in in.lj # ditto on 8 Phis :pre
[Required hardware/software:]
Kokkos support within LAMMPS must be built with a C++11 compatible
compiler. If using gcc, version 4.7.2 or later is required.
To build with Kokkos support for CPUs, your compiler must support the
NOTE: To build with Kokkos support for OpenMP threading, your compiler must support the
OpenMP interface. You should have one or more multi-core CPUs so that
multiple threads can be launched by each MPI task running on a CPU.
To build with Kokkos support for NVIDIA GPUs, NVIDIA CUDA software
use a C++11 compatible compiler and
set KOKKOS_ARCH variable in /src/MAKE/OPTIONS/Makefile.kokkos_omp
cd lammps/src
make yes-kokkos
make kokkos_omp :pre
[Compile for Intel KNL Xeon Phi (Intel Compiler, OpenMPI):]
use a C++11 compatible compiler and
cd lammps/src
make yes-kokkos
make kokkos_phi :pre
[Compile for CPUs and GPUs (with OpenMPI or MPICH):]
NOTE: To build with Kokkos support for NVIDIA GPUs, NVIDIA CUDA software
version 7.5 or later must be installed on your system. See the
discussion for the "GPU"_accelerate_gpu.html package for details of
how to check and do this.
use a C++11 compatible compiler and
set KOKKOS_ARCH variable in /src/MAKE/OPTIONS/Makefile.kokkos_cuda_mpi for both GPU and CPU
cd lammps/src
make yes-kokkos
make kokkos_cuda_mpi :pre
[Alternative Methods of Compiling:]
Alternatively, the KOKKOS package can be built by specifying Kokkos variables
on the make command line. For example:
make mpi KOKKOS_DEVICES=OpenMP KOKKOS_ARCH=SNB # set the KOKKOS_DEVICES and KOKKOS_ARCH variable explicitly
make kokkos_cuda_mpi KOKKOS_ARCH=Pascal60,Power8 # set the KOKKOS_ARCH variable explicitly :pre
Setting the KOKKOS_DEVICES and KOKKOS_ARCH variables on the
make command line requires a GNU-compatible make command. Try
"gmake" if your system's standard make complains.
NOTE: If you build using make line variables and re-build LAMMPS twice
with different KOKKOS options and the *same* target, then you *must* perform a "make clean-all"
or "make clean-machine" before each build. This is to force all the
KOKKOS-dependent files to be re-compiled with the new options.
[Running LAMMPS with the KOKKOS package:]
All Kokkos operations occur within the
context of an individual MPI task running on a single node of the
machine. The total number of MPI tasks used by LAMMPS (one or
multiple per compute node) is set in the usual manner via the mpirun
or mpiexec commands, and is independent of Kokkos. E.g. the mpirun
command in OpenMPI does this via its
-np and -npernode switches. Ditto for MPICH via -np and -ppn.
[Running on a multi-core CPU:]
Here is a quick overview of how to use the KOKKOS package
for CPU acceleration, assuming one or more 16-core nodes.
mpirun -np 16 lmp_kokkos_mpi_only -k on -sf kk -in in.lj # 1 node, 16 MPI tasks/node, no multi-threading
mpirun -np 2 -ppn 1 lmp_kokkos_omp -k on t 16 -sf kk -in in.lj # 2 nodes, 1 MPI task/node, 16 threads/task
mpirun -np 2 lmp_kokkos_omp -k on t 8 -sf kk -in in.lj # 1 node, 2 MPI tasks/node, 8 threads/task
mpirun -np 32 -ppn 4 lmp_kokkos_omp -k on t 4 -sf kk -in in.lj # 8 nodes, 4 MPI tasks/node, 4 threads/task :pre
To run using the KOKKOS package, use the "-k on", "-sf kk" and "-pk kokkos" "command-line switches"_Section_start.html#start_7 in your mpirun command.
You must use the "-k on" "command-line
switch"_Section_start.html#start_7 to enable the KOKKOS package. It
takes additional arguments for hardware settings appropriate to your
system. Those arguments are "documented
here"_Section_start.html#start_7. For OpenMP use:
-k on t Nt :pre
The "t Nt" option specifies how many OpenMP threads per MPI
task to use with a node. The default is Nt = 1, which is MPI-only mode.
Note that the product of MPI tasks * OpenMP
threads/task should not exceed the physical number of cores (on a
node), otherwise performance will suffer. If hyperthreading is enabled, then
the product of MPI tasks * OpenMP threads/task should not exceed the
physical number of cores * hardware threads.
The "-k on" switch also issues a "package kokkos" command (with no
additional arguments) which sets various KOKKOS options to default
values, as discussed on the "package"_package.html command doc page.
The "-sf kk" "command-line switch"_Section_start.html#start_7
will automatically append the "/kk" suffix to styles that support it.
In this manner no modification to the input script is needed. Alternatively,
one can run with the KOKKOS package by editing the input script as described below.
NOTE: The default for the "package kokkos"_package.html command is
to use "full" neighbor lists and set the Newton flag to "off" for both
pairwise and bonded interactions. However, when running on CPUs, it
will typically be faster to use "half" neighbor lists and set the
Newton flag to "on", just as is the case for non-accelerated pair
styles. It can also be faster to use non-threaded communication.
Use the "-pk kokkos" "command-line switch"_Section_start.html#start_7 to
change the default "package kokkos"_package.html
options. See its doc page for details and default settings. Experimenting with
its options can provide a speed-up for specific calculations. For example:
mpirun -np 16 lmp_kokkos_mpi_only -k on -sf kk -pk kokkos newton on neigh half comm no -in in.lj # Newton on, Half neighbor list, non-threaded comm :pre
If the "newton"_newton.html command is used in the input
script, it can also override the Newton flag defaults.
[Core and Thread Affinity:]
When using multi-threading, it is important for
performance to bind both MPI tasks to physical cores, and threads to
physical cores, so they do not migrate during a simulation.
If you are not certain MPI tasks are being bound (check the defaults
for your MPI installation), binding can be forced with these flags:
OpenMPI 1.8: mpirun -np 2 --bind-to socket --map-by socket ./lmp_openmpi ...
Mvapich2 2.0: mpiexec -np 2 --bind-to socket --map-by socket ./lmp_mvapich ... :pre
For binding threads with KOKKOS OpenMP, use thread affinity
environment variables to force binding. With OpenMP 3.1 (gcc 4.7 or
later, intel 12 or later) setting the environment variable
OMP_PROC_BIND=true should be sufficient. In general, for best performance
with OpenMP 4.0 or better set OMP_PROC_BIND=spread and OMP_PLACES=threads.
For binding threads with the
KOKKOS pthreads option, compile LAMMPS the KOKKOS HWLOC=yes option
as described below.
[Running on Knight's Landing (KNL) Intel Xeon Phi:]
Here is a quick overview of how to use the KOKKOS package
for the Intel Knight's Landing (KNL) Xeon Phi:
KNL Intel Phi chips have 68 physical cores. Typically 1 to 4 cores
are reserved for the OS, and only 64 or 66 cores are used. Each core
has 4 hyperthreads,so there are effectively N = 256 (4*64) or
N = 264 (4*66) cores to run on. The product of MPI tasks * OpenMP threads/task should not exceed this limit,
otherwise performance will suffer. Note that with the KOKKOS package you do not need to
specify how many KNLs there are per node; each
KNL is simply treated as running some number of MPI tasks.
Examples of mpirun commands that follow these rules are shown below.
Intel KNL node with 68 cores (272 threads/node via 4x hardware threading):
mpirun -np 64 lmp_kokkos_phi -k on t 4 -sf kk -in in.lj # 1 node, 64 MPI tasks/node, 4 threads/task
mpirun -np 66 lmp_kokkos_phi -k on t 4 -sf kk -in in.lj # 1 node, 66 MPI tasks/node, 4 threads/task
mpirun -np 32 lmp_kokkos_phi -k on t 8 -sf kk -in in.lj # 1 node, 32 MPI tasks/node, 8 threads/task
mpirun -np 512 -ppn 64 lmp_kokkos_phi -k on t 4 -sf kk -in in.lj # 8 nodes, 64 MPI tasks/node, 4 threads/task :pre
The -np setting of the mpirun command sets the number of MPI
tasks/node. The "-k on t Nt" command-line switch sets the number of
threads/task as Nt. The product of these two values should be N, i.e.
256 or 264.
NOTE: The default for the "package kokkos"_package.html command is
to use "full" neighbor lists and set the Newton flag to "off" for both
pairwise and bonded interactions. When running on KNL, this
will typically be best for pair-wise potentials. For manybody potentials,
using "half" neighbor lists and setting the
Newton flag to "on" may be faster. It can also be faster to use non-threaded communication.
Use the "-pk kokkos" "command-line switch"_Section_start.html#start_7 to
change the default "package kokkos"_package.html
options. See its doc page for details and default settings. Experimenting with
its options can provide a speed-up for specific calculations. For example:
mpirun -np 64 lmp_kokkos_phi -k on t 4 -sf kk -pk kokkos comm no -in in.lj # Newton off, full neighbor list, non-threaded comm
mpirun -np 64 lmp_kokkos_phi -k on t 4 -sf kk -pk kokkos newton on neigh half comm no -in in.reax # Newton on, half neighbor list, non-threaded comm :pre
NOTE: MPI tasks and threads should be bound to cores as described above for CPUs.
NOTE: To build with Kokkos support for Intel Xeon Phi coprocessors such as Knight's Corner (KNC), your
system must be configured to use them in "native" mode, not "offload"
mode like the USER-INTEL package supports.
[Running on GPUs:]
Use the "-k" "command-line switch"_Section_commands.html#start_7 to
specify the number of GPUs per node. Typically the -np setting
of the mpirun command should set the number of MPI
tasks/node to be equal to the # of physical GPUs on the node.
You can assign multiple MPI tasks to the same GPU with the
KOKKOS package, but this is usually only faster if significant portions
of the input script have not been ported to use Kokkos. Using CUDA MPS
is recommended in this scenario. As above for multi-core CPUs (and no GPU), if N is the number
of physical cores/node, then the number of MPI tasks/node should not exceed N.
-k on g Ng :pre
Here are examples of how to use the KOKKOS package for GPUs,
assuming one or more nodes, each with two GPUs:
mpirun -np 2 lmp_kokkos_cuda_openmpi -k on g 2 -sf kk -in in.lj # 1 node, 2 MPI tasks/node, 2 GPUs/node
mpirun -np 32 -ppn 2 lmp_kokkos_cuda_openmpi -k on g 2 -sf kk -in in.lj # 16 nodes, 2 MPI tasks/node, 2 GPUs/node (32 GPUs total) :pre
NOTE: The default for the "package kokkos"_package.html command is
to use "full" neighbor lists and set the Newton flag to "off" for both
pairwise and bonded interactions, along with threaded communication.
When running on Maxwell or Kepler GPUs, this will typically be best. For Pascal GPUs,
using "half" neighbor lists and setting the
Newton flag to "on" may be faster. For many pair styles, setting the neighbor binsize
equal to the ghost atom cutoff will give speedup.
Use the "-pk kokkos" "command-line switch"_Section_start.html#start_7 to
change the default "package kokkos"_package.html
options. See its doc page for details and default settings. Experimenting with
its options can provide a speed-up for specific calculations. For example:
mpirun -np 2 lmp_kokkos_cuda_openmpi -k on g 2 -sf kk -pk kokkos binsize 2.8 -in in.lj # Set binsize = neighbor ghost cutoff
mpirun -np 2 lmp_kokkos_cuda_openmpi -k on g 2 -sf kk -pk kokkos newton on neigh half binsize 2.8 -in in.lj # Newton on, half neighborlist, set binsize = neighbor ghost cutoff :pre
NOTE: For good performance of the KOKKOS package on GPUs, you must
have Kepler generation GPUs (or later). The Kokkos library exploits
texture cache options not supported by Telsa generation GPUs (or
older).
To build with Kokkos support for Intel Xeon Phi coprocessors, your
sysmte must be configured to use them in "native" mode, not "offload"
mode like the USER-INTEL package supports.
NOTE: When using a GPU, you will achieve the best performance if your
input script does not use fix or compute styles which are not yet
Kokkos-enabled. This allows data to stay on the GPU for multiple
timesteps, without being copied back to the host CPU. Invoking a
non-Kokkos fix or compute, or performing I/O for
"thermo"_thermo_style.html or "dump"_dump.html output will cause data
to be copied back to the CPU incurring a performance penalty.
[Building LAMMPS with the KOKKOS package:]
NOTE: To get an accurate timing breakdown between time spend in pair,
kspace, etc., you must set the environment variable CUDA_LAUNCH_BLOCKING=1.
However, this will reduce performance and is not recommended for production runs.
You must choose at build time whether to build for CPUs (OpenMP),
GPUs, or Phi.
[Run with the KOKKOS package by editing an input script:]
You can do any of these in one line, using the suitable make command
line flags as described in "Section 4"_Section_packages.html of the
manual. If run from the src directory, these
commands will create src/lmp_kokkos_omp, lmp_kokkos_cuda_mpi, and
lmp_kokkos_phi. Note that the OMP and PHI options use
src/MAKE/Makefile.mpi as the starting Makefile.machine. The CUDA
option uses src/MAKE/OPTIONS/Makefile.kokkos_cuda_mpi.
The latter two steps can be done using the "-k on", "-pk kokkos" and
"-sf kk" "command-line switches"_Section_start.html#start_6
respectively. Or the effect of the "-pk" or "-sf" switches can be
Alternatively the effect of the "-sf" or "-pk" switches can be
duplicated by adding the "package kokkos"_package.html or "suffix
kk"_suffix.html commands respectively to your input script.
kk"_suffix.html commands to your input script.
Or you can follow these steps:
CPU-only (run all-MPI or with OpenMP threading):
cd lammps/src
make yes-kokkos
make kokkos_omp :pre
CPU-only (only MPI, no threading):
cd lammps/src
make yes-kokkos
make kokkos_mpi_only :pre
Intel Xeon Phi (Intel Compiler, Intel MPI):
cd lammps/src
make yes-kokkos
make kokkos_phi :pre
CPUs and GPUs (with MPICH or OpenMPI):
cd lammps/src
make yes-kokkos
make kokkos_cuda_mpi :pre
These examples set the KOKKOS-specific OMP, MIC, CUDA variables on the
make command line which requires a GNU-compatible make command. Try
"gmake" if your system's standard make complains.
NOTE: If you build using make line variables and re-build LAMMPS twice
with different KOKKOS options and the *same* target, e.g. g++ in the
first two examples above, then you *must* perform a "make clean-all"
or "make clean-machine" before each build. This is to force all the
KOKKOS-dependent files to be re-compiled with the new options.
NOTE: Currently, there are no precision options with the KOKKOS
package. All compilation and computation is performed in double
precision.
There are other allowed options when building with the KOKKOS package.
As above, they can be set either as variables on the make command line
or in Makefile.machine. This is the full list of options, including
those discussed above, Each takes a value shown below. The
default value is listed, which is set in the
lib/kokkos/Makefile.kokkos file.
#Default settings specific options
#Options: force_uvm,use_ldg,rdc
KOKKOS_DEVICES, values = {OpenMP}, {Serial}, {Pthreads}, {Cuda}, default = {OpenMP}
KOKKOS_ARCH, values = {KNC}, {SNB}, {HSW}, {Kepler}, {Kepler30}, {Kepler32}, {Kepler35}, {Kepler37}, {Maxwell}, {Maxwell50}, {Maxwell52}, {Maxwell53}, {ARMv8}, {BGQ}, {Power7}, {Power8}, default = {none}
KOKKOS_DEBUG, values = {yes}, {no}, default = {no}
KOKKOS_USE_TPLS, values = {hwloc}, {librt}, default = {none}
KOKKOS_CUDA_OPTIONS, values = {force_uvm}, {use_ldg}, {rdc} :ul
KOKKOS_DEVICE sets the parallelization method used for Kokkos code
(within LAMMPS). KOKKOS_DEVICES=OpenMP means that OpenMP will be
used. KOKKOS_DEVICES=Pthreads means that pthreads will be used.
KOKKOS_DEVICES=Cuda means an NVIDIA GPU running CUDA will be used.
If KOKKOS_DEVICES=Cuda, then the lo-level Makefile in the src/MAKE
directory must use "nvcc" as its compiler, via its CC setting. For
best performance its CCFLAGS setting should use -O3 and have a
KOKKOS_ARCH setting that matches the compute capability of your NVIDIA
hardware and software installation, e.g. KOKKOS_ARCH=Kepler30. Note
the minimal required compute capability is 2.0, but this will give
significantly reduced performance compared to Kepler generation GPUs
with compute capability 3.x. For the LINK setting, "nvcc" should not
be used; instead use g++ or another compiler suitable for linking C++
applications. Often you will want to use your MPI compiler wrapper
for this setting (i.e. mpicxx). Finally, the lo-level Makefile must
also have a "Compilation rule" for creating *.o files from *.cu files.
See src/Makefile.cuda for an example of a lo-level Makefile with all
of these settings.
KOKKOS_USE_TPLS=hwloc binds threads to hardware cores, so they do not
migrate during a simulation. KOKKOS_USE_TPLS=hwloc should always be
used if running with KOKKOS_DEVICES=Pthreads for pthreads. It is not
necessary for KOKKOS_DEVICES=OpenMP for OpenMP, because OpenMP
provides alternative methods via environment variables for binding
threads to hardware cores. More info on binding threads to cores is
given in "Section 5.3"_Section_accelerate.html#acc_3.
KOKKOS_ARCH=KNC enables compiler switches needed when compiling for an
Intel Phi processor.
KOKKOS_USE_TPLS=librt enables use of a more accurate timer mechanism
on most Unix platforms. This library is not available on all
platforms.
KOKKOS_DEBUG is only useful when developing a Kokkos-enabled style
within LAMMPS. KOKKOS_DEBUG=yes enables printing of run-time
debugging information that can be useful. It also enables runtime
bounds checking on Kokkos data structures.
KOKKOS_CUDA_OPTIONS are additional options for CUDA.
For more information on Kokkos see the Kokkos programmers' guide here:
/lib/kokkos/doc/Kokkos_PG.pdf.
[Run with the KOKKOS package from the command line:]
The mpirun or mpiexec command sets the total number of MPI tasks used
by LAMMPS (one or multiple per compute node) and the number of MPI
tasks used per node. E.g. the mpirun command in MPICH does this via
its -np and -ppn switches. Ditto for OpenMPI via -np and -npernode.
When using KOKKOS built with host=OMP, you need to choose how many
OpenMP threads per MPI task will be used (via the "-k" command-line
switch discussed below). Note that the product of MPI tasks * OpenMP
threads/task should not exceed the physical number of cores (on a
node), otherwise performance will suffer.
When using the KOKKOS package built with device=CUDA, you must use
exactly one MPI task per physical GPU.
When using the KOKKOS package built with host=MIC for Intel Xeon Phi
coprocessor support you need to insure there are one or more MPI tasks
per coprocessor, and choose the number of coprocessor threads to use
per MPI task (via the "-k" command-line switch discussed below). The
product of MPI tasks * coprocessor threads/task should not exceed the
maximum number of threads the coprocessor is designed to run,
otherwise performance will suffer. This value is 240 for current
generation Xeon Phi(TM) chips, which is 60 physical cores * 4
threads/core. Note that with the KOKKOS package you do not need to
specify how many Phi coprocessors there are per node; each
coprocessors is simply treated as running some number of MPI tasks.
You must use the "-k on" "command-line
switch"_Section_start.html#start_6 to enable the KOKKOS package. It
takes additional arguments for hardware settings appropriate to your
system. Those arguments are "documented
here"_Section_start.html#start_6. The two most commonly used
options are:
-k on t Nt g Ng :pre
The "t Nt" option applies to host=OMP (even if device=CUDA) and
host=MIC. For host=OMP, it specifies how many OpenMP threads per MPI
task to use with a node. For host=MIC, it specifies how many Xeon Phi
threads per MPI task to use within a node. The default is Nt = 1.
Note that for host=OMP this is effectively MPI-only mode which may be
fine. But for host=MIC you will typically end up using far less than
all the 240 available threads, which could give very poor performance.
The "g Ng" option applies to device=CUDA. It specifies how many GPUs
per compute node to use. The default is 1, so this only needs to be
specified is you have 2 or more GPUs per compute node.
The "-k on" switch also issues a "package kokkos" command (with no
additional arguments) which sets various KOKKOS options to default
values, as discussed on the "package"_package.html command doc page.
Use the "-sf kk" "command-line switch"_Section_start.html#start_6,
which will automatically append "kk" to styles that support it. Use
the "-pk kokkos" "command-line switch"_Section_start.html#start_6 if
you wish to change any of the default "package kokkos"_package.html
optionns set by the "-k on" "command-line
switch"_Section_start.html#start_6.
Note that the default for the "package kokkos"_package.html command is
to use "full" neighbor lists and set the Newton flag to "off" for both
pairwise and bonded interactions. This typically gives fastest
performance. If the "newton"_newton.html command is used in the input
script, it can override the Newton flag defaults.
However, when running in MPI-only mode with 1 thread per MPI task, it
will typically be faster to use "half" neighbor lists and set the
Newton flag to "on", just as is the case for non-accelerated pair
styles. You can do this with the "-pk" "command-line
switch"_Section_start.html#start_6.
[Or run with the KOKKOS package by editing an input script:]
The discussion above for the mpirun/mpiexec command and setting
appropriate thread and GPU values for host=OMP or host=MIC or
device=CUDA are the same.
The discussion above for building LAMMPS with the KOKKOS package, the mpirun/mpiexec command, and setting
appropriate thread are the same.
You must still use the "-k on" "command-line
switch"_Section_start.html#start_6 to enable the KOKKOS package, and
switch"_Section_start.html#start_7 to enable the KOKKOS package, and
specify its additional arguments for hardware options appropriate to
your system, as documented above.
Use the "suffix kk"_suffix.html command, or you can explicitly add a
You can use the "suffix kk"_suffix.html command, or you can explicitly add a
"kk" suffix to individual styles in your input script, e.g.
pair_style lj/cut/kk 2.5 :pre
You only need to use the "package kokkos"_package.html command if you
wish to change any of its option defaults, as set by the "-k on"
"command-line switch"_Section_start.html#start_6.
"command-line switch"_Section_start.html#start_7.
[Using OpenMP threading and CUDA together (experimental):]
With the KOKKOS package, both OpenMP multi-threading and GPUs can be used
together in a few special cases. In the Makefile, the KOKKOS_DEVICES variable must
include both "Cuda" and "OpenMP", as is the case for /src/MAKE/OPTIONS/Makefile.kokkos_cuda_mpi
KOKKOS_DEVICES=Cuda,OpenMP :pre
The suffix “/kk” is equivalent to “/kk/device”, and for Kokkos CUDA,
using the “-sf kk” in the command line gives the default CUDA version everywhere.
However, if the “/kk/host” suffix is added to a specific style in the input
script, the Kokkos OpenMP (CPU) version of that specific style will be used instead.
Set the number of OpenMP threads as "t Nt" and the number of GPUs as "g Ng"
-k on t Nt g Ng :pre
For example, the command to run with 1 GPU and 8 OpenMP threads is then:
mpiexec -np 1 lmp_kokkos_cuda_openmpi -in in.lj -k on g 1 t 8 -sf kk :pre
Conversely, if the “-sf kk/host” is used in the command line and then the
“/kk” or “/kk/device” suffix is added to a specific style in your input script,
then only that specific style will run on the GPU while everything else will
run on the CPU in OpenMP mode. Note that the execution of the CPU and GPU
styles will NOT overlap, except for a special case:
A kspace style and/or molecular topology (bonds, angles, etc.) running on
the host CPU can overlap with a pair style running on the GPU. First compile
with “--default-stream per-thread” added to CCFLAGS in the Kokkos CUDA Makefile.
Then explicitly use the “/kk/host” suffix for kspace and bonds, angles, etc.
in the input file and the "kk" suffix (equal to "kk/device") on the command line.
Also make sure the environment variable CUDA_LAUNCH_BLOCKING is not set to "1"
so CPU/GPU overlap can occur.
[Speed-ups to expect:]
@ -363,7 +371,7 @@ package. :l
When running large number of atoms per GPU, KOKKOS is typically faster
than the GPU package. :l
When running on Intel Xeon Phi, KOKKOS is not as fast as
When running on Intel hardware, KOKKOS is not as fast as
the USER-INTEL package, which is optimized for that hardware. :l
:ule
@ -371,123 +379,78 @@ See the "Benchmark page"_http://lammps.sandia.gov/bench.html of the
LAMMPS web site for performance of the KOKKOS package on different
hardware.
[Guidelines for best performance:]
[Advanced Kokkos options:]
Here are guidline for using the KOKKOS package on the different
hardware configurations listed above.
There are other allowed options when building with the KOKKOS package.
As above, they can be set either as variables on the make command line
or in Makefile.machine. This is the full list of options, including
those discussed above. Each takes a value shown below. The
default value is listed, which is set in the
/lib/kokkos/Makefile.kokkos file.
Many of the guidelines use the "package kokkos"_package.html command
See its doc page for details and default settings. Experimenting with
its options can provide a speed-up for specific calculations.
KOKKOS_DEVICES, values = {Serial}, {OpenMP}, {Pthreads}, {Cuda}, default = {OpenMP}
KOKKOS_ARCH, values = {KNC}, {SNB}, {HSW}, {Kepler30}, {Kepler32}, {Kepler35}, {Kepler37}, {Maxwell50}, {Maxwell52}, {Maxwell53}, {Pascal60}, {Pascal61}, {ARMv80}, {ARMv81}, {ARMv81}, {ARMv8-ThunderX}, {BGQ}, {Power7}, {Power8}, {Power9}, {KNL}, {BDW}, {SKX}, default = {none}
KOKKOS_DEBUG, values = {yes}, {no}, default = {no}
KOKKOS_USE_TPLS, values = {hwloc}, {librt}, {experimental_memkind}, default = {none}
KOKKOS_CXX_STANDARD, values = {c++11}, {c++1z}, default = {c++11}
KOKKOS_OPTIONS, values = {aggressive_vectorization}, {disable_profiling}, default = {none}
KOKKOS_CUDA_OPTIONS, values = {force_uvm}, {use_ldg}, {rdc}, {enable_lambda}, default = {enable_lambda} :ul
[Running on a multi-core CPU:]
KOKKOS_DEVICES sets the parallelization method used for Kokkos code
(within LAMMPS). KOKKOS_DEVICES=Serial means that no threading will be used.
KOKKOS_DEVICES=OpenMP means that OpenMP threading will be
used. KOKKOS_DEVICES=Pthreads means that pthreads will be used.
KOKKOS_DEVICES=Cuda means an NVIDIA GPU running CUDA will be used.
If N is the number of physical cores/node, then the number of MPI
tasks/node * number of threads/task should not exceed N, and should
typically equal N. Note that the default threads/task is 1, as set by
the "t" keyword of the "-k" "command-line
switch"_Section_start.html#start_6. If you do not change this, no
additional parallelism (beyond MPI) will be invoked on the host
CPU(s).
KOKKOS_ARCH enables compiler switches needed when compiling for a
specific hardware:
You can compare the performance running in different modes:
ARMv80 = ARMv8.0 Compatible CPU
ARMv81 = ARMv8.1 Compatible CPU
ARMv8-ThunderX = ARMv8 Cavium ThunderX CPU
SNB = Intel Sandy/Ivy Bridge CPUs
HSW = Intel Haswell CPUs
BDW = Intel Broadwell Xeon E-class CPUs
SKX = Intel Sky Lake Xeon E-class HPC CPUs (AVX512)
KNC = Intel Knights Corner Xeon Phi
KNL = Intel Knights Landing Xeon Phi
Kepler30 = NVIDIA Kepler generation CC 3.0
Kepler32 = NVIDIA Kepler generation CC 3.2
Kepler35 = NVIDIA Kepler generation CC 3.5
Kepler37 = NVIDIA Kepler generation CC 3.7
Maxwell50 = NVIDIA Maxwell generation CC 5.0
Maxwell52 = NVIDIA Maxwell generation CC 5.2
Maxwell53 = NVIDIA Maxwell generation CC 5.3
Pascal60 = NVIDIA Pascal generation CC 6.0
Pascal61 = NVIDIA Pascal generation CC 6.1
BGQ = IBM Blue Gene/Q CPUs
Power8 = IBM POWER8 CPUs
Power9 = IBM POWER9 CPUs :ul
run with 1 MPI task/node and N threads/task
run with N MPI tasks/node and 1 thread/task
run with settings in between these extremes :ul
KOKKOS_USE_TPLS=hwloc binds threads to hardware cores, so they do not
migrate during a simulation. KOKKOS_USE_TPLS=hwloc should always be
used if running with KOKKOS_DEVICES=Pthreads for pthreads. It is not
necessary for KOKKOS_DEVICES=OpenMP for OpenMP, because OpenMP
provides alternative methods via environment variables for binding
threads to hardware cores. More info on binding threads to cores is
given in "Section 5.3"_Section_accelerate.html#acc_3.
Examples of mpirun commands in these modes are shown above.
KOKKOS_USE_TPLS=librt enables use of a more accurate timer mechanism
on most Unix platforms. This library is not available on all
platforms.
When using KOKKOS to perform multi-threading, it is important for
performance to bind both MPI tasks to physical cores, and threads to
physical cores, so they do not migrate during a simulation.
KOKKOS_DEBUG is only useful when developing a Kokkos-enabled style
within LAMMPS. KOKKOS_DEBUG=yes enables printing of run-time
debugging information that can be useful. It also enables runtime
bounds checking on Kokkos data structures.
If you are not certain MPI tasks are being bound (check the defaults
for your MPI installation), binding can be forced with these flags:
KOKKOS_CXX_STANDARD and KOKKOS_OPTIONS are typically not changed when building LAMMPS.
OpenMPI 1.8: mpirun -np 2 -bind-to socket -map-by socket ./lmp_openmpi ...
Mvapich2 2.0: mpiexec -np 2 -bind-to socket -map-by socket ./lmp_mvapich ... :pre
For binding threads with the KOKKOS OMP option, use thread affinity
environment variables to force binding. With OpenMP 3.1 (gcc 4.7 or
later, intel 12 or later) setting the environment variable
OMP_PROC_BIND=true should be sufficient. For binding threads with the
KOKKOS pthreads option, compile LAMMPS the KOKKOS HWLOC=yes option
(see "this section"_Section_packages.html#KOKKOS of the manual for
details).
[Running on GPUs:]
Insure the -arch setting in the machine makefile you are using,
e.g. src/MAKE/Makefile.cuda, is correct for your GPU hardware/software.
(see "this section"_Section_packages.html#KOKKOS of the manual for
details).
The -np setting of the mpirun command should set the number of MPI
tasks/node to be equal to the # of physical GPUs on the node.
Use the "-k" "command-line switch"_Section_commands.html#start_6 to
specify the number of GPUs per node, and the number of threads per MPI
task. As above for multi-core CPUs (and no GPU), if N is the number
of physical cores/node, then the number of MPI tasks/node * number of
threads/task should not exceed N. With one GPU (and one MPI task) it
may be faster to use less than all the available cores, by setting
threads/task to a smaller value. This is because using all the cores
on a dual-socket node will incur extra cost to copy memory from the
2nd socket to the GPU.
Examples of mpirun commands that follow these rules are shown above.
NOTE: When using a GPU, you will achieve the best performance if your
input script does not use any fix or compute styles which are not yet
Kokkos-enabled. This allows data to stay on the GPU for multiple
timesteps, without being copied back to the host CPU. Invoking a
non-Kokkos fix or compute, or performing I/O for
"thermo"_thermo_style.html or "dump"_dump.html output will cause data
to be copied back to the CPU.
You cannot yet assign multiple MPI tasks to the same GPU with the
KOKKOS package. We plan to support this in the future, similar to the
GPU package in LAMMPS.
You cannot yet use both the host (multi-threaded) and device (GPU)
together to compute pairwise interactions with the KOKKOS package. We
hope to support this in the future, similar to the GPU package in
LAMMPS.
[Running on an Intel Phi:]
Kokkos only uses Intel Phi processors in their "native" mode, i.e.
not hosted by a CPU.
As illustrated above, build LAMMPS with OMP=yes (the default) and
MIC=yes. The latter insures code is correctly compiled for the Intel
Phi. The OMP setting means OpenMP will be used for parallelization on
the Phi, which is currently the best option within Kokkos. In the
future, other options may be added.
Current-generation Intel Phi chips have either 61 or 57 cores. One
core should be excluded for running the OS, leaving 60 or 56 cores.
Each core is hyperthreaded, so there are effectively N = 240 (4*60) or
N = 224 (4*56) cores to run on.
The -np setting of the mpirun command sets the number of MPI
tasks/node. The "-k on t Nt" command-line switch sets the number of
threads/task as Nt. The product of these 2 values should be N, i.e.
240 or 224. Also, the number of threads/task should be a multiple of
4 so that logical threads from more than one MPI task do not run on
the same physical core.
Examples of mpirun commands that follow these rules are shown above.
KOKKOS_CUDA_OPTIONS are additional options for CUDA. The LAMMPS KOKKOS package must be compiled
with the {enable_lambda} option when using GPUs.
[Restrictions:]
As noted above, if using GPUs, the number of MPI tasks per compute
node should equal to the number of GPUs per compute node. In the
future Kokkos will support assigning multiple MPI tasks to a single
GPU.
Currently Kokkos does not support AMD GPUs due to limits in the
available backend programming models. Specifically, Kokkos requires
extensive C++ support from the Kernel language. This is expected to
change in the future.
Currently, there are no precision options with the KOKKOS
package. All compilation and computation is performed in double
precision.