domdec (c46b2)

Domain Decomposition

Domain decomposition (abreviated "domdec") is a method of parallelizing
Molecular Dynamics (MD) simulation. In Domain decomposition, the simulation box
is divided into Nx x Ny x Nz sub-boxes. Each CPU (or core) is assigned a home
sub-box. Each CPU is responsible for updating the coordinates of the atoms
residing in its home sub-box. The non-bonded forces are calculated for all
atom pairs in the home box plus around volume Rcut around the home box in
the positive x, y, and z direction. After the force calculation, the required
forces are communicated using MPI to the sub-boxes surrounding the home box.
The communication between the sub-boxes is implemented using the "Eighth shell"
method.

Special Notice:

The DOMDEC code for doing molecular dynamics
simulations with CHARMM is an evolving, highly scalable molecular
dynamics engine that has been released in the distribution version,
c37b1, without the usual year of the testing in the developmental
version because of the important speed up it can provide via
multiprocessor parallization relative to the previously available MD
codes in CHARMM. (For a discussion and benchmarks, see the News item
in www.charmm.org.) Although DOMDEC has been thoroughly tested, it is
likely that input script configurations that work fine in conventional
that will be found as more people use the code. It is suggested,
therefore, that before doing long runs with DOMDEC, it be confirmed
that DOMDEC gives the correct results by comparing the results of a
test run with those obtained with one of the standard MD codes in
conditions rather than the image facility in CHARMM. We suggest that
DOMDEC is the method of choice for long standard dynamics NVT or NPT
runs with PME on up to 256 processors. Additional features will be
announced in updates to the documentation.

If you use DOMDEC in your research, please cite this manuscript:

"New Faster CHARMM Molecular Dynamics Engine",
A.-P. Hynninen, and M. F. Crowley,
Journal of Computational Chemistry 2014, 35, 406-413


* Description | Short description
* Limitations | Which features are enabled and which not
* Syntax | Syntax of the domdec command
* Examples | Examples of using domdec
* Installing | Enabling domdec in charmm


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In order to use the Particle-Mesh Ewald (PME) electrostatics, CHARMM must be
compiled with pref keywords COLFFT. Domdec splits
the CPUs given by the mpirun -command into direct and reciprocal CPUs. The
direct CPUs are responsible for bonded and non-bonded force calculation,
neighbor list search, etc. The reciprocal CPUs are responsible for calculating
the reciprocal part of the PME sum.

The Isotropic Periodic Sum (IPS) method has been implemented to work with
DOMDEC and DOMDEC_GPU. Using IPS can get ride of reciprocal calcuation and
improve the scaling for massive parellel computing.


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Current limitations of DOMDec:
- Must use COLFFT keyword in pref.dat in order to have correct Ewald
electrostatics.
- In order to use SHAKE, must use "shake fast" and have FSSHK keyword in
pref.dat
- Must use Leapfrog Verlet integrator (in module dynamc.src):
- LEAP, LANG, and CPT all work.
- PERT and TSM do not work.
- Only supports orthogonal simulation boxes.
- Only supports 3-atom solvent models (TIP3, SPC), e.g. no support for TIP4.
- Heavy atoms with hydrogen bonds must be immediately before the hydrogens in the
topology.
- Dynamic Load Balancing is in beta phase for Constant Pressure simulations,
if you have trouble, switch off Dynamic Load Balancing when performing constant
pressure simulations.
- Minimization (mini -command) does not work with DOMDEC. The best way around this
is to first do minimization and then turn on DOMDEC for dynamics.
- DOMDEC cannot be turned OFF within the script where the DOMDEC -command was
given. For example you cannot run dynamics with DOMDEC, turn it off, and run
dynamics without DOMDEC.
- IMAGe recentering command only operates on molecules that are a single DOMDEC
group. For example, water molecules will be correctly recentered but most larger
molecule are not.

- Currently supports following constraints:
* Distance matrix (DMCO)
* Umbrella potential (RXNC)
* Adaptive Umbrella Sampling (ADUMB)
* Restrained distances (RESD)
* Imposed distance restraints (NOE)
* Absolute harmonic constraints (CONS HARM ABSO)
* Dihedral constraints (CONS DIHE)
* Center of mass constraints (CONS HMCM)

Tips for improving performance:
- Saving trajectory (NSAVC, NSAVV) or restart file (ISVFRQ) during dynamics
requires a all-to-all communication which, when done often, slows down the
simulation. In most systems, setting NSAVC, NSAVV, and ISVFRQ to a value greater
than 100 is adviced.
- Make sure the SSE instructions are being used. Look for
"Using SSE version of non-bonded force loops" in the output.
- Use FFTW or MKL library.
- Try a different number of reciprocal cores to find the optimal value.


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[SYNTAX DOMDec]

Way to invoke domdec:

--------------------1-----------------------------
Add CHARMM script command

DOMDec [NDIR NX NY NZ]
[GPU {ON | OFF}]
[DLB {ON | OFF}]
[SPLIt {ON | OFF}]
[PPANg N]
[SINGle]
[DOUBle]
[TEST]

to ENERGY command.

NDIR NX NY NZ
Defines the spatial division among processors/cores of the direct space
calculation into NX x NY x NZ sub-boxes where each core gets a sub-box.
In this way, NX x NY x NZ cores of the direct nonbond space calculation
and the remaining cores are reserved for the reciprocal space calculation
if PME is requested. For example, NDIR 2 2 2 will divide the simulation
box into 2 x 2 x 2 (=8) sub-boxes. If the mpirun command asked for 12
cores, then 4 cores would be reserved for simultaneous reciprocal-space
calculation

GPU {ON | OFF}
Turns GPU computation on/off. In order to use GPU, CHARMM must be compiled
with domdec_gpu switch, see below.

GPUId {ID}
Manually set the GPU ID to be used in GPU computations. Where
ID = 0..."Number of GPUS on node" - 1. Setting -1 (the default) results
in automatic picking of GPU.

DLB {ON | OFF}
Turns on direct space dynamic load balancing.

SPLIt {ON | OFF}
Turns direct/reciprocal split on/off. Turning split off can give better
performance with small CPU counts.

PPANg N
Sets the number of points per Angstrom used for the lookup tables.
Default value is 200. Note that using a higher value will result in higher
precision, but slower simulation. This parameter is used mostly for
testing purposes, when one wants to compare results between DOMDEC and
traditional CHARMM.

SINGle
Performs force calculation in single precision. This can be used to
get extra performance. Note that forces are accumulated in double
precision. For FFTW users: PME simulations in single precision require
FFTW that is compiled in single precision. If available, use MKL since
it contains both double and single precision libraries by default.

DOUBle
Performs force calculation in double precision. This is the default
setting.

TEST
Runs DOMDEC unit tests:
- Nonbonded lookup table build and functional correctness test.

If NDIR is not defined, program will guess the values based on a simple
algorithm where the number of reciprocal cores is set to 1/4th of the number
of total cores.

******************
NOTES:
******************

The sub-box sizes are limited by the cut-off and the number of
sub-boxes as follows:

BOXX = system box size in X direction
NX = number of sub-boxes in X direction
RCUT = non-bonded cut-off + radius of the largest group
BOXX/NX = sub-box size in X direction

Then, for NX >= 2, the sub-box size in X direction must satisfy:
BOXX/NX <= BOXX - 2*RCUT

If your system violates this restriction, you can try reducing NX to 1 or by
increasing NX.

******************
NOTES on using GPU:
******************

DOMDec GPU support is enabled with "DOMDEC GPU ON" command.
When using the GPU code, use as many or less MPI nodes as there are GPUs.
That is, oversubscribing the GPUs is not currently allowed.

For example, if you have 2 GPUs on each physical node, run it using:
mpirun -n 2 ...

This will use both GPUs on the node.

You can also run it using:
mpirun -n 1 ...

This will just use one of the GPUs.

For example, say you want to run on 4 nodes (called gpu1,
gpu2, gpu3, gpu4) where each has 2 GPUs. However, you only wish to use a
single GPU on each node (which is usually optimal).
In order to run on all 4 nodes, do:

Intel MPI:
----------
mpirun -n 4 -machinefile machinefile.txt ...

where machinefile.txt reads:

gpu1:1
gpu2:1
gpu3:1
gpu4:1

OpenMPI:
--------
mpirun -n 4 -x OMP_NUM_THREADS=8 [--bind-to none] --hostfile machinefile.txt ...

where machinefile.txt reads:

gpu1 slots=1
gpu2 slots=1
gpu3 slots=1
gpu4 slots=1

The "--bind-to none" option is needed for OpenMPI 1.7.4 and newer. It makes sure
that OpenMPI does not bind all OpenMP threads to a single core.
Note, for OpenMPI you must set OMP_NUM_THREADS.

For other MPI implementations, consult your documentation.

On multi MPI node simulations, the performance bottleneck is the MPI communication.
Therefore, it is usually optimal to use only a single GPU per node when running
DOMDEC_GPU over multiple MPI nodes.

On single MPI node simulations, using 2 GPUs vs 1 GPU (on that node) might give you
better performance assuming you have a powerful enough CPU.

For better performance, it is important that you do not use more CPU threads
than you have physical cores in your CPU.


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Examples of using Domdec

******************
Example with PME:
******************
PME electrostatics is used (CHARMM compiled with COLFFT):

ENERGY DOMD NDIR 2 2 2

This command divides the simulation box into 2 x 2 x 2 (=8) sub-boxes. The
remaining 2 CPUs are assigned as reciprocal CPUs responsible for the PME
reciprocal calculation.

******************
Example without PME:
******************
No PME is used:

ENERGY DOMD NDIR 2 2 2

This command divides the simulation box into 2 x 2 x 2 (=8) sub-boxes, just
like in the example 1. However, no reciprocal CPUs are assigned.

DLB ON/OFF turns dynamic load balancing (DLB) on or off. Using DLB is adviced
as it improves performance. By default DLB is ON.


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Examples using DOMDec:

Example 1:
----------

! System setup (psf) is done here

energy eps 1.0 cutnb 11 cutim 11 ctofnb 9 ctonnb 7.5 vswi -
Ewald kappa 0.320 pmEwald order 4 fftx 64 ffty 64 fftz 64 -
domdec ndir 2 2 2 dlb on

shake fast bonh tol 1.0e-8 para

dynamics leap start timestep 0.002 nstep 100


Example 2:
----------

! Using IPS for nonbonded interaction calculation

energy eps 1.0 cutnb 11 cutim 11 ctofnb 9 -
eips vips pxyz netcg -
domdec ndir 2 2 2 dlb on

shake fast bonh tol 1.0e-8 para

dynamics leap start timestep 0.002 nstep 100

Example 3:
----------

! Using IPS for nonbonded interaction calculation with GPU

energy eps 1.0 cutnb 11 cutim 11 ctofnb 9 -
eips vips pxyz netcg -
domdec ndir 2 2 2 dlb on gpu on

shake fast bonh tol 1.0e-8 para

dynamics leap start timestep 0.002 nstep 100


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Installing CHARMM with DOMDEC enabled:

To get you a working executable,
MPI libraries must be installed and mpif90 in your path. You can alter the
executable limits at run time (using dimension script command,
» dimension ). If the correct MPI binaries
occur first in your PATH, then to compile using the configure script,
you usually do not need to add extra command line options to enable DOMDEC.
Use the normal procedure given your compilers (» cmake ).

If you wish to compile DOMDEC with GPU support, add the '-u' option to the
options that you normally use with the configure script. For example:
$ ./configure -u ...
The '-u' option is a synonym for the configure option '--with-domdec_gpu'.

Notes on compiling with PGI:

PGI C compiler cannot be used to compile the SSE force kernels in
source/nbonds/enb_core_sse.c. If PGI compiler is used on this file, a warning
is issued compile time and the code resorts using the slower Fortran versions
of the force kernels.

Notes on compiling with Pathscale:

FFTW3 include file fftw3.f03 does not compile correctly with Pathscale
compiler version 3.2.99. This is why FFTW and PATHSCALE pref keywords are
declared mutually exclusive.