mdrun Main Table of Contents

VERSION 4.6.4 Wed 13 Nov 2013

Description

The mdrun program is the main computational chemistry engine within GROMACS. Obviously, it performs Molecular Dynamics simulations, but it can also perform Stochastic Dynamics, Energy Minimization, test particle insertion or (re)calculation of energies. Normal mode analysis is another option. In this case mdrun builds a Hessian matrix from single conformation. For usual Normal Modes-like calculations, make sure that the structure provided is properly energy-minimized. The generated matrix can be diagonalized by g_nmeig.

The mdrun program reads the run input file (-s) and distributes the topology over nodes if needed. mdrun produces at least four output files. A single log file (-g) is written, unless the option -seppot is used, in which case each node writes a log file. The trajectory file (-o), contains coordinates, velocities and optionally forces. The structure file (-c) contains the coordinates and velocities of the last step. The energy file (-e) contains energies, the temperature, pressure, etc, a lot of these things are also printed in the log file. Optionally coordinates can be written to a compressed trajectory file (-x).

The option -dhdl is only used when free energy calculation is turned on.

A simulation can be run in parallel using two different parallelization schemes: MPI parallelization and/or OpenMP thread parallelization. The MPI parallelization uses multiple processes when mdrun is compiled with a normal MPI library or threads when mdrun is compiled with the GROMACS built-in thread-MPI library. OpenMP threads are supported when mdrun is compiled with OpenMP. Full OpenMP support is only available with the Verlet cut-off scheme, with the (older) group scheme only PME-only processes can use OpenMP parallelization. In all cases mdrun will by default try to use all the available hardware resources. With a normal MPI library only the options -ntomp (with the Verlet cut-off scheme) and -ntomp_pme, for PME-only processes, can be used to control the number of threads. With thread-MPI there are additional options -nt, which sets the total number of threads, and -ntmpi, which sets the number of thread-MPI threads. Note that using combined MPI+OpenMP parallelization is almost always slower than single parallelization, except at the scaling limit, where especially OpenMP parallelization of PME reduces the communication cost. OpenMP-only parallelization is much faster than MPI-only parallelization on a single CPU(-die). Since we currently don't have proper hardware topology detection, mdrun compiled with thread-MPI will only automatically use OpenMP-only parallelization when you use up to 4 threads, up to 12 threads with Intel Nehalem/Westmere, or up to 16 threads with Intel Sandy Bridge or newer CPUs. Otherwise MPI-only parallelization is used (except with GPUs, see below).

To quickly test the performance of the new Verlet cut-off scheme with old .tpr files, either on CPUs or CPUs+GPUs, you can use the -testverlet option. This should not be used for production, since it can slightly modify potentials and it will remove charge groups making analysis difficult, as the .tpr file will still contain charge groups. For production simulations it is highly recommended to specify cutoff-scheme = Verlet in the .mdp file.

With GPUs (only supported with the Verlet cut-off scheme), the number of GPUs should match the number of MPI processes or MPI threads, excluding PME-only processes/threads. With thread-MPI, unless set on the command line, the number of MPI threads will automatically be set to the number of GPUs detected. To use a subset of the available GPUs, or to manually provide a mapping of GPUs to PP ranks, you can use the -gpu_id option. The argument of -gpu_id is a string of digits (without delimiter) representing device id-s of the GPUs to be used. For example, "02" specifies using GPUs 0 and 2 in the first and second PP ranks per compute node respectively. To select different sets of GPU-s on different nodes of a compute cluster, use the GMX_GPU_ID environment variable instead. The format for GMX_GPU_ID is identical to -gpu_id, with the difference that an environment variable can have different values on different compute nodes. Multiple MPI ranks on each node can share GPUs. This is accomplished by specifying the id(s) of the GPU(s) multiple times, e.g. "0011" for four ranks sharing two GPUs in this node. This works within a single simulation, or a multi-simulation, with any form of MPI.

When using PME with separate PME nodes or with a GPU, the two major compute tasks, the non-bonded force calculation and the PME calculation run on different compute resources. If this load is not balanced, some of the resources will be idle part of time. With the Verlet cut-off scheme this load is automatically balanced when the PME load is too high (but not when it is too low). This is done by scaling the Coulomb cut-off and PME grid spacing by the same amount. In the first few hundred steps different settings are tried and the fastest is chosen for the rest of the simulation. This does not affect the accuracy of the results, but it does affect the decomposition of the Coulomb energy into particle and mesh contributions. The auto-tuning can be turned off with the option -notunepme.

mdrun pins (sets affinity of) threads to specific cores, when all (logical) cores on a compute node are used by mdrun, even when no multi-threading is used, as this usually results in significantly better performance. If the queuing systems or the OpenMP library pinned threads, we honor this and don't pin again, even though the layout may be sub-optimal. If you want to have mdrun override an already set thread affinity or pin threads when using less cores, use -pin on. With SMT (simultaneous multithreading), e.g. Intel Hyper-Threading, there are multiple logical cores per physical core. The option -pinstride sets the stride in logical cores for pinning consecutive threads. Without SMT, 1 is usually the best choice. With Intel Hyper-Threading 2 is best when using half or less of the logical cores, 1 otherwise. The default value of 0 do exactly that: it minimizes the threads per logical core, to optimize performance. If you want to run multiple mdrun jobs on the same physical node,you should set -pinstride to 1 when using all logical cores. When running multiple mdrun (or other) simulations on the same physical node, some simulations need to start pinning from a non-zero core to avoid overloading cores; with -pinoffset you can specify the offset in logical cores for pinning.

When mdrun is started using MPI with more than 1 process or with thread-MPI with more than 1 thread, MPI parallelization is used. By default domain decomposition is used, unless the -pd option is set, which selects particle decomposition.

With domain decomposition, the spatial decomposition can be set with option -dd. By default mdrun selects a good decomposition. The user only needs to change this when the system is very inhomogeneous. Dynamic load balancing is set with the option -dlb, which can give a significant performance improvement, especially for inhomogeneous systems. The only disadvantage of dynamic load balancing is that runs are no longer binary reproducible, but in most cases this is not important. By default the dynamic load balancing is automatically turned on when the measured performance loss due to load imbalance is 5% or more. At low parallelization these are the only important options for domain decomposition. At high parallelization the options in the next two sections could be important for increasing the performace.

When PME is used with domain decomposition, separate nodes can be assigned to do only the PME mesh calculation; this is computationally more efficient starting at about 12 nodes. The number of PME nodes is set with option -npme, this can not be more than half of the nodes. By default mdrun makes a guess for the number of PME nodes when the number of nodes is larger than 11 or performance wise not compatible with the PME grid x dimension. But the user should optimize npme. Performance statistics on this issue are written at the end of the log file. For good load balancing at high parallelization, the PME grid x and y dimensions should be divisible by the number of PME nodes (the simulation will run correctly also when this is not the case).

This section lists all options that affect the domain decomposition.

Option -rdd can be used to set the required maximum distance for inter charge-group bonded interactions. Communication for two-body bonded interactions below the non-bonded cut-off distance always comes for free with the non-bonded communication. Atoms beyond the non-bonded cut-off are only communicated when they have missing bonded interactions; this means that the extra cost is minor and nearly indepedent of the value of -rdd. With dynamic load balancing option -rdd also sets the lower limit for the domain decomposition cell sizes. By default -rdd is determined by mdrun based on the initial coordinates. The chosen value will be a balance between interaction range and communication cost.

When inter charge-group bonded interactions are beyond the bonded cut-off distance, mdrun terminates with an error message. For pair interactions and tabulated bonds that do not generate exclusions, this check can be turned off with the option -noddcheck.

When constraints are present, option -rcon influences the cell size limit as well. Atoms connected by NC constraints, where NC is the LINCS order plus 1, should not be beyond the smallest cell size. A error message is generated when this happens and the user should change the decomposition or decrease the LINCS order and increase the number of LINCS iterations. By default mdrun estimates the minimum cell size required for P-LINCS in a conservative fashion. For high parallelization it can be useful to set the distance required for P-LINCS with the option -rcon.

The -dds option sets the minimum allowed x, y and/or z scaling of the cells with dynamic load balancing. mdrun will ensure that the cells can scale down by at least this factor. This option is used for the automated spatial decomposition (when not using -dd) as well as for determining the number of grid pulses, which in turn sets the minimum allowed cell size. Under certain circumstances the value of -dds might need to be adjusted to account for high or low spatial inhomogeneity of the system.

The option -gcom can be used to only do global communication every n steps. This can improve performance for highly parallel simulations where this global communication step becomes the bottleneck. For a global thermostat and/or barostat the temperature and/or pressure will also only be updated every -gcom steps. By default it is set to the minimum of nstcalcenergy and nstlist.

With -rerun an input trajectory can be given for which forces and energies will be (re)calculated. Neighbor searching will be performed for every frame, unless nstlist is zero (see the .mdp file).

ED (essential dynamics) sampling and/or additional flooding potentials are switched on by using the -ei flag followed by an .edi file. The .edi file can be produced with the make_edi tool or by using options in the essdyn menu of the WHAT IF program. mdrun produces a .xvg output file that contains projections of positions, velocities and forces onto selected eigenvectors.

When user-defined potential functions have been selected in the .mdp file the -table option is used to pass mdrun a formatted table with potential functions. The file is read from either the current directory or from the GMXLIB directory. A number of pre-formatted tables are presented in the GMXLIB dir, for 6-8, 6-9, 6-10, 6-11, 6-12 Lennard-Jones potentials with normal Coulomb. When pair interactions are present, a separate table for pair interaction functions is read using the -tablep option.

When tabulated bonded functions are present in the topology, interaction functions are read using the -tableb option. For each different tabulated interaction type the table file name is modified in a different way: before the file extension an underscore is appended, then a 'b' for bonds, an 'a' for angles or a 'd' for dihedrals and finally the table number of the interaction type.

The options -px and -pf are used for writing pull COM coordinates and forces when pulling is selected in the .mdp file.

With -multi or -multidir, multiple systems can be simulated in parallel. As many input files/directories are required as the number of systems. The -multidir option takes a list of directories (one for each system) and runs in each of them, using the input/output file names, such as specified by e.g. the -s option, relative to these directories. With -multi, the system number is appended to the run input and each output filename, for instance topol.tpr becomes topol0.tpr, topol1.tpr etc. The number of nodes per system is the total number of nodes divided by the number of systems. One use of this option is for NMR refinement: when distance or orientation restraints are present these can be ensemble averaged over all the systems.

With -replex replica exchange is attempted every given number of steps. The number of replicas is set with the -multi or -multidir option, described above. All run input files should use a different coupling temperature, the order of the files is not important. The random seed is set with -reseed. The velocities are scaled and neighbor searching is performed after every exchange.

Finally some experimental algorithms can be tested when the appropriate options have been given. Currently under investigation are: polarizability and X-ray bombardments.

The option -membed does what used to be g_membed, i.e. embed a protein into a membrane. The data file should contain the options that where passed to g_membed before. The -mn and -mp both apply to this as well.

The option -pforce is useful when you suspect a simulation crashes due to too large forces. With this option coordinates and forces of atoms with a force larger than a certain value will be printed to stderr.

Checkpoints containing the complete state of the system are written at regular intervals (option -cpt) to the file -cpo, unless option -cpt is set to -1. The previous checkpoint is backed up to state_prev.cpt to make sure that a recent state of the system is always available, even when the simulation is terminated while writing a checkpoint. With -cpnum all checkpoint files are kept and appended with the step number. A simulation can be continued by reading the full state from file with option -cpi. This option is intelligent in the way that if no checkpoint file is found, Gromacs just assumes a normal run and starts from the first step of the .tpr file. By default the output will be appending to the existing output files. The checkpoint file contains checksums of all output files, such that you will never loose data when some output files are modified, corrupt or removed. There are three scenarios with -cpi:

* no files with matching names are present: new output files are written

* all files are present with names and checksums matching those stored in the checkpoint file: files are appended

* otherwise no files are modified and a fatal error is generated

With -noappend new output files are opened and the simulation part number is added to all output file names. Note that in all cases the checkpoint file itself is not renamed and will be overwritten, unless its name does not match the -cpo option.

With checkpointing the output is appended to previously written output files, unless -noappend is used or none of the previous output files are present (except for the checkpoint file). The integrity of the files to be appended is verified using checksums which are stored in the checkpoint file. This ensures that output can not be mixed up or corrupted due to file appending. When only some of the previous output files are present, a fatal error is generated and no old output files are modified and no new output files are opened. The result with appending will be the same as from a single run. The contents will be binary identical, unless you use a different number of nodes or dynamic load balancing or the FFT library uses optimizations through timing.

With option -maxh a simulation is terminated and a checkpoint file is written at the first neighbor search step where the run time exceeds -maxh*0.99 hours.

When mdrun receives a TERM signal, it will set nsteps to the current step plus one. When mdrun receives an INT signal (e.g. when ctrl+C is pressed), it will stop after the next neighbor search step (with nstlist=0 at the next step). In both cases all the usual output will be written to file. When running with MPI, a signal to one of the mdrun processes is sufficient, this signal should not be sent to mpirun or the mdrun process that is the parent of the others.

When mdrun is started with MPI, it does not run niced by default.

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