Getting good performance from mdrun

The GROMACS build system and the gmx mdrun tool has a lot of built-in and configurable intelligence to detect your hardware and make pretty effective use of that hardware. For a lot of casual and serious use of gmx mdrun, the automatic machinery works well enough. But to get the most from your hardware to maximize your scientific quality, read on!

Hardware background information

Modern computer hardware is complex and heterogeneous, so we need to discuss a little bit of background information and set up some definitions. Experienced HPC users can skip this section.

core

A hardware compute unit that actually executes instructions. There is normally more than one core in a processor, often many more.

cache

A special kind of memory local to core(s) that is much faster to access than main memory, kind of like the top of a human’s desk, compared to their filing cabinet. There are often several layers of caches associated with a core.

socket

A group of cores that share some kind of locality, such as a shared cache. This makes it more efficient to spread computational work over cores within a socket than over cores in different sockets. Modern processors often have more than one socket.

node

A group of sockets that share coarser-level locality, such as shared access to the same memory without requiring any network hardware. A normal laptop or desktop computer is a node. A node is often the smallest amount of a large compute cluster that a user can request to use.

thread

A stream of instructions for a core to execute. There are many different programming abstractions that create and manage spreading computation over multiple threads, such as OpenMP, pthreads, winthreads, CUDA, OpenCL, and OpenACC. Some kinds of hardware can map more than one software thread to a core; on Intel x86 processors this is called “hyper-threading.” Normally, gmx mdrun will not benefit from such mapping.

affinity

On some kinds of hardware, software threads can migrate between cores to help automatically balance workload. Normally, the performance of gmx mdrun will degrade dramatically if this is permitted, so gmx mdrun will by default set the affinity of its threads to their cores, unless the user or software environment has already done so. Setting thread affinity is sometimes called “pinning” threads to cores.

MPI

The dominant multi-node parallelization-scheme, which provides a standardized language in which programs can be written that work across more than one node.

rank

In MPI, a rank is the smallest grouping of hardware used in the multi-node parallelization scheme. That grouping can be controlled by the user, and might correspond to a core, a socket, a node, or a group of nodes. The best choice varies with the hardware, software and compute task. Sometimes an MPI rank is called an MPI process.

GPU

A graphics processing unit, which is often faster and more efficient than conventional processors for particular kinds of compute workloads. A GPU is always associated with a particular node, and often a particular socket within that node.

OpenMP

A standardized technique supported by many compilers to share a compute workload over multiple cores. Often combined with MPI to achieve hybrid MPI/OpenMP parallelism.

CUDA

A programming-language extension developed by Nvidia for use in writing code for their GPUs.

SIMD

Modern CPU cores have instructions that can execute large numbers of floating-point instructions in a single cycle.

GROMACS background information

The algorithms in gmx mdrun and their implementations are most relevant when choosing how to make good use of the hardware. For details, see the Reference Manual. The most important of these are

Domain Decomposition

The domain decomposition (DD) algorithm decomposes the (short-ranged) component of the non-bonded interactions into domains that share spatial locality, which permits efficient code to be written. Each domain handles all of the particle-particle (PP) interactions for its members, and is mapped to a single rank. Within a PP rank, OpenMP threads can share the workload, or the work can be off-loaded to a GPU. The PP rank also handles any bonded interactions for the members of its domain. A GPU may perform work for more than one PP rank, but it is normally most efficient to use a single PP rank per GPU and for that rank to have thousands of particles. When the work of a PP rank is done on the CPU, mdrun will make extensive use of the SIMD capabilities of the core. There are various command-line options <controlling-the-domain-decomposition-algorithm to control the behaviour of the DD algorithm.

Particle-mesh Ewald

The particle-mesh Ewald (PME) algorithm treats the long-ranged components of the non-bonded interactions (Coulomb and/or Lennard-Jones). Either all, or just a subset of ranks may participate in the work for computing long-ranged component (often inaccurately called simple the “PME” component). Because the algorithm uses a 3D FFT that requires global communication, its performance gets worse as more ranks participate, which can mean it is fastest to use just a subset of ranks (e.g. one-quarter to one-half of the ranks). If there are separate PME ranks, then the remaining ranks handle the PP work. Otherwise, all ranks do both PP and PME work.

Running mdrun within a single node

gmx mdrun can be configured and compiled in several different ways that are efficient to use within a single node. The default configuration using a suitable compiler will deploy a multi-level hybrid parallelism that uses CUDA, OpenMP and the threading platform native to the hardware. For programming convenience, in GROMACS, those native threads are used to implement on a single node the same MPI scheme as would be used between nodes, but much more efficient; this is called thread-MPI. From a user’s perspective, real MPI and thread-MPI look almost the same, and GROMACS refers to MPI ranks to mean either kind, except where noted. A real external MPI can be used for gmx mdrun within a single node, but runs more slowly than the thread-MPI version.

By default, gmx mdrun will inspect the hardware available at run time and do its best to make fairly efficient use of the whole node. The log file, stdout and stderr are used to print diagnostics that inform the user about the choices made and possible consequences.

A number of command-line parameters are available to vary the default behavior.

-nt

The total number of threads to use. The default, 0, will start as many threads as available cores. Whether the threads are thread-MPI ranks, or OpenMP threads within such ranks depends on other settings.

-ntmpi

The total number of thread-MPI ranks to use. The default, 0, will start one rank per GPU (if present), and otherwise one rank per core.

-ntomp

The total number of OpenMP threads per rank to start. The default, 0, will start one thread on each available core. Alternatively, mdrun will honor the appropriate system environment variable (e.g. OMP_NUM_THREADS) if set.

-npme

The total number of ranks to dedicate to the long-ranged component of PME, if used. The default, -1, will dedicate ranks only if the total number of threads is at least 12, and will use around one-third of the ranks for the long-ranged component.

-ntomp_pme

When using PME with separate PME ranks, the total number of OpenMP threads per separate PME ranks. The default, 0, copies the value from -ntomp.

-gpu_id

A string that specifies the ID numbers of the GPUs to be used by corresponding PP ranks on this node. For example, “0011” specifies that the lowest two PP ranks use GPU 0, and the other two use GPU 1.

-pin

Can be set to “auto,” “on” or “off” to control whether mdrun will attempt to set the affinity of threads to cores. Defaults to “auto,” which means that if mdrun detects that all the cores on the node are being used for mdrun, then it should behave like “on,” and attempt to set the affinities (unless they are already set by something else).

-pinoffset

If -pin on, specifies the logical core number to which mdrun should pin the first thread. When running more than one instance of mdrun on a node, use this option to to avoid pinning threads from different mdrun instances to the same core.

-pinstride

If -pin on, specifies the stride in logical core numbers for the cores to which mdrun should pin its threads. When running more than one instance of mdrun on a node, use this option to to avoid pinning threads from different mdrun instances to the same core. Use the default, 0, to minimize the number of threads per physical core - this lets mdrun manage the hardware-, OS- and configuration-specific details of how to map logical cores to physical cores.

-ddorder

Can be set to “interleave,” “pp_pme” or “cartesian.” Defaults to “interleave,” which means that any separate PME ranks will be mapped to MPI ranks in an order like PP, PP, PME, PP, PP, PME, ... etc. This generally makes the best use of the available hardware. “pp_pme” maps all PP ranks first, then all PME ranks. “cartesian” is a special-purpose mapping generally useful only on special torus networks with accelerated global communication for Cartesian communicators. Has no effect if there are no separate PME ranks.

-nb

Can be set to “auto”, “cpu”, “gpu”, “cpu_gpu.” Defaults to “auto,” which uses a compatible GPU if available. Setting “cpu” requires that no GPU is used. Setting “gpu” requires that a compatible GPU be available and will be used. Setting “cpu_gpu” permits the CPU to execute a GPU-like code path, which will run slowly on the CPU and should only be used for debugging.

Examples for mdrun on one node

gmx mdrun

Starts mdrun using all the available resources. mdrun will automatically choose a fairly efficient division into thread-MPI ranks, OpenMP threads and assign work to compatible GPUs. Details will vary with hardware and the kind of simulation being run.

gmx mdrun -nt 8

Starts mdrun using 8 threads, which might be thread-MPI or OpenMP threads depending on hardware and the kind of simulation being run.

gmx mdrun -ntmpi 2 -ntomp 4

Starts mdrun using eight total threads, with four thread-MPI ranks and two OpenMP threads per core. You should only use these options when seeking optimal performance, and must take care that the ranks you create can have all of their OpenMP threads run on the same socket. The number of ranks must be a multiple of the number of sockets, and the number of cores per node must be a multiple of the number of threads per rank.

gmx mdrun -gpu_id 12

Starts mdrun using GPUs with IDs 1 and 2 (e.g. because GPU 0 is dedicated to running a display). This requires two thread-MPI ranks, and will split the available CPU cores between them using OpenMP threads.

gmx mdrun -ntmpi 4 -gpu_id "1122"

Starts mdrun using four thread-MPI ranks, and maps them to GPUs with IDs 1 and 2. The CPU cores available will be split evenly between the ranks using OpenMP threads.

gmx mdrun -nt 6 -pin on -pinoffset 0
gmx mdrun -nt 6 -pin on -pinoffset 3

Starts two mdrun processes, each with six total threads. Threads will have their affinities set to particular logical cores, beginning from the logical core with rank 0 or 3, respectively. The above would work well on an Intel CPU with six physical cores and hyper-threading enabled. Use this kind of setup only if restricting mdrun to a subset of cores to share a node with other processes.

gmx mpirun_mpi -np 2

When using an gmx mdrun compiled with external MPI, this will start two ranks and as many OpenMP threads as the hardware and MPI setup will permit. If the MPI setup is restricted to one node, then the resulting gmx mdrun will be local to that node.

Running mdrun on more than one node

This requires configuring GROMACS to build with an external MPI library. By default, this mdrun executable will be named gmx mdrun. All of the considerations for running single-node mdrun still apply, except that -ntmpi and -nt cause a fatal error, and instead the number of ranks is controlled by the MPI environment. Settings such as -npme are much more important when using multiple nodes. Configuring the MPI environment to produce one rank per core is generally good until one approaches the strong-scaling limit. At that point, using OpenMP to spread the work of an MPI rank over more than one core is needed to continue to improve absolute performance. The location of the scaling limit depends on the processor, presence of GPUs, network, and simulation algorithm, but it is worth measuring at around ~200 particles/core if you need maximum throughput.

There are further command-line parameters that are relevant in these cases.

-tunepme

Defaults to “on.” If “on,” will optimize various aspects of the PME and DD algorithms, shifting load between ranks and/or GPUs to maximize throughput

-dlb

Can be set to “auto,” “no,” or “yes.” Defaults to “auto.” Doing Dynamic Load Balancing between MPI ranks is needed to maximize performance. This is particularly important for molecular systems with heterogeneous particle or interaction density. When a certain threshold for performance loss is exceeded, DLB activates and shifts particles between ranks to improve performance.

-gcom

During the simulation gmx mdrun must communicate between all ranks to compute quantities such as kinetic energy. By default, this happens whenever plausible, and is influenced by a lot of [.mdp options](#mdp-options). The period between communication phases must be a multiple of nstlist, and defaults to the minimum of nstcalcenergy and nstlist. mdrun -gcom sets the number of steps that must elapse between such communication phases, which can improve performance when running on a lot of nodes. Note that this means that _e.g._ temperature coupling algorithms will effectively remain at constant energy until the next global communication phase.

Note that -tunepme has more effect when there is more than one node, because the cost of communication for the PP and PME ranks differs. It still shifts load between PP and PME ranks, but does not change the number of separate PME ranks in use.

Note also that -dlb and -tunepme can interfere with each other, so if you experience performance variation that could result from this, you may wish to tune PME separately, and run the result with mdrun -notunepme -dlb yes.

The gmx tune_pme utility is available to search a wider range of parameter space, including making safe modifications to the tpr file, and varying -npme. It is only aware of the number of ranks created by the MPI environment, and does not explicitly manage any aspect of OpenMP during the optimization.

Examples for mdrun on more than one node

The examples and explanations for for single-node mdrun are still relevant, but -nt is no longer the way to choose the number of MPI ranks.

mpirun -np 16 gmx mdrun_mpi

Starts gmx mdrun with 16 ranks, which are mapped to the hardware by the MPI library, e.g. as specified in an MPI hostfile. The available cores will be automatically split among ranks using OpenMP threads, depending on the hardware and any environment settings such as OMP_NUM_THREADS.

mpirun -np 16 gmx mdrun_mpi -npme 5

Starts gmx mdrun with 16 ranks, as above, and require that 5 of them are dedicated to the PME component.

mpirun -np 11 gmx mdrun_mpi -ntomp 2 -npme 6 -ntomp_pme 1

Starts gmx mdrun with 11 ranks, as above, and require that six of them are dedicated to the PME component with one OpenMP thread each. The remaining five do the PP component, with two OpenMP threads each.

mpirun -np 4 gmx mdrun -ntomp 6 -gpu_id 00

Starts gmx mdrun on a machine with two nodes, using four total ranks, each rank with six OpenMP threads, and both ranks on a node sharing GPU with ID 0.

mpirun -np 8 gmx mdrun -ntomp 3 -gpu_id 0000

Starts gmx mdrun on a machine with two nodes, using eight total ranks, each rank with three OpenMP threads, and all four ranks on a node sharing GPU with ID 0. This may or may not be faster than the previous setup on the same hardware.

mpirun -np 20 gmx mdrun_mpi -ntomp 4 -gpu_id 0

Starts gmx mdrun with 20 ranks, and assigns the CPU cores evenly across ranks each to one OpenMP thread. This setup is likely to be suitable when there are ten nodes, each with one GPU, and each node has two sockets.

mpirun -np 20 gmx mdrun_mpi -gpu_id 00

Starts gmx mdrun with 20 ranks, and assigns the CPU cores evenly across ranks each to one OpenMP thread. This setup is likely to be suitable when there are ten nodes, each with one GPU, and each node has two sockets.

mpirun -np 20 gmx mdrun_mpi -gpu_id 01

Starts gmx mdrun with 20 ranks. This setup is likely to be suitable when there are ten nodes, each with two GPUs.

mpirun -np 40 gmx mdrun_mpi -gpu_id 0011

Starts gmx mdrun with 40 ranks. This setup is likely to be suitable when there are ten nodes, each with two GPUs, and OpenMP performs poorly on the hardware.

Controlling the domain decomposition algorithm

This section lists all the options that affect how the domain decomposition algorithm decomposes the workload to the available parallel hardware.

-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. Particles 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 independent 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 gmx mdrun based on the initial coordinates. The chosen value will be a balance between interaction range and communication cost.

-ddcheck

On by default. When inter charge-group bonded interactions are beyond the bonded cut-off distance, gmx 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.

-rcon

When constraints are present, option -rcon influences the cell size limit as well. Particles 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 gmx 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 -rcon.

-dds

Sets the minimum allowed x, y and/or z scaling of the cells with dynamic load balancing. gmx 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.

Finding out how to run mdrun better

TODO In future patch: red flags in log files, how to interpret wallcycle output

TODO In future patch: import wiki page stuff on performance checklist; maybe here, maybe elsewhere

Running mdrun with GPUs

TODO In future patch: any tips not covered above

Running the OpenCL version of mdrun

The current version works with GCN-based AMD GPUs, and NVIDIA CUDA GPUs. Make sure that you have the latest drivers installed. The minimum OpenCL version required is 1.1. See also the known limitations.

The same -gpu_id option (or GMX_GPU_ID environment variable) used to select CUDA devices, or to define a mapping of GPUs to PP ranks, is used for OpenCL devices.

The following devices are known to work correctly:

• AMD: FirePro W5100, HD 7950, FirePro W9100, Radeon R7 240, Radeon R7 M260, Radeon R9 290
• NVIDIA: GeForce GTX 660M, GeForce GTX 660Ti, GeForce GTX 750Ti, GeForce GTX 780, GTX Titan

Building the OpenCL program can take a few seconds when gmx mdrun starts up, because the kernels that run on the GPU can only be compiled at run time. This is not normally a problem for long production MD, but you might prefer to do some kinds of work on just the CPU (e.g. see -nb above).

Some other OpenCL management environment variables may be of interest to developers.

Known limitations of the OpenCL support

Limitations in the current OpenCL support of interest to GROMACS users:

• Using more than one GPU on a node is supported only with thread MPI
• Sharing a GPU between multiple PP ranks is not supported
• No Intel devices (CPUs, GPUs or Xeon Phi) are supported
• Due to blocking behavior of clEnqueue functions in the NVIDIA driver, there is almost no performance gain when using NVIDIA GPUs. A bug report has already been filed about this issue. A possible workaround would be to have a separate thread for issuing GPU commands. However this hasn’t been implemented yet.

Limitations of interest to GROMACS developers:

• The current implementation is not compatible with OpenCL devices that are not using warp/wavefronts or for which the warp/wavefront size is not a multiple of 32
• Some Ewald tabulated kernels are known to produce incorrect results, so (correct) analytical kernels are used instead.