4.23. Use of distributed grids within style classes — LAMMPS documentation

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# 4.23. Use of distributed grids within style classes

Added in version 22Dec2022.

The LAMMPS source code includes two classes which facilitate the creation and use of distributed grids. These are the Grid2d and Grid3d classes in the src/grid2d.cpp.h and src/grid3d.cpp.h files respectively. As the names imply, they are used for 2d or 3d simulations, as defined by the dimension command.

The Howto_grid page gives an overview of how distributed grids are defined from a user perspective, lists LAMMPS commands which use them, and explains how grid cell data is referenced from an input script. Please read that page first as it motivates the coding details discussed here.

This doc page is for users who wish to write new styles (input script commands) which use distributed grids. There are a variety of material models and analysis methods which use atoms (or coarse-grained particles) and grids in tandem.

A distributed grid means each processor owns a subset of the grid cells. In LAMMPS, the subset for each processor will be a sub-block of grid cells with low and high index bounds in each dimension of the grid. The union of the sub-blocks across all processors is the global grid.

More specifically, a grid point is defined for each cell (by default the center point), and a processor owns a grid cell if its point is within the processor’s spatial subdomain. The union of processor subdomains is the global simulation box. If a grid point is on the boundary of two subdomains, the lower processor owns the grid cell. A processor may also store copies of ghost cells which surround its owned cells.

## 4.23.1. Style commands

Style commands which can define and use distributed grids include the compute, fix, pair, and kspace styles. If you wish grid cell data to persist across timesteps, then use a fix. If you wish grid cell data to be accessible by other commands, then use a fix or compute. Currently in LAMMPS, the pair_style amoeba, kspace_style pppm, and kspace_style msm commands use distributed grids but do not require either of these capabilities; they thus create and use distributed grids internally. Note that a pair style which needs grid cell data to persist could be coded to work in tandem with a fix style which provides that capability.

The size of a grid is specified by the number of grid cells in each dimension of the simulation domain. In any dimension the size can be any value >= 1. Thus a 10x10x1 grid for a 3d simulation is effectively a 2d grid, where each grid cell spans the entire z-dimension. A 1x100x1 grid for a 3d simulation is effectively a 1d grid, where grid cells are a series of thin xz slabs in the y-dimension. It is even possible to define a 1x1x1 3d grid, though it may be inefficient to use it in a computational sense.

Note that the choice of grid size is independent of the number of processors or their layout in a grid of processor subdomains which overlays the simulations domain. Depending on the distributed grid size, a single processor may own many 1000s or no grid cells.

A command can define multiple grids, each of a different size. Each grid is an instantiation of the Grid2d or Grid3d class.

The command also defines what data it will store for each grid it creates and it allocates the multidimensional array(s) needed to store the data. No grid cell data is stored within the Grid2d or Grid3d classes.

If a single value per grid cell is needed, the data array will have the same dimension as the grid, i.e. a 2d array for a 2d grid, likewise for 3d. If multiple values per grid cell are needed, the data array will have one more dimension than the grid, i.e. a 3d array for a 2d grid, or 4d array for a 3d grid. A command can choose to define multiple data arrays for each grid it defines.

## 4.23.2. Grid data allocation and access

The simplest way for a command to allocate and access grid cell data is to use the create_offset() methods provided by the Memory class. Arguments for these methods can be values returned by the setup_grid() method (described below), which define the extent of the grid cells (owned+ghost) the processor owns. These 4 methods allocate memory for 2d (first two) and 3d (second two) grid data. The two methods that end in “_offset” allocate an array which stores a single value per grid cell. The two that end in “_last” allocate an array which stores Nvalues per grid cell.

    // single value per cell for a 2d grid = 2d array
    memory->create2d_offset(data2d_one, nylo_out, nyhi_out,
                            nxlo_out, nxhi_out, "data2d_one");

    // nvalues per cell for a 2d grid = 3d array
    memory->create3d_offset_last(data2d_multi, nylo_out, nyhi_out,
                                 nxlo_out, nxhi_out, nvalues, "data2d_multi");

    // single value per cell for a 3d grid = 3d array
    memory->create3d_offset(data3d_one, nzlo_out, nzhi_out, nylo_out,
                            nyhi_out, nxlo_out, nxhi_out, "data3d_one");

    // nvalues per cell for a 3d grid = 4d array
    memory->create4d_offset_last(data3d_multi, nzlo_out, nzhi_out, nylo_out,
                                 nyhi_out, nxlo_out, nxhi_out, nvalues,
                                 "data3d_multi");

Note that these multidimensional arrays are allocated as contiguous chunks of memory where the x-index of the grid varies fastest, then y, and the z-index slowest. For multiple values per grid cell, the Nvalues are contiguous, so their index varies even faster than the x-index.

The key point is that the “offset” methods create arrays which are indexed by the range of indices which are the bounds of the sub-block of the global grid owned by this processor. This means loops like these can be written in the caller code to loop over owned grid cells, where the “i” loop bounds are the range of owned grid cells for the processor. These are the bounds returned by the setup_grid() method:

    for (int iy = iylo; iy <= iyhi; iy++)
      for (int ix = ixlo; ix <= ixhi; ix++)
        data2d_one[iy][ix] = 0.0;

    for (int iy = iylo; iy <= iyhi; iy++)
      for (int ix = ixlo; ix <= ixhi; ix++)
        for (int m = 0; m < nvalues; m++)
          data2d_multi[iy][ix][m] = 0.0;

    for (int iz = izlo; iz <= izhi; iz++)
      for (int iy = iylo; iy <= iyhi; iy++)
        for (int ix = ixlo; ix <= ixhi; ix++)
          data3d_one[iz][iy][ix] = 0.0;

    for (int iz = izlo; iz <= izhi; iz++)
      for (int iy = iylo; iy <= iyhi; iy++)
        for (int ix = ixlo; ix <= ixhi; ix++)
           for (int m = 0; m < nvalues; m++)
              data3d_multi[iz][iy][ix][m] = 0.0;

Simply replacing the “i” bounds with “o” bounds, also returned by the setup_grid() method, would alter this code to loop over owned+ghost cells (the entire allocated grid).

## 4.23.3. Grid class constructors

The following subsections describe the public methods of the Grid3d class which a style command can invoke. The Grid2d methods are similar; simply remove arguments which refer to the z-dimension.

There are 2 constructors which can be used. They differ in the extra i/o xyz lo/hi arguments:

    Grid3d(class LAMMPS *lmp, MPI_Comm gcomm, int gnx, int gny, int gnz)
    Grid3d(class LAMMPS *lmp, MPI_Comm gcomm, int gnx, int gny, int gnz,
           int ixlo, int ixhi, int iylo, int iyhi, int izlo, int izhi,
           int oxlo, int oxhi, int oylo, int oyhi, int ozlo, int ozhi)

Both constructors take the LAMMPS instance pointer and a communicator over which the grid will be distributed. Typically this is the world communicator the LAMMPS instance is using. The kspace_style msm command creates a series of grids, each of different size, which are partitioned across different sub-communicators of processors. Both constructors are also passed the global grid size: gnx by gny by gnz.

The first constructor is used when the caller wants the Grid class to partition the global grid across processors; the Grid class defines which grid cells each processor owns and also which it stores as ghost cells. A subsequent call to setup_grid(), discussed below, returns this info to the caller.

The second constructor allows the caller to define the extent of owned and ghost cells, and pass them to the Grid class. The 6 arguments which start with “i” are the inclusive lower and upper index bounds of the owned (inner) grid cells this processor owns in each of the 3 dimensions within the global grid. Owned grid cells are indexed from 0 to N-1 in each dimension.

The 6 arguments which start with “o” are the inclusive bounds of the owned+ghost (outer) grid cells it stores. If the ghost cells are on the other side of a periodic boundary, then these indices may be < 0 or >= N in any dimension, so that oxlo <= ixlo and ixhi >= ixhi is always the case.

For example, if Nx = 100, then a processor might pass ixlo=50, ixhi=60, oxlo=48, oxhi=62 to the Grid class. Or ixlo=0, ixhi=10, oxlo=-2, oxhi=13. If a processor owns no grid cells in a dimension, then the ihi value should be specified as one less than the ilo value.

Note that the only reason to use the second constructor is if the logic for assigning ghost cells is too complex for the Grid class to compute, using the various set() methods described next. Currently only the kspace_style pppm/electrode and kspace_style msm commands use the second constructor.

## 4.23.4. Grid class set methods

The following methods affect how the Grid class computes which owned and ghost cells are assigned to each processor. Set_shift_grid() is the only method which influences owned cell assignment; all the rest influence ghost cell assignment. These methods are only used with the first constructor; they are ignored if the second constructor is used. These methods must be called before the setup_grid() method is invoked, because they influence its operation.

    void set_shift_grid(double shift);
    void set_distance(double distance);
    void set_stencil_atom(int lo, int hi);
    void set_shift_atom(double shift_lo, double shift_hi);
    void set_stencil_grid(int lo, int hi);
    void set_zfactor(double factor);

Processors own a grid cell if a point within the grid cell is inside the processor’s subdomain. By default this is the center point of the grid cell. The set_shift_grid() method can change this. The shift argument is a value from 0.0 to 1.0 (inclusive) which is the offset of the point within the grid cell in each dimension. The default is 0.5 for the center of the cell. A value of 0.0 is the lower left corner point; a value of 1.0 is the upper right corner point. There is typically no need to change the default as it is optimal for minimizing the number of ghost cells needed.

If a processor maps its particles to grid cells, it needs to allow for its particles being outside its subdomain between reneighboring. The distance argument of the set_distance() method sets the furthest distance outside a processor’s subdomain which a particle can move. Typically this is half the neighbor skin distance, assuming reneighboring is done appropriately. This distance is used in determining how many ghost cells a processor needs to store to enable its particles to be mapped to grid cells. The default value is 0.0.

Some commands, like the kspace_style pppm command, map values (charge in the case of PPPM) to a stencil of grid cells beyond the grid cell the particle is in. The stencil extent may be different in the low and high directions. The set_stencil_atom() method defines the maximum values of those 2 extents, assumed to be the same in each of the 3 dimensions. Both the lo and hi values are specified as positive integers. The default values are both 0.

Some commands, like the kspace_style pppm command, shift the position of an atom when mapping it to a grid cell, based on the size of the stencil used to map values to the grid (charge in the case of PPPM). The lo and hi arguments of the set_shift_atom() method are the minimum shift in the low direction and the maximum shift in the high direction, assumed to be the same in each of the 3 dimensions. The shifts should be fractions of a grid cell size with values between 0.0 and 1.0 inclusive. The default values are both 0.0. See the src/pppm.cpp file for examples of these lo/hi values for regular and staggered grids.

Some methods like the fix ttm/grid command, perform finite difference kinds of operations on the grid, to diffuse electron heat in the case of the two-temperature model (TTM). This operation uses ghost grid values beyond the owned grid values the processor updates. The set_stencil_grid() method defines the extent of this stencil in both directions, assumed to be the same in each of the 3 dimensions. Both the lo and hi values are specified as positive integers. The default values are both 0.

The kspace_style pppm commands allow a grid to be defined which overlays a volume which extends beyond the simulation box in the z dimension. This is for the purpose of modeling a 2d-periodic slab (non-periodic in z) as if it were a larger 3d periodic system, extended (with empty space) in the z dimension. The kspace_modify slab command is used to specify the ratio of the larger volume to the simulation volume; a volume ratio of ~3 is typical. For this kind of model, the PPPM caller sets the global grid size gnz ~3x larger than it would be otherwise. This same ratio is passed by the PPPM caller as the factor argument to the Grid class via the set_zfactor() method (set_yfactor() for 2d grids). The Grid class will then assign ownership of the 1/3 of grid cells that overlay the simulation box to the processors which also overlay the simulation box. The remaining 2/3 of the grid cells are assigned to processors whose subdomains are adjacent to the upper z boundary of the simulation box.

## 4.23.5. Grid class setup_grid method

The setup_grid() method is called after the first constructor (above) to partition the grid across processors, which determines which grid cells each processor owns. It also calculates how many ghost grid cells in each dimension and each direction each processor needs to store.

Note that this method is NOT called if the second constructor above is used. In that case, the caller assigns owned and ghost cells to each processor.

Also note that this method must be invoked after any set_*() methods have been used, since they can influence the assignment of owned and ghost cells.

    void setup_grid(int &ixlo, int &ixhi, int &iylo, int &iyhi, int &izlo, int &izhi,
                    int &oxlo, int &oxhi, int &oylo, int &oyhi, int &ozlo, int &ozhi)

The 6 return arguments which start with “i” are the inclusive lower and upper index bounds of the owned (inner) grid cells this processor owns in each of the 3 dimensions within the global grid. Owned grid cells are indexed from 0 to N-1 in each dimension.

The 6 return arguments which start with “o” are the inclusive bounds of the owned+ghost cells it owns. If the ghost cells are on the other side of a periodic boundary, then these indices may be < 0 or >= N in any dimension, so that oxlo <= ixlo and ixhi >= ixhi is always the case.

## 4.23.6. More grid class set methods

The following 2 methods can be used to override settings made by the constructors above. If used, they must be called called before the setup_comm() method is invoked, since it uses the settings that these methods override. In LAMMPS these methods are called by by the kspace_style msm command for the grids it instantiates using the 2nd constructor above.

    void set_proc_neighs(int pxlo, int pxhi, int pylo, int pyhi, int pzlo, int pzhi)
    void set_caller_grid(int fxlo, int fxhi, int fylo, int fyhi, int fzlo, int fzhi)

The set_proc_neighs() method sets the processor IDs of the 6 neighboring processors for each processor. Normally these would match the processor grid neighbors which LAMMPS creates to overlay the simulation box (the default). However, MSM excludes non-participating processors from coarse grid communication when less processors are used. This method allows MSM to override the default values.

The set_caller_grid() method species the size of the data arrays the caller allocates. Normally these would match the extent of the ghost grid cells (the default). However the MSM caller allocates a larger data array (more ghost cells) for its finest-level grid, for use in other operations besides owned/ghost cell communication. This method allows MSM to override the default values.

## 4.23.7. Grid class get methods

The following methods allow the caller to query the settings for a specific grid, whether it created the grid or another command created it.

    void get_size(int &nxgrid, int &nygrid, int &nzgrid);
    void get_bounds_owned(int &xlo, int &xhi, int &ylo, int &yhi, int &zlo, int &zhi)
    void get_bounds_ghost(int &xlo, int &xhi, int &ylo, int &yhi, int &zlo, int &zhi)

The get_size() method returns the size of the global grid in each dimension.

The get_bounds_owned() method return the inclusive index bounds of the grid cells this processor owns. The values range from 0 to N-1 in each dimension. These values are the same as the “i” values returned by setup_grid().

The get_bounds_ghost() method return the inclusive index bounds of the owned+ghost grid cells this processor stores. The owned cell indices range from 0 to N-1, so these indices may be less than 0 or greater than or equal to N in each dimension. These values are the same as the “o” values returned by setup_grid().

## 4.23.8. Grid class owned/ghost communication

If needed by the command, the following methods setup and perform communication of grid data to/from neighboring processors. The forward_comm() method sends owned grid cell data to the corresponding ghost grid cells on other processors. The reverse_comm() method sends ghost grid cell data to the corresponding owned grid cells on another processor. The caller can choose to sum ghost grid cell data to the owned grid cell or simply copy it.

    void setup_comm(int &nbuf1, int &nbuf2)
    void forward_comm(int caller, void *ptr, int which, int nper, int nbyte,
                      void *buf1, void *buf2, MPI_Datatype datatype);
    void reverse_comm(int caller, void *ptr, int which, int nper, int nbyte,
                      void *buf1, void *buf2, MPI_Datatype datatype)
    int ghost_adjacent();

The setup_comm() method must be called one time before performing forward or reverse communication (multiple times if needed). It returns two integers, which should be used to allocate two buffers. The nbuf1 and nbuf2 values are the number of grid cells whose data will be stored in two buffers by the Grid class when forward or reverse communication is performed. The caller should thus allocate them to a size large enough to hold all the data used in any single forward or reverse communication operation it performs. Note that the caller may allocate and communicate multiple data arrays for a grid it instantiates. This size includes the bytes needed for the data type of the grid data it stores, e.g. double precision values.

The forward_comm() and reverse_comm() methods send grid cell data from owned to ghost cells, or ghost to owned cells, respectively, as described above. The caller argument should be one of these values – Grid3d::COMPUTE, Grid3d::FIX, Grid3d::KSPACE, Grid3d::PAIR – depending on the style of the caller class. The ptr argument is the “this” pointer to the caller class. These two arguments are used to call back to pack()/unpack() functions in the caller class, as explained below.

The which argument is a flag the caller can set which is passed to the caller’s pack()/unpack() methods. This allows a single callback method to pack/unpack data for several different flavors of forward/reverse communication, e.g. operating on different grids or grid data.

The nper argument is the number of values per grid cell to be communicated. The nbyte argument is the number of bytes per value, e.g. 8 for double-precision values. The buf1 and buf2 arguments are the two allocated buffers described above. So long as they are allocated for the maximum size communication, they can be re-used for any forward_comm()/reverse_comm() call. The datatype argument is the MPI_Datatype setting, which should match the buffer allocation and the nbyte argument. E.g. MPI_DOUBLE for buffers storing double precision values.

To use the forward_grid() method, the caller must provide two callback functions; likewise for use of the reverse_grid() methods. These are the 4 functions, their arguments are all the same.

    void pack_forward_grid(int which, void *vbuf, int nlist, int *list);
    void unpack_forward_grid(int which, void *vbuf, int nlist, int *list);
    void pack_reverse_grid(int which, void *vbuf, int nlist, int *list);
    void unpack_reverse_grid(int which, void *vbuf, int nlist, int *list);

The which argument is set to the which value of the forward_comm() or reverse_comm() calls. It allows the pack/unpack function to select what data values to pack/unpack. Vbuf is the buffer to pack/unpack the data to/from. It is a void pointer so that the caller can cast it to whatever data type it chooses, e.g. double precision values. Nlist is the number of grid cells to pack/unpack and list is a vector (nlist in length) of offsets to where the data for each grid cell resides in the caller’s data arrays, which is best illustrated with an example from the src/EXTRA-FIX/fix_ttm_grid.cpp class which stores the scalar electron temperature for 3d system in a 3d grid (one value per grid cell):

    void FixTTMGrid::pack_forward_grid(int /*which*/, void *vbuf, int nlist, int *list)
    {
      auto buf = (double *) vbuf;
      double *src = &T_electron[nzlo_out][nylo_out][nxlo_out];
      for (int i = 0; i < nlist; i++) buf[i] = src[list[i]];
    }

In this case, the which argument is not used, vbuf points to a buffer of doubles, and the electron temperature is stored by the FixTTMGrid class in a 3d array of owned+ghost cells called T_electron. That array is allocated by the memory->create_3d_offset() method described above so that the first grid cell it stores is indexed as T_electron[nzlo_out][nylo_out][nxlo_out]. The nlist values in list are integer offsets from that first grid cell. Setting src to the address of the first cell allows those offsets to be used to access the temperatures to pack into the buffer.

Here is a similar portion of code from the src/fix_ave_grid.cpp class which can store two kinds of data, a scalar count of atoms in a grid cell, and one or more grid-cell-averaged atom properties. The code from its unpack_reverse_grid() function for 2d grids and multiple per-atom properties per grid cell (nvalues) is shown here:

    void FixAveGrid::unpack_reverse_grid(int /*which*/, void *vbuf, int nlist, int *list)
    {
      auto buf = (double *) vbuf;
      double *count,*data,*values;
      count = &count2d[nylo_out][nxlo_out];
      data = &array2d[nylo_out][nxlo_out][0];
      m = 0;
      for (i = 0; i < nlist; i++) {
        count[list[i]] += buf[m++];
        values = &data[nvalues*list[i]];
        for (j = 0; j < nvalues; j++)
         values[j] += buf[m++];
      }
    }

Both the count and the multiple values per grid cell are communicated in vbuf. Note that data is now a pointer to the first value in the first grid cell. And values points to where the first value in data is for an offset of grid cells, calculated by multiplying nvalues by list[i]. Finally, because this is reverse communication, the communicated buffer values are summed to the caller values.

The ghost_adjacent() method returns a 1 if every processor can perform the necessary owned/ghost communication with only its nearest neighbor processors (4 in 2d, 6 in 3d). It returns a 0 if any processor’s ghost cells extend further than nearest neighbor processors.

This can be checked by callers who have the option to change the global grid size to ensure more efficient nearest-neighbor-only communication if they wish. In this case, they instantiate a grid of a given size (resolution), then invoke setup_comm() followed by ghost_adjacent(). If the ghost cells are not adjacent, they destroy the grid instance and start over with a higher-resolution grid. Several of the kspace_style pppm command variants have this option.

## 4.23.9. Grid class remap methods for load balancing

The following methods are used when a load-balancing operation, triggered by the balance or fix balance commands, changes the partitioning of the simulation domain into processor subdomains.

In order to work with load-balancing, any style command (compute, fix, pair, or kspace style) which allocates a grid and stores per-grid data should define a reset_grid() method; it takes no arguments. It will be called by the two balance commands after they have reset processor subdomains and migrated atoms (particles) to new owning processors. The reset_grid() method will typically perform some or all of the following operations. See the src/fix_ave_grid.cpp and src/EXTRA_FIX/fix_ttm_grid.cpp files for examples of reset_grid() methods, as well as the pack_remap_grid() and unpack_remap_grid() functions.

First, the reset_grid() method can instantiate new grid(s) of the same global size, then call setup_grid() to partition them via the new processor subdomains. At this point, it can invoke the identical() method which compares the owned and ghost grid cell index bounds between two grids, the old grid passed as a pointer argument, and the new grid whose identical() method is being called. It returns 1 if the indices match on all processors, otherwise 0. If they all match, then the new grids can be deleted; the command can continue to use the old grids.

If not, then the command should allocate new grid data array(s) which depend on the new partitioning. If the command does not need to persist its grid data from the old partitioning to the new one, then the command can simply delete the old data array(s) and grid instance(s). It can then return.

If the grid data does need to persist, then the data for each grid needs to be “remapped” from the old grid partitioning to the new grid partitioning. The setup_remap() and remap() methods are used for that purpose.

    int identical(Grid3d *old);
    void setup_remap(Grid3d *old, int &nremap_buf1, int &nremap_buf2)
    void remap(int caller, void *ptr, int which, int nper, int nbyte,
               void *buf1, void *buf2, MPI_Datatype datatype)

The arguments to these methods are identical to those for the setup_comm() and forward_comm() or reverse_comm() methods. However the returned nremap_buf1 and nremap2_buf values will be different than the nbuf1 and nbuf2 values. They should be used to allocate two different remap buffers, separate from the owned/ghost communication buffers.

To use the remap() method, the caller must provide two callback functions:

    void pack_remap_grid(int which, void *vbuf, int nlist, int *list);
    void unpack_remap_grid(int which, void *vbuf, int list, int *list);

Their arguments are identical to those for the pack_forward_grid() and unpack_forward_grid() callback functions (or the reverse variants) discussed above. Normally, both these methods pack/unpack all the data arrays for a given grid. The which argument of the remap() method sets the which value for the pack/unpack functions. If the command instantiates multiple grids (of different sizes), it can be used within the pack/unpack methods to select which grid’s data is being remapped.

Note that the pack_remap_grid() function must copy values from the OLD grid data arrays into the vbuf buffer. The unpack_remap_grid() function must copy values from the vbuf buffer into the NEW grid data arrays.

After the remap operation for grid cell data has been performed, the reset_grid() method can deallocate the two remap buffers it created, and can then exit.

## 4.23.10. Grid class I/O methods

There are two I/O methods in the Grid classes which can be used to read and write grid cell data to files. The caller can decide on the precise format of each file, e.g. whether header lines are prepended or comment lines are allowed. Fundamentally, the file should contain one line per grid cell for the entire global grid. Each line should contain identifying info as to which grid cell it is, e.g. a unique grid cell ID or the ix,iy,iz indices of the cell within a 3d grid. The line should also contain one or more data values which are stored within the grid data arrays created by the command

For grid cell IDs, the LAMMPS convention is that the IDs run from 1 to N, where N = Nx * Ny for 2d grids and N = Nx * Ny * Nz for 3d grids. The x-index of the grid cell varies fastest, then y, and the z-index varies slowest. So for a 10x10x10 grid the cell IDs from 901-1000 would be in the top xy layer of the z dimension.

The read_file() method does something simple. It reads a chunk of consecutive lines from the file and passes them back to the caller to process. The caller provides a unpack_read_grid() function for this purpose. The function checks the grid cell ID or indices and only stores grid cell data for the grid cells it owns.

The write_file() method does something slightly more complex. Each processor packs the data for its owned grid cells into a buffer. The caller provides a pack_write_grid() function for this purpose. The write_file() method then loops over all processors and each sends its buffer one at a time to processor 0, along with the 3d (or 2d) index bounds of its grid cell data within the global grid. Processor 0 calls back to the unpack_write_grid() function provided by the caller with the buffer. The function writes one line per grid cell to the file.

See the src/EXTRA_FIX/fix_ttm_grid.cpp file for examples of now both these methods are used to read/write electron temperature values from/to a file, as well as for implementations of the the pack/unpack functions described below.

Here are the details of the two I/O methods and the 3 callback functions. See the src/fix_ave_grid.cpp file for examples of all of them.

    void read_file(int caller, void *ptr, FILE *fp, int nchunk, int maxline)
    void write_file(int caller, void *ptr, int which,
                    int nper, int nbyte, MPI_Datatype datatype

The caller argument in both methods should be one of these values – Grid3d::COMPUTE, Grid3d::FIX, Grid3d::KSPACE, Grid3d::PAIR – depending on the style of the caller class. The ptr argument in both methods is the “this” pointer to the caller class. These 2 arguments are used to call back to pack()/unpack() functions in the caller class, as explained below.

For the read_file() method, the fp argument is a file pointer to the file to be read from, opened on processor 0 by the caller. Nchunk is the number of lines to read per chunk, and maxline is the maximum number of characters per line. The Grid class will allocate a buffer for storing chunks of lines based on these values.

For the write_file() method, the which argument is a flag the caller can set which is passed back to the caller’s pack()/unpack() methods. If the command instantiates multiple grids (of different sizes), this flag can be used within the pack/unpack methods to select which grid’s data is being written out (presumably to different files). the nper argument is the number of values per grid cell to be written out. The nbyte argument is the number of bytes per value, e.g. 8 for double-precision values. The datatype argument is the MPI_Datatype setting, which should match the nbyte argument. E.g. MPI_DOUBLE for double precision values.

To use the read_grid() method, the caller must provide one callback function. To use the write_grid() method, it provides two callback functions:

    int unpack_read_grid(int nlines, char *buffer)
    void pack_write_grid(int which, void *vbuf)
    void unpack_write_grid(int which, void *vbuf, int *bounds)

For unpack_read_grid() the nlines argument is the number of lines of character data read from the file and contained in buffer. The lines each include a newline character at the end. When the function processes the lines, it may choose to skip some of them (header or comment lines). It returns an integer count of the number of grid cell lines it processed. This enables the Grid class read_file() method to know when it has read the correct number of lines.

For pack_write_grid() and unpack_write_grid(), the vbuf argument is the buffer to pack/unpack data to/from. It is a void pointer so that the caller can cast it to whatever data type it chooses, e.g. double precision values. the which argument is set to the which value of the write_file() method. It allows the caller to choose which grid data to operate on.

For unpack_write_grid(), the bounds argument is a vector of 4 or 6 integer grid indices (4 for 2d, 6 for 3d). They are the xlo,xhi,ylo,yhi,zlo,zhi index bounds of the portion of the global grid which the vbuf holds owned grid cell data values for. The caller should loop over the values in vbuf with a double loop (2d) or triple loop (3d), similar to the code snippets listed above. The x-index varies fastest, then y, and the z-index slowest. If there are multiple values per grid cell, the index for those values varies fastest of all. The caller can add the x,y,z indices of the grid cell (or the corresponding grid cell ID) to the data value(s) written as one line to the output file.

## 4.23.11. Style class grid access methods

A style command can enable its grid cell data to be accessible from other commands. For example fix ave/grid or dump grid or dump grid/vtk. Those commands access the grid cell data by using a grid reference in their input script syntax, as described on the Howto_grid doc page. They look like this:

- c_ID:gname:dname

- c_ID:gname:dname[I]

- f_ID:gname:dname

- f_ID:gname:dname[I]

Each grid command instantiates has a unique gname, defined by the command. Likewise each grid cell data structure (scalar or vector) associated with a grid has a unique dname, also defined by the command.

To provide access to its grid cell data, a style command needs to implement the following 4 methods:

    int get_grid_by_name(const std::string &name, int &dim);
    void *get_grid_by_index(int index);
    int get_griddata_by_name(int igrid, const std::string &name, int &ncol);
    void *get_griddata_by_index(int index);

Currently only computes and fixes can implement these methods. If it does so, the compute of fix should also set the variable pergrid_flag to 1. See any of the compute or fix commands which set “pergrid_flag = 1” for examples of how these 4 functions can be implemented.

The get_grid_by_name() method takes a grid name as input and returns two values. The dim argument is returned as 2 or 3 for the dimensionality of the grid. The function return is a grid index from 0 to G-1 where G is the number of grids the command instantiates. A value of -1 is returned if the grid name is not recognized.

The get_grid_by_index() method is called after the get_grid_by_name() method, using the grid index it returned as its argument. This method will return a pointer to the Grid2d or Grid3d class. The caller can use this to query grid attributes, such as the global size of the grid, to ensure it is of the expected size.

The get_griddata_by_name() method takes a grid index igrid and a data name as input. It returns two values. The ncol argument is returned as a 0 if the grid data is a single value (scalar) per grid cell, or an integer M > 0 if there are M values (vector) per grid cell. Note that even if M = 1, it is still a 1-length vector, not a scalar. The function return is a data index from 0 to D-1 where D is the number of data sets associated with that grid by the command. A value of -1 is returned if the data name is not recognized.

The get_griddata_by_index() method is called after the get_griddata_by_name() method, using the data index it returned as its argument. This method will return a pointer to the multidimensional array which stores the requested data.

As in the discussion above of the Memory class create_offset() methods, the dimensionality of the array associated with the returned pointer depends on whether it is a 2d or 3d grid and whether there is a single or multiple values stored for each grid cell:

- single value per cell for a 2d grid = 2d array pointer

- multiple values per cell for a 2d grid = 3d array pointer

- single value per cell for a 3d grid = 3d array pointer

- multiple values per cell for a 3d grid = 4d array pointer

The caller will typically access the data by casting the void pointer to the corresponding array pointer and using nested loops in x,y,z between owned or ghost index bounds returned by the get_bounds_owned() or get_bounds_ghost() methods to index into the array. Example code snippets with this logic were listed above,

## 4.23.12. Final notes

Finally, here are some additional issues to pay attention to for writing any style command which uses distributed grids via the Grid2d or Grid3d class.

The command destructor should delete all instances of the Grid class, any buffers it allocated for forward/reverse or remap communication, and any data arrays it allocated to store grid cell data.

If a command is intended to work for either 2d or 3d simulations, then it should have logic to instantiate either 2d or 3d grids and their associated data arrays, depending on the dimension of the simulation box. The fix ave/grid command is an example of such a command.

When a command maps its particles to the grid and updates grid cell values, it should check that it is not updating or accessing a grid cell value outside the range of its owned+ghost cells, and generate an error message if that is the case. This could happen, for example, if a particle has moved further than half the neighbor skin distance, because the neighbor list update criterion are not adequate to prevent it from happening. See the src/KSPACE/pppm.cpp file and its particle_map() method for an example of this kind of error check.