module mpp_domains_mod¶

Overview¶

mpp_domains_mod is a set of simple calls for domain decomposition and domain updates on rectilinear grids. It requires the module module mpp_mod, upon which it is built.

Scalable implementations of finite-difference codes are generally based on decomposing the model domain into subdomains that are distributed among processors. These domains will then be obliged to exchange data at their boundaries if data dependencies are merely nearest-neighbour, or may need to acquire information from the global domain if there are extended data dependencies, as in the spectral transform. The domain decomposition is a key operation in the development of parallel codes.

mpp_domains_mod provides a domain decomposition and domain update API for rectilinear grids, built on top of the module mpp_mod API for message passing. Features of mpp_domains_mod include:

Simple, minimal API, with free access to underlying API for more complicated stuff.

Design toward typical use in climate/weather CFD codes.

Domains¶

I have assumed that domain decomposition will mainly be in 2 horizontal dimensions, which will in general be the two fastest-varying indices. There is a separate implementation of 1D decomposition on the fastest-varying index, and 1D decomposition on the second index, treated as a special case of 2D decomposition, is also possible. We define domain as the grid associated with a task. We define the compute domain as the set of gridpoints that are computed by a task, and the data domain as the set of points that are required by the task for the calculation. There can in general be more than 1 task per PE, though often the number of domains is the same as the processor count. We define the global domain as the global computational domain of the entire model (i.e, the same as the computational domain if run on a single processor). 2D domains are defined using a derived type domain2D, constructed as follows (see comments in code for more details):

type, public :: domain_axis_spec
    private
    integer :: begin, end, size, max_size
    logical :: is_global
    end type domain_axis_spec
    type, public :: domain1D
    private
    type(domain_axis_spec) :: compute, data, global, active
    logical :: mustputb, mustgetb, mustputf, mustgetf, folded
    type(domain1D), pointer, dimension(:) :: list
    integer :: pe              !PE to which this domain is assigned
    integer :: pos
    end type domain1D
domaintypes of higher rank can be constructed from type domain1D
typically we only need 1 and 2D, but could need higher (e.g 3D LES)
some elements are repeated below if they are needed once per domain
    type, public :: domain2D
    private
    type(domain1D) :: x
    type(domain1D) :: y
    type(domain2D), pointer, dimension(:) :: list
    integer :: pe              !PE to which this domain is assigned
    integer :: pos
    end type domain2D
    type(domain1D), public :: NULL_DOMAIN1D
    type(domain2D), public :: NULL_DOMAIN2D

The domain2D type contains all the necessary information to define the global, compute and data domains of each task, as well as the PE associated with the task. The PEs from which remote data may be acquired to update the data domain are also contained in a linked list of neighbours.

Other modules used¶

mpp_mod

Public interface¶

use mpp_domains_mod [, only:  mpp_define_domains,
                              mpp_update_domains,
                              mpp_redistribute,
                              mpp_global_field,
                              mpp_global_max,
                              mpp_global_sum,
                              operator,
                              mpp_get_compute_domain,
                              mpp_get_compute_domains,
                              mpp_get_data_domain,
                              mpp_get_global_domain,
                              mpp_define_layout,
                              mpp_get_pelist,
                              mpp_get_layout,
                              mpp_domains_init,
                              mpp_domains_set_stack_size,
                              mpp_domains_exit,
                              mpp_get_domain_components ]

mpp_define_domains:: Set up a domain decomposition.
mpp_update_domains:: Halo updates.
mpp_redistribute:: Reorganization of distributed global arrays.
mpp_global_field:: Fill in a global array from domain-decomposed arrays.
mpp_global_max:: Global max/min of domain-decomposed arrays.
mpp_global_sum:: Global sum of domain-decomposed arrays.
operator:: Equality/inequality operators for domaintypes.
mpp_get_compute_domain:: These routines retrieve the axis specifications associated with the compute domains.
mpp_get_compute_domains:: Retrieve the entire array of compute domain extents associated with a decomposition.
mpp_get_data_domain:: These routines retrieve the axis specifications associated with the data domains.
mpp_get_global_domain:: These routines retrieve the axis specifications associated with the global domains.
mpp_define_layout:: Retrieve layout associated with a domain decomposition.
mpp_get_pelist:: Retrieve list of PEs associated with a domain decomposition.
mpp_get_layout:: Retrieve layout associated with a domain decomposition.
mpp_domains_init:: Initialize domain decomp package.
mpp_domains_set_stack_size:: Set user stack size.
mpp_domains_exit:: Exit mpp_domains_mod.
mpp_get_domain_components:: Retrieve 1D components of 2D decomposition.

Public data¶

None.

Public routines¶

Mpp_define_domains

call mpp_define_domains ( global_indices, ndivs, domain, & pelist, flags, halo, extent, maskmap )

call mpp_define_domains ( global_indices, layout, domain, pelist, & xflags, yflags, xhalo, yhalo, & xextent, yextent, maskmap, name )

DESCRIPTION

There are two forms for the mpp_define_domains call. The 2D version is generally to be used but is built by repeated calls to the 1D version, also provided.

INPUT

`global_indices`	Defines the global domain. [integer, dimension(2)] [integer, dimension(4)]
`ndivs`	Is the number of domain divisions required. [integer]
`pelist`	List of PEs to which the domains are to be assigned. [integer, dimension(0:)] [integer, dimension(0:)]
`flags`	An optional flag to pass additional information about the desired domain topology. Useful flags in a 1D decomposition include `GLOBAL_DATA_DOMAIN` and `CYCLIC_GLOBAL_DOMAIN`. Flags are integers: multiple flags may be added together. The flag values are public parameters available by use association. [integer]
`halo`	Width of the halo. [integer]
`extent`	Normally `mpp_define_domains` attempts an even division of the global domain across `ndivs` domains. The `extent` array can be used by the user to pass a custom domain division. The `extent` array has `ndivs` elements and holds the compute domain widths, which should add up to cover the global domain exactly. [integer, dimension(0:)]
`maskmap`	Some divisions may be masked (`maskmap=.FALSE.`) to exclude them from the computation (e.g for ocean model domains that are all land). The `maskmap` array is dimensioned `ndivs` and contains `.TRUE.` values for any domain that must be included in the computation (default all). The `pelist` array length should match the number of domains included in the computation. [logical, dimension(0:)] [logical, dimension(:,:)]
`layout`	[integer, dimension(2)]
`xflags, yflags`	[integer]
`xhalo, yhalo`	[integer]
`xextent, yextent`	[integer, dimension(0:)]
`name`	[character(len=*)]

INPUT/OUTPUT

domain

Holds the resulting domain decomposition. [type(domain1D)] [type(domain2D)]

NOTE

For example:

call mpp_define_domains( (/1,100/), 10, domain, &
     flags=GLOBAL_DATA_DOMAIN+CYCLIC_GLOBAL_DOMAIN, halo=2 )

defines 10 compute domains spanning the range [1,100] of the global domain. The compute domains are non-overlapping blocks of 10. All the data domains are global, and with a halo of 2 span the range [-1:102]. And since the global domain has been declared to be cyclic, domain(9)%next => domain(0) and domain(0)%prev => domain(9). A field is allocated on the data domain, and computations proceed on the compute domain. A call to ` <#mpp_update_domains>`__ would fill in the values in the halo region:

call mpp_get_data_domain( domain, isd, ied ) !returns -1 and 102
    call mpp_get_compute_domain( domain, is, ie ) !returns (1,10) on PE 0 ...
    allocate( a(isd:ied) )
    do i = is,ie
       a(i) = <perform computations>
    end do
    call mpp_update_domains( a, domain )

The call to mpp_update_domains fills in the regions outside the compute domain. Since the global domain is
cyclic, the values at i=(-1,0) are the same as at i=(99,100); and i=(101,102) are the same as
i=(1,2).
The 2D version is just an extension of this syntax to two dimensions.
The 2D version of the above should generally be used in codes, including 1D-decomposed ones, if there is a
possibility of future evolution toward 2D decomposition. The arguments are similar to the 1D case, except that
now we have optional arguments flags, halo, extent and maskmap along two axes.
flags can now take an additional possible value to fold one or more edges. This is done by using flags
FOLD_WEST_EDGE, FOLD_EAST_EDGE, FOLD_SOUTH_EDGE or FOLD_NORTH_EDGE. When a fold exists (e.g
cylindrical domain), vector fields reverse sign upon crossing the fold. This parity reversal is performed only
in the vector version of ` <#mpp_update_domains>`__. In addition, shift operations may need to be applied to
vector fields on staggered grids, also described in the vector interface to mpp_update_domains.
name is the name associated with the decomposition, e.g 'Ocean model'. If this argument is present,
mpp_define_domains will print the domain decomposition generated to stdlog.
Examples:

call mpp_define_domains( (/1,100,1,100/), (/2,2/), domain, xhalo=1 )

will create the following domain layout:

               |---------|-----------|-----------|-------------|
               |domain(1)|domain(2)  |domain(3)  |domain(4)    |
|--------------|---------|-----------|-----------|-------------|
|Compute domain|1,50,1,50|51,100,1,50|1,50,51,100|51,100,51,100|
|--------------|---------|-----------|-----------|-------------|
|Data domain   |0,51,1,50|50,101,1,50|0,51,51,100|50,101,51,100|
|--------------|---------|-----------|-----------|-------------|

Again, we allocate arrays on the data domain, perform computations on the compute domain, and call mpp_update_domains to update the halo region.

If we wished to perfom a 1D decomposition along Y on the same global domain, we could use:

call mpp_define_domains( (/1,100,1,100/), layout=(/4,1/), domain, xhalo=1 )

This will create the following domain layout:

               |----------|-----------|-----------|------------|
               |domain(1) |domain(2)  |domain(3)  |domain(4)   |
|--------------|----------|-----------|-----------|------------|
|Compute domain|1,100,1,25|1,100,26,50|1,100,51,75|1,100,76,100|
|--------------|----------|-----------|-----------|------------|
|Data domain   |0,101,1,25|0,101,26,50|0,101,51,75|1,101,76,100|
|--------------|----------|-----------|-----------|------------|

Mpp_update_domains
```
call mpp_update_domains ( field, domain, flags )
```
```
call mpp_update_domains ( fieldx, fieldy, domain, flags, gridtype )
```
DESCRIPTION
mpp_update_domains is used to perform a halo update of a domain-decomposed array on each PE. MPP_TYPE_ can be of type complex, integer, logical or real; of 4-byte or 8-byte kind; of rank up to 5. The vector version (with two input data fields) is only present for real types. For 2D domain updates, if there are halos present along both x and y, we can choose to update one only, by specifying flags=XUPDATE or flags=YUPDATE. In addition, one-sided updates can be performed by setting flags to any combination of WUPDATE, EUPDATE, SUPDATE and NUPDATE, to update the west, east, north and south halos respectively. Any combination of halos may be used by adding the requisite flags, e.g: flags=XUPDATE+SUPDATE or flags=EUPDATE+WUPDATE+SUPDATE will update the east, west and south halos. If a call to mpp_update_domains involves at least one E-W halo and one N-S halo, the corners involved will also be updated, i.e, in the example above, the SE and SW corners will be updated. If flags is not supplied, that is equivalent to flags=XUPDATE+YUPDATE. The vector version is passed the x and y components of a vector field in tandem, and both are updated upon return. They are passed together to treat parity issues on various grids. For example, on a cubic sphere projection, the x and y components may be interchanged when passing from an equatorial cube face to a polar face. For grids with folds, vector components change sign on crossing the fold. Special treatment at boundaries such as folds is also required for staggered grids. The following types of staggered grids are recognized: 1) AGRID: values are at grid centers. 2) BGRID_NE: vector fields are at the NE vertex of a grid cell, i.e: the array elements u(i,j) and v(i,j) are actually at (i+½,j+½) with respect to the grid centers. 3) BGRID_SW: vector fields are at the SW vertex of a grid cell, i.e: the array elements u(i,j) and v(i,j) are actually at (i-½,j-½) with respect to the grid centers. 4) CGRID_NE: vector fields are at the N and E faces of a grid cell, i.e: the array elements u(i,j) and v(i,j) are actually at (i+½,j) and (i,j+½) with respect to the grid centers. 5) CGRID_SW: vector fields are at the S and W faces of a grid cell, i.e: the array elements u(i,j) and v(i,j) are actually at (i-½,j) and (i,j-½) with respect to the grid centers. The gridtypes listed above are all available by use association as integer parameters. The scalar version of mpp_update_domains assumes that the values of a scalar field are always at AGRID locations, and no special boundary treatment is required. If vector fields are at staggered locations, the optional argument gridtype must be appropriately set for correct treatment at boundaries. It is safe to apply vector field updates to the appropriate arrays irrespective of the domain topology: if the topology requires no special treatment of vector fields, specifying gridtype will do no harm. mpp_update_domains internally buffers the date being sent and received into single messages for efficiency. A turnable internal buffer area in memory is provided for this purpose by mpp_domains_mod. The size of this buffer area can be set by the user by calling mpp_domains_set_stack_size in module mpp_domains_mod.
Mpp_redistribute
```
call mpp_redistribute ( domain_in, field_in, domain_out, field_out )
```
DESCRIPTION
mpp_redistribute is used to reorganize a distributed array. MPP_TYPE_ can be of type integer, complex, or real; of 4-byte or 8-byte kind; of rank up to 5.

INPUT

field_in

field_in is dimensioned on the data domain of domain_in.

OUTPUT

field_out

field_out on the data domain of domain_out.
Mpp_global_field
```
call mpp_global_field ( domain, local, global, flags )
```
DESCRIPTION
mpp_global_field is used to get an entire domain-decomposed array on each PE. MPP_TYPE_ can be of type complex, integer, logical or real; of 4-byte or 8-byte kind; of rank up to 5. All PEs in a domain decomposition must call mpp_global_field, and each will have a complete global field at the end. Please note that a global array of rank 3 or higher could occupy a lot of memory.

INPUT

domain

local

local is dimensioned on either the compute domain or the data domain of domain.

flags

flags can be given the value XONLY or YONLY, to specify a globalization on one axis only.

OUTPUT

global

global is dimensioned on the corresponding global domain.
Mpp_global_max
```
mpp_global_max ( domain, field, locus )
```
DESCRIPTION
mpp_global_max is used to get the maximum value of a domain-decomposed array on each PE. MPP_TYPE_ can be of type integer or real; of 4-byte or 8-byte kind; of rank up to 5. The dimension of locus must equal the rank of field. All PEs in a domain decomposition must call mpp_global_max, and each will have the result upon exit. The function mpp_global_min, with an identical syntax. is also available.

INPUT

domain

field

field is dimensioned on either the compute domain or the data domain of domain.

OUTPUT

locus

locus, if present, can be used to retrieve the location of the maximum (as in the MAXLOC intrinsic of f90).

Mpp_global_sum

call mpp_global_sum ( domain, field, flags )

DESCRIPTION

mpp_global_sum is used to get the sum of a domain-decomposed array on each PE. MPP_TYPE_ can be of type integer, complex, or real; of 4-byte or 8-byte kind; of rank up to 5.

INPUT

`domain`
`field`	`field` is dimensioned on either the compute domain or the data domain of `domain`.
`flags`	`flags`, if present, must have the value `BITWISE_EXACT_SUM`. This produces a sum that is guaranteed to produce the identical result irrespective of how the domain is decomposed. This method does the sum first along the ranks beyond 2, and then calls ` <#mpp_global_field>`__ to produce a global 2D array which is then summed. The default method, which is considerably faster, does a local sum followed by mpp_sum in module mpp_mod across the domain decomposition.

NOTE

All PEs in a domain decomposition must call mpp_global_sum, and each will have the result upon exit.

Operator
DESCRIPTION
The module provides public operators to check for equality/inequality of domaintypes, e.g:
type(domain1D) :: a, b type(domain2D) :: c, d ... if( a.NE.b )then ... end if if( c==d )then ... end if
Domains are considered equal if and only if the start and end indices of each of their component global, data and compute domains are equal.
Mpp_get_compute_domain
```
call mpp_get_compute_domain
```
DESCRIPTION
The domain is a derived type with private elements. These routines retrieve the axis specifications associated with the compute domains The 2D version of these is a simple extension of 1D.
Mpp_get_compute_domains
```
call mpp_get_compute_domains ( domain, xbegin, xend, xsize, & ybegin, yend, ysize )
```
DESCRIPTION
Retrieve the entire array of compute domain extents associated with a decomposition.

INPUT
domain

OUTPUT
xbegin,ybegin, xend,yend, xsize,ysize
Mpp_get_data_domain
```
call mpp_get_data_domain
```
DESCRIPTION
The domain is a derived type with private elements. These routines retrieve the axis specifications associated with the data domains. The 2D version of these is a simple extension of 1D.
Mpp_get_global_domain
```
call mpp_get_global_domain
```
DESCRIPTION
The domain is a derived type with private elements. These routines retrieve the axis specifications associated with the global domains. The 2D version of these is a simple extension of 1D.
Mpp_define_layout
```
call mpp_define_layout ( global_indices, ndivs, layout )
```
DESCRIPTION
Given a global 2D domain and the number of divisions in the decomposition (ndivs: usually the PE count unless some domains are masked) this calls returns a 2D domain layout. By default, mpp_define_layout will attempt to divide the 2D index space into domains that maintain the aspect ratio of the global domain. If this cannot be done, the algorithm favours domains that are longer in x than y, a preference that could improve vector performance.

INPUT

global_indices

[integer, dimension(4)]

ndivs

[integer]

OUTPUT

layout

[integer, dimension(2)]
Mpp_get_pelist

DESCRIPTION
The 1D version of this call returns an array of the PEs assigned to this 1D domain decomposition. In addition the optional argument pos may be used to retrieve the 0-based position of the domain local to the calling PE, i.e domain%list(pos)%pe is the local PE, as returned by mpp_pe in module mpp_mod. The 2D version of this call is identical to 1D version.

INPUT

domain

[type(domain1D)] [type(domain2D)]

OUTPUT

pelist

[integer, dimension(:)] [integer, dimension(:)]

pos

[integer] [integer]
Mpp_get_layout
```
call mpp_get_layout ( domain, layout )
```
DESCRIPTION
The 1D version of this call returns the number of divisions that was assigned to this decomposition axis. The 2D version of this call returns an array of dimension 2 holding the results on two axes.

INPUT

domain

[type(domain1D)] [type(domain2D)]

OUTPUT

layout

[integer] [integer, dimension(2)]
Mpp_domains_init
```
call mpp_domains_init (flags)
```
DESCRIPTION
Called to initialize the mpp_domains_mod package. flags can be set to MPP_VERBOSE to have mpp_domains_mod keep you informed of what it’s up to. MPP_DEBUG returns even more information for debugging. mpp_domains_init will call mpp_init, to make sure module mpp_mod is initialized. (Repeated calls to mpp_init do no harm, so don’t worry if you already called it).

INPUT

flags

[integer]
Mpp_domains_set_stack_size
```
call mpp_domains_set_stack_size (n)
```
DESCRIPTION
This sets the size of an array that is used for internal storage by mpp_domains. This array is used, for instance, to buffer the data sent and received in halo updates. This call has implied global synchronization. It should be placed somewhere where all PEs can call it.

INPUT

n

[integer]
Mpp_domains_exit
```
call mpp_domains_exit ()
```
DESCRIPTION
Serves no particular purpose, but is provided should you require to re-initialize mpp_domains_mod, for some odd reason.
Mpp_get_domain_components
```
call mpp_get_domain_components ( domain, x, y )
```
DESCRIPTION
It is sometime necessary to have direct recourse to the domain1D types that compose a domain2D object. This call retrieves them.

INPUT

domain

[type(domain2D)]

OUTPUT

x,y

[type(domain1D)]

Data sets¶

None.

Error messages¶

None.

References¶

None.

Compiler specifics¶

Any module or program unit using mpp_domains_mod must contain the line

use mpp_domains_mod

mpp_domains_mod uses module mpp_mod, and therefore is subject to the compiling and linking requirements of module mpp_mod.

Precompiler options¶

mpp_domains_mod uses standard f90, and has no special requirements. There are some OS-dependent pre-processor directives that you might need to modify on non-SGI/Cray systems and compilers. The portability, as described in module mpp_mod obviously is a constraint, since this module is built on top of it. Contact me, Balaji, SGI/GFDL, with questions.

Loader options¶

The source consists of the main source file and also requires the following include files: GFDL users can check it out of the main CVS repository as part of the CVS module. The current public tag is . External users can download the latest package . Public access to the GFDL CVS repository will soon be made available.

Test PROGRAM¶

None.

Notes¶

None.