lammps/doc/processors.txt

"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c

:link(lws,http://lammps.sandia.gov)
:link(ld,Manual.html)
:link(lc,Section_commands.html#comm)

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processors command :h3

[Syntax:]

processors Px Py Pz keyword args ... :pre

Px,Py,Pz = # of processors in each dimension of a 3d grid :ulb,l
zero or more keyword/arg pairs may be appended :l
keyword = {grid} or {numa} or {part} :l
  {grid} arg = {cart} or {cart/reorder} or {xyz} or {xzy} or {yxz} or {yzx} or {zxy} or {zyx}
     cart = use MPI_Cart() methods to layout 3d grid of procs with reorder = 0
     cart/reorder = use MPI_Cart() methods to layout 3d grid of procs with reorder = 1
     xyz,xzy,yxz,yzx,zxy,zyx = layout 3d grid of procs in IJK order, where I varies fastest, then J, and K slowest
  {numa} arg = none
  {part} args = Psend Precv cstyle
    Psend = partition # (1 to Np) which will send its processor layout
    Precv = partition # (1 to Np) which will recv the processor layout
    cstyle = {multiple}
      {multiple} = Psend layout will be multiple of Precv layout in each dimension :pre
:ule

[Examples:]

processors 2 4 4
processors * * 5
processors * * * grid xyz
processors * * * numa
processors * * * part 1 2 multiple :pre

[Description:]

Specify how processors are mapped as a 3d logical grid to the global
simulation box.  This involves 2 steps.  First if there are P
processors it means choosing a factorization P = Px by Py by Pz so
that there are Px processors in the x dimension, and similarly for the
y and z dimensions.  Second, the P processors (with MPI ranks 0 to
P-1) are mapped to the logical grid so that each grid cell is a
processor.  The arguments to this command control each of these 2
steps.

The Px, Py, Pz parameters affect the factorization.  Any of the 3
parameters can be specified with an asterisk "*", which means LAMMPS
will choose the number of processors in that dimension.  It will do
this based on the size and shape of the global simulation box so as to
minimize the surface-to-volume ratio of each processor's sub-domain.

Since LAMMPS does not load-balance by changing the grid of 3d
processors on-the-fly, this choosing explicit values for Px or Py or
Pz can be used to override the LAMMPS default if it is known to be
sub-optimal for a particular problem.  For example, a problem where
the extent of atoms will change dramatically in a particular dimension
over the course of the simulation.

The product of Px, Py, Pz must equal P, the total # of processors
LAMMPS is running on.  For a "2d simulation"_dimension.html, Pz must
equal 1.  If multiple partitions are being used then P is the number
of processors in this partition; see "this
section"_Section_start.html#start_6 for an explanation of the
-partition command-line switch.

Note that if you run on a large, prime number of processors P, then a
grid such as 1 x P x 1 will be required, which may incur extra
communication costs due to the high surface area of each processor's
sub-domain.

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The {grid} keyword affects how processor IDs are mapped to the 3d grid
of processors.

The {cart} style uses the family of MPI Cartesian functions to do
this, namely MPI_Cart_create(), MPI_Cart_get(), MPI_Cart_shift(), and
MPI_Cart_rank().  It invokes the MPI_Cart_create() function with its
reorder flag = 0, so that MPI is not free to reorder the processors.

The {cart/reorder} style does the same thing as the {cart} style
except it sets the reorder flag to 1, so that MPI is free to reorder
processors if it desires.

The {xyz}, {xzy}, {yxz}, {yzx}, {zxy}, and {zyx} styles are all
similar.  If the style is IJK, then it explicitly maps the P
processors to the grid so that the processor ID in the I direction
varies fastest, the processor ID in the J direction varies next
fastest, and the processor ID in the K direction varies slowest.  For
example, if you select style {xyz} and you have a 2x2x2 grid of 8
processors, the assignments of the 8 octants of the simulation domain
will be:

proc 0 = lo x, lo y, lo z octant
proc 1 = hi x, lo y, lo z octant
proc 2 = lo x, hi y, lo z octant
proc 3 = hi x, hi y, lo z octant
proc 4 = lo x, lo y, hi z octant
proc 5 = hi x, lo y, hi z octant
proc 6 = lo x, hi y, hi z octant
proc 7 = hi x, hi y, hi z octant :pre

Note that, in principle, an MPI implementation on a particular machine
should be aware of both the machine's network topology and the
specific subset of processors and nodes that were assigned to your
simulation.  Thus its MPI_Cart calls can optimize the assignment of
MPI processes to the 3d grid to minimize communication costs.  However
in practice, few if any MPI implementations actually do this.  So it
is likely that the {cart} and {cart/reorder} styles simply give the
same result as one of the IJK styles.

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The {numa} keyword affects both the factorization of P into Px,Py,Pz
and the mapping of processors to the 3d grid.

It will perform a two-level factorization of the simulation box to
minimize inter-node communication.  This can improve parallel
efficiency by reducing network traffic.  When this keyword is set, the
simulation box is first divided across nodes.  Then within each node,
the subdomain is further divided between the cores of each node.

The numa setting will be ignored if (a) there are less than 4 cores
per node, or (b) the number of MPI processes is not divisible by the
number of cores used per node, or (c) only 1 node is allocated, or (d)
any of the Px or Py of Pz values is greater than 1.

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The {part} keyword can be useful when running in multi-partition mode,
e.g. with the "run_style verlet/split"_run_style.html command.  It
specifies a dependency bewteen a sending partition {Psend} and a
receiving partition {Precv} which is enforced when each is setting up
their own mapping of the partitions processors to the simulation box.
Each of {Psend} and {Precv} must be integers from 1 to Np, where Np is
the number of partitions you have defined via the "-partition
command-line switch"__Section_start.html#start_6.

A "dependency" means that the sending partition will create its 3d
logical grid as Px by Py by Pz and after it has done this, it will
send the Px,Py,Pz values to the receiving partition.  The receiving
partition will wait to receive these values before creating its own 3d
logical grid and will use the sender's Px,Py,Pz values as a
constraint.  The nature of the constraint is determined by the
{cstyle} argument.

For a {cstyle} of {multiple}, each dimension of the sender's processor
grid is required to be an integer multiple of the corresponding
dimension in the receiver's processor grid.  This is a requirement of
the "run_style verlet/split"_run_style.html command.

For example, assume the sending partition creates a 4x6x10 grid = 240
processor grid.  If the receiving partition is running on 80
processors, it could create a 4x2x10 grid, but it will not create a
2x4x10 grid, since in the y-dimension, 6 is not an integer multiple of
4.

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Note that you can use the "partition"_partition.html command to
specify different processor grids for different partitions, e.g.

partition yes 1 processors 4 4 4
partition yes 2 processors 2 3 2 :pre

IMPORTANT NOTE: If you use the "partition"_partition.html command to
invoke different "processsors" commands on different partitions, and
you also use the {part} keyword, then you must insure that both the
sending and receiving partitions invoke the "processors" command that
connects the 2 partitions via the {part} keyword.  LAMMPS cannot
easily check for this, but your simulation will likely hang in its
setup phase if this error has been made.

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[Restrictions:]

This command cannot be used after the simulation box is defined by a
"read_data"_read_data.html or "create_box"_create_box.html command.
It can be used before a restart file is read to change the 3d
processor grid from what is specified in the restart file.

[Related commands:] none

[Default:]

The option defaults are Px Py Pz = * * *, grid = cart, numa = 0.