2006-09-22 00:22:34 +08:00
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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
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<HR>
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<H3>processors command
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</H3>
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<P><B>Syntax:</B>
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</P>
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2011-12-08 07:43:51 +08:00
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<PRE>processors Px Py Pz keyword args ...
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2006-09-22 00:22:34 +08:00
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</PRE>
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<UL><LI>Px,Py,Pz = # of processors in each dimension of a 3d grid
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<LI>zero or more keyword/arg pairs may be appended
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<LI>keyword = <I>part</I>
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<PRE> <I>part</I> args = Psend Precv cstyle
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Psend = partition # (1 to Np) which will send its processor layout
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Precv = partition # (1 to Np) which will recv the processor layout
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cstyle = <I>multiple</I>
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<I>multiple</I> = Psend layout will be multiple of Precv layout in each dimension
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</PRE>
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2006-09-22 00:22:34 +08:00
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</UL>
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<P><B>Examples:</B>
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</P>
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2010-05-21 00:03:29 +08:00
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<PRE>processors 2 4 4
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processors * * 5
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processors * * * part 1 2 multiple
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Specify how processors are mapped as a 3d logical grid to the global
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simulation box, namely Px by Py by Pz.
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</P>
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<P>Any of the Px, Py, Pz parameters can be specified with an asterisk
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"*", which means LAMMPS will choose the number of processors in that
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dimension. It will do this based on the size and shape of the global
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simulation box so as to minimize the surface-to-volume ratio of each
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processor's sub-domain.
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2006-09-22 00:22:34 +08:00
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</P>
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<P>Since LAMMPS does not load-balance by changing the grid of 3d
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processors on-the-fly, this command can be used to override the LAMMPS
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default if it is known to be sub-optimal for a particular problem.
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For example, a problem where the atom's extent will change
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dramatically in a particular dimension over the course of the
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simulation.
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2006-09-22 00:22:34 +08:00
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</P>
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<P>The product of Px, Py, Pz must equal P, the total # of processors
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LAMMPS is running on. For a <A HREF = "dimension.html">2d simulation</A>, Pz must
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equal 1. If multiple partitions are being used then P is the number
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2011-08-26 00:46:23 +08:00
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of processors in this partition; see <A HREF = "Section_start.html#start_6">this
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section</A> for an explanation of the
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-partition command-line switch.
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</P>
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<P>Note that if you run on a large, prime number of processors P, then a
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grid such as 1 x P x 1 will be required, which may incur extra
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communication costs.
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2006-09-22 00:22:34 +08:00
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</P>
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2011-12-08 07:43:51 +08:00
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<HR>
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<P>The <I>part</I> keyword can be useful when running in multi-partition mode,
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e.g. with the <A HREF = "run_style.html<A HREF = "Section_start.html#start_6">-partition">>run_style verlet/split</A> command. It
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specifies a dependency bewteen a sending partition <I>Psend</I> and a
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receiving partition <I>Precv</I> which is enforced when each is setting up
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their own mapping of the partitions processors to the simulation box.
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Each of <I>Psend</I> and <I>Precv</I> must be integers from 1 to Np, where Np is
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the number of partitions you have defined via the <A HREF = </A>
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command-line switch</A>.
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</P>
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<P>A "dependency" means that the sending partition will create its 3d
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logical grid as Px by Py by Pz and after it has done this, it will
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send the Px,Py,Pz values to the receiving partition. The receiving
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partition will wait to receive these values before creating its own 3d
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logical grid and will use the sender's Px,Py,Pz values as a
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constraint. The nature of the constraint is determined by the
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<I>cstyle</I> argument.
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</P>
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<P>For a <I>cstyle</I> of <I>multiple</I>, each dimension of the sender's processor
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grid is required to be an integer multiple of the corresponding
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dimension in the receiver's processor grid. This is a requirement of
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the <A HREF = "run_style.html">run_style verlet/split</A> command.
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</P>
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<P>For example, assume the sending partition creates a 4x6x10 grid = 240
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processor grid. If the receiving partition is running on 80
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processors, it could create a 4x2x10 grid, but it will not create a
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2x4x10 grid, since in the y-dimension, 6 is not an integer multiple of
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4.
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</P>
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<HR>
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<P>Note that you can use the <A HREF = "partition.html">partition</A> command to
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specify different processor grids for different partitions, e.g.
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</P>
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<PRE>partition yes 1 processors 4 4 4
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partition yes 2 processors 2 3 2
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</PRE>
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<P>IMPORTANT NOTE: If you use the <A HREF = "partition.html">partition</A> command to
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invoke different "processsors" commands on different partitions, and
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you also use the <I>part</I> keyword, then you must insure that both the
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sending and receiving partitions invoke the "processors" command that
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connects the 2 partitions via the <I>part</I> keyword. LAMMPS cannot
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easily check for this, but your simulation will likely hang in its
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setup phase if this error has been made.
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</P>
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2011-12-08 08:00:59 +08:00
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<HR>
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2006-09-22 00:22:34 +08:00
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<P><B>Restrictions:</B>
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</P>
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<P>This command cannot be used after the simulation box is defined by a
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<A HREF = "read_data.html">read_data</A> or <A HREF = "create_box.html">create_box</A> command.
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It can be used before a restart file is read to change the 3d
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processor grid from what is specified in the restart file.
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</P>
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<P><B>Related commands:</B> none
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</P>
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<P><B>Default:</B>
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</P>
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2010-05-21 00:03:29 +08:00
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<P>Px Py Pz = * * *
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</P>
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</HTML>
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