final tweaks

doc/src/Developer_comm_ops.rst

@@ -29,33 +29,36 @@ is updated (e.g. their positions) and this updated information needs to
 be **copied** to the corresponding ghost atoms.
 
 And second, *reverse communication* which sends ghost atom information
-from each processor to the owning processor to **accumulate** (sum) the
-values with the corresponding owned atoms.  The need for this arises
-when data is computed and also stored with ghost atoms (e.g. forces when
-using a "half" neighbor list) and thus those terms need to be added to
-their corresponding atoms on the process where they are "owned" atoms.
-Please note, that with the :doc:`newton off <newton>` setting this does
-not happen and the neighbor lists are constructed that these pairs are
-computed on both MPI processes containing one of the atoms and only the
-data pertaining to the local atom is stored.
+from each processor to the owning processor to **accumulate** (sum)
+the values with the corresponding owned atoms.  The need for this
+arises when data is computed and also stored with ghost atoms
+(e.g. forces when using a "half" neighbor list) and thus those terms
+need to be added to their corresponding atoms on the process where
+they are "owned" atoms.  Please note, that with the :doc:`newton off
+<newton>` setting this does not happen and the neighbor lists are
+constructed so that these interactions are computed on both MPI
+processes containing one of the atoms and only the data pertaining to
+the local atom is stored.
 
 The time-integration classes in LAMMPS invoke these operations each
 timestep via the *forward_comm()* and *reverse_comm()* methods in the
 *Comm* class.  Which per-atom data is communicated depends on the
 currently used :doc:`atom style <atom_style>` and whether
-:doc:`comm_modify vel <comm_modify>` setting is "no" (default) or "yes".
+:doc:`comm_modify vel <comm_modify>` setting is "no" (default) or
+"yes".
 
 Similarly, *Pair* style classes can invoke the *forward_comm(this)*
 and *reverse_comm(this)* methods in the *Comm* class to perform the
-same operations on per-atom data that is stored within the pair style
-class. Note that this function requires passing the ``this`` pointer
-as the first argument to be able to call the "pack" and "unpack" functions
-discussed below.  An example for the use of these functions are many-body pair
-styles like the embedded-atom method (EAM) which compute intermediate
-values in the first part of the compute() function that need to be
-stored on both, owned and ghost atoms for the second part of the force
-computing.  The *Comm* class methods perform the MPI communication for
-buffers of per-atom data.  They "call back" to the *Pair* class so it can *pack*
+same operations on per-atom data that is generated and stored within
+the pair style class. Note that this function requires passing the
+``this`` pointer as the first argument to enable the *Comm* class to
+call the "pack" and "unpack" functions discussed below.  An example of
+the use of these functions are many-body pair styles like the
+embedded-atom method (EAM) which compute intermediate values in the
+first part of the compute() function that need to be stored by both
+owned and ghost atoms for the second part of the force computation.
+The *Comm* class methods perform the MPI communication for buffers of
+per-atom data.  They "call back" to the *Pair* class so it can *pack*
 or *unpack* the buffer with data the *Pair* class owns.  There are 4
 such methods that the *Pair* class must define, assuming it uses both
 forward and reverse communication:
@@ -65,32 +68,37 @@ forward and reverse communication:
 * pack_reverse_comm()
 * unpack_reverse_comm()
 
-The arguments to these methods include the buffer and a list of atoms to
-pack or unpack.  The *Pair* class also must set the *comm_forward* and
-*comm_reverse* variables which store the number of values it will store
-in the communication buffers for each operation.  This means it can
-choose to store multiple per-atom values in the buffer, if desired, and
-they will be communicated together to minimize communication overhead.
-The communication buffers are defined as arrays containing ``double``
-values.  To correctly store integers that may be 64-bit (bigint,
-tagint, imageint) in the buffer, you need to use the `ubuf union
-<Communication buffer coding with ubuf>`_.
+The arguments to these methods include the buffer and a list of atoms
+to pack or unpack.  The *Pair* class also must set the *comm_forward*
+and *comm_reverse* variables which store the number of values stored
+in the communication buffers for each operation.  This means, if
+desired, it can choose to store multiple per-atom values in the
+buffer, and they will be communicated together to minimize
+communication overhead.  The communication buffers are defined vectors
+containing ``double`` values.  To correctly store integers that may be
+64-bit (bigint, tagint, imageint) in the buffer, you need to use the
+`ubuf union <Communication buffer coding with ubuf>`_ construct.
 
-The *Fix*, *Compute*, and *Dump* classes can also invoke the same kind of
-forward and reverse communication operations using *Comm* class methods.
-The same pack/unpack methods and comm_forward/comm_reverse variables
-must be defined by the *Fix* or *Compute* class.
+The *Fix*, *Compute*, and *Dump* classes can also invoke the same kind
+of forward and reverse communication operations using the same *Comm*
+class methods.  Likewise the same pack/unpack methods and
+comm_forward/comm_reverse variables must be defined by the calling
+*Fix*, *Compute*, or *Dump* class.
 
 For *Fix* classes there is an optional second argument to the
-*forward_comm()* and *reverse_comm()* call which are used when there are
-communications with buffer sizes different from the value set in the
-comm_forward/comm_reverse variables.  For this to work a class member
-variable must be set to indicate to the pack/unpack routines which
-which data needs to be packed into and unpacked from the buffer.
+*forward_comm()* and *reverse_comm()* call which can be used when the
+fix performs multiple modes of communication, with different numbers
+of values per atom.  The fix should set the *comm_forward* and
+*comm_reverse* variables to the maximum value, but can invoke the
+communication for a particulard mode with a smaller value.  For this
+to work, the *pack_forward_comm()*, etc methods typically use a class
+member variable to choose which values to pack/unpack into/from the
+buffer.
 
-Finally, for reverse communications in *Fix* classes there is also
-the *reverse_comm_variable()* method that allows the communication to have
-a different amount of data per-atom.  It invokes these corresponding callback methods:
+Finally, for reverse communications in *Fix* classes there is also the
+*reverse_comm_variable()* method that allows the communication to have
+a different amount of data per-atom.  It invokes these corresponding
+callback methods:
 
 * pack_reverse_comm_size()
 * unpack_reverse_comm_size()
@@ -133,8 +141,8 @@ processor sends to the first processor, and the first processor
 receives from the last processor.
 
 Invoking the *ring()* method passes each processor's buffer in *P*
-steps around the ring.  At each step a *callback* method (provided as
-an argument to ring()) in the caller is invoked.  This allows each
+steps around the ring.  At each step a *callback* method, provided as
+an argument to ring(), in the caller is invoked.  This allows each
 processor to examine the data buffer provided by every other
 processor.  It may extract values needed by its atoms from the
 buffers, or it may alter placeholder values in the buffer.  In the
@@ -145,18 +153,18 @@ Note that the *ring* operation is similar to an MPI_Alltoall()
 operation where every processor effectively sends and receives data to
 every other processor.  The difference is that the *ring* operation
 does it one step at a time, so the total volume of data does not need
-to be stored by every processor.  However, *ring* is also less
-efficient than MPI_Alltoall() because of the *P* stages required.  So
-it is typically only suitable for small data buffers and occasional
-operations that are not time-critical.
+to be stored by every processor.  However, the *ring* operation is
+also less efficient than MPI_Alltoall() because of the *P* stages
+required.  So it is typically only suitable for small data buffers and
+occasional operations that are not time-critical.
 
 Irregular operation
 ===================
 
-The *irregular* operation is provided by the *Irregular* class.
-Irregular communication is when each processor knows what data it
-needs to send to what processor, but does not know what processors are
-sending it data.  An example for LAMMPS is when load-balancing is
+The *irregular* operation is provided by the *Irregular* class.  What
+LAMMPS terms irregular communication is when each processor knows what
+data it needs to send to what processor, but does not know what
+processors are sending it data.  An example is when load-balancing is
 performed and each processor needs to send some of its atoms to new
 processors.
 
@@ -194,7 +202,7 @@ communicated to a new owning processor.
 Rendezvous operation
 ====================
 
-Finally, the *rendezvous* operation is invoked vie the *rendezvous()*
+Finally, the *rendezvous* operation is invoked via the *rendezvous()*
 method in the *Comm* class.  Depending on how much communication is
 needed and how many processors a LAMMPS simulation is running on, it
 can be a much more efficient choice than the *ring()* method.  It uses
@@ -203,14 +211,15 @@ communication.  The rendezvous algorithm is described in detail in
 :ref:`(Plimpton) <Plimpton>`, including some LAMMPS use cases.
 
 For the *rendezvous()* method, each processor specifies a list of *N*
-datums to send, each to a specified processor.  Internally, this
-communication is performed as an irregular operation.  The received
-datums are returned to the caller via invocation of *callback*
-function, provided as an argument to rendezvous().  The caller can
-then process the received datums and (optionally) assemble a new list
-of datums to communicate to a new list of specific processors.  When
-the callback function exits, the *rendezvous()* method performs a
-second irregular communication on the new list of datums.
+datums to send and which processor to send each of them to.
+Internally, this communication is performed as an irregular operation.
+The received datums are returned to the caller via invocation of
+*callback* function, provided as an argument to *rendezvous()*.  The
+caller can then process the received datums and (optionally) assemble
+a new list of datums to communicate to a new list of specific
+processors.  When the callback function exits, the *rendezvous()*
+method performs a second irregular communication on the new list of
+datums.
 
 Examples in LAMMPS of use of the *rendezvous* operation are the
 :doc:`fix rigid/small <fix_rigid>` and :doc:`fix shake