Add a new MachEnv class to hold various machine and messaging parameters

components/omega/doc/devGuide/MachEnv.md

@@ -0,0 +1,69 @@
+(omega-dev-mach-env)=
+
+## Machine Environment (MachEnv)
+
+The MachEnv class maintains a number of parameters associated with
+the parallel machine environment. These include message-passing parameters
+like MPI communicators and task information, the number of threads
+if threaded, vector length for CPUs, and GPU and node information as
+needed. Multiple environments can be defined to support running portions
+of the model on subsets of tasks. 
+
+On model initiation, the initialization routine `OMEGA::MachEnv::init()`
+is called to set up the default MachEnv, which can be retrieved at
+any time using:
+```c++
+OMEGA::MachEnv DefEnv = OMEGA::MachEnv::getDefaultEnv();
+```
+that returns a pointer to the default environment.
+Once an environment has been retrieved, the individual data members
+are retrieved using various get functions:
+```c++
+  MPI_Comm Comm   = DefEnv->getComm();
+  int MyTask      = DefEnv->getMyTask();
+  int NumTasks    = DefEnv->getNumTasks();
+  int MasterTask  = DefEnv->getMasterTask();
+```
+There is also a logical function `isMasterTask` that can be used
+for work that should only be done on the master task. By default,
+the master task is defined as task 0 in the environment, but if
+the master task is overloaded, there is a `setMasterTask` that can
+redefine any other task in the group as the master.
+
+If OMEGA has been built with OpenMP threading, a `getNumThreads`
+function is available; it returns 1 if threading is not on.
+The MachEnv also has a public parameter `OMEGA::VecLength` that can
+be used to tune the vector length for CPU architectures. For
+GPU builds, this VecLength is set to 1.
+
+As noted previously, additional environments can be defined for
+subsets of a parent environment. There are three interfaces for
+creating an environment:
+```c++
+  OMEGA::MachEnv MyNewEnv = 
+      OMEGA::MachEnv::createNewEnv(Name, ParentEnv, NewSize);
+  OMEGA::MachEnv MyNewEnv = 
+      OMEGA::MachEnv::createNewEnv(Name, ParentEnv, NewSize, Begin, Stride);
+  OMEGA::MachEnv MyNewEnv = 
+      OMEGA::MachEnv::createNewEnv(Name, ParentEnv, NewSize, Tasks);
+```
+The first creates a new environment from the first `NewSize` contiguous
+tasks in the parent machine environment. The second creates a new
+environment from a strided set of tasks starting at the `Begin` Task
+(eg all odd tasks would have Begin=1 and Stride=2). The final
+interface creates a subset containing the selected tasks stored in
+a vector `Tasks`. Each new environment is given a `Name` and can be
+retrieved by name using `OMEGA::MachEnv::getEnv(Name)`. Once retrieved,
+the other get functions listed above are used to get each data member.
+Because the new environments are subsets of the default environment, one
+additional function `OMEGA::MachEnv::isMember()` is provided so that
+non-member tasks can be excluded from calculations. The retrieval functions
+call from non-member tasks will return invalid values. Finally, there is
+a `removeEnv` function that can delete any defined environment.
+
+As a class that is basically a container for the environment parameters,
+the implementation is a simple class with several scalar data members and
+the retrieval/creation functions noted above. To track all defined
+environments, a c++ map container is used to pair a name with a pointer to
+a defined environment.  
+

components/omega/doc/index.md

@@ -23,6 +23,7 @@ Development is taking place at https://github.com/E3SM-Project/Omega.
 userGuide/QuickStart
 userGuide/OmegaBuild
 userGuide/DataTypes
+userGuide/MachEnv
 ```
 
 ```{toctree}
@@ -31,6 +32,7 @@ userGuide/DataTypes
 
 devGuide/QuickStart
 devGuide/DataTypes
+devGuide/MachEnv
 devGuide/CondaEnv
 devGuide/Docs
 devGuide/BuildDocs

components/omega/doc/userGuide/MachEnv.md

@@ -0,0 +1,24 @@
+(omega-user-mach-env)=
+
+## Machine Environment (MachEnv)
+
+Within OMEGA, many aspects of the machine environment are stored
+in a class called MachEnv. These include message-passing parameters
+like MPI communicators and task information, number of threads
+if threaded, vector length for CPUs, GPU and node information as
+needed. Multiple environments are supported in case portions of the
+code need to run on subsets of tasks or in different contexts. A
+default environment is defined early in the initialization of the
+model and can be retrieved as described in the
+[Developer's Guide](#omega-dev-mach-env).
+
+The user is not expected to set any parameters in MachEnv. All
+quantities are derived from either the job launch command
+(eg mpirun or srun) that defines the number of MPI tasks and tasks
+layouts or from machine parameters enforced during the build based
+on supported machine xml configurations.
+The latter include the pre-processing parameters 
+`-DOMEGA_VECTOR_LENGTH=xx` and `-DOMEGA_THREADED` that define an
+optimal vector length for CPU code and turn on OpenMP threading
+if desired.
+