NAME

       MPIL_Trace_on -  LAM/MPI-specific function to enable run-time tracing

SYNOPSIS

       #include <mpi.h>
       int MPIL_Trace_on(void)

NOTES

       These  functions  give the application the flexibility to generate traces only during certain interesting
       phases of the application's execution.  This technique can considerably reduce  the  size  of  the  trace
       files and burden of displaying them.

       Both  functions  are collective over the MPI_COMM_WORLD communicator.  In typical usage, the -toff option
       of mpirun (1) would be used to enable tracing, but start with the runtime switch in the off position.  At
       the   beginning   of   an  interesting  phase  of  program  execution,  MPIL_Trace_on  would  be  called.
       MPIL_Trace_off would be called after the interesting phase.  Tracing can be turned on and off many times.
       Each  period of tracing eventually forms a trace segment in the trace file extracted by lamtrace (1).  If
       the on/off functions are never used and tracing is enabled with the -ton option of mpirun (1),  a  single
       trace segment is produced.

       The on/off functions have no effect if tracing is not enabled by mpirun (1) with either the -ton or -toff
       switches.  Thus, an application can be littered with these functions but run without trace collection and
       very little additional overhead due to the no-operation function calls.

       This  is  a LAM/MPI-specific function and is intended mainly for debugging.  If this function is used, it
       should be used in conjunction with the LAM_MPI C preprocessor macro

       #if LAM_MPI
       MPIL_Trace_on();
       #endif

LIMITATIONS

       After the volume of generated traces exceeds a preset limit, the oldest traces are discarded in favour of
       new traces.  Avoiding discarded traces is further incentive to use MPIL_Trace_on and MPIL_Trace_off .

NOTES FOR FORTRAN

       All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK ) have an additional argument ierr at the
       end of the argument list.  ierr is an integer and has the same meaning as the return value of the routine
       in C.  In Fortran, MPI routines are subroutines, and are invoked with the call statement.

       All MPI objects (e.g., MPI_Datatype , MPI_Comm ) are of type INTEGER in Fortran.

ERRORS

       If an error occurs in an MPI function, the current MPI error handler is called to handle it.  By default,
       this error handler aborts the MPI job.  The error handler may be changed with  MPI_Errhandler_set  ;  the
       predefined  error  handler  MPI_ERRORS_RETURN  may be used to cause error values to be returned (in C and
       Fortran; this error handler is less useful in with the C++ MPI bindings.  The  predefined  error  handler
       MPI::ERRORS_THROW_EXCEPTIONS  should be used in C++ if the error value needs to be recovered).  Note that
       MPI does not guarantee that an MPI program can continue past an error.

       All MPI routines (except MPI_Wtime and MPI_Wtick ) return an error value; C routines as the value of  the
       function and Fortran routines in the last argument.  The C++ bindings for MPI do not return error values;
       instead, error values are communicated  by  throwing  exceptions  of  type  MPI::Exception  (but  not  by
       default).  Exceptions are only thrown if the error value is not MPI::SUCCESS .

       Note that if the MPI::ERRORS_RETURN handler is set in C++, while MPI functions will return upon an error,
       there will be no way to recover what the actual error value was.
       MPI_SUCCESS
              - No error; MPI routine completed successfully.
       MPI_ERR_OTHER
              - Other error; use MPI_Error_string to get more information about this error code.

SEE ALSO

       MPI_Trace_off, mpirun

LOCATION

       mpil_trace.c