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