MPIL_Trace_on - LAM/MPI-specific function to enable run-time tracing
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
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
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
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.
- No error; MPI routine completed successfully.
- Other error; use MPI_Error_string to get more information
about this error code.