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 consider
ably reduce the size of the trace files and burden of dis
playing them.
Both functions are collective over the MPI_COMM_WORLD com
municator. 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
the argument list. ierr is an integer and has the same
meaning as the return value of the routine in C. In For
tran, 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 val
ues to be returned (in C and Fortran; this error handler
is less useful in with the C++ MPI bindings. The prede
fined 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++ bind
ings 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
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