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user guide: add time measurement sections

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Rodrigo Arias 2021-02-05 19:20:48 +01:00
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@ -693,6 +693,75 @@ another experiment (the results of the initialization). The
initialization results will be cached if the derivation is kept
invariant, when modifying the computation phase parameters.
.\" ===================================================================
.NH 2
Measurement of the execution time
.LP
The programs must measure the wall time of the computation phase following a
set of rules. The way in which the wall time is measured is very important to
get accurate results. The measured time must be implemented by using a
monotonic clock which is able to correct the drift of the oscillator of
the internal clock due to changes in temperature. This clock must be
measured in C and C++ with:
.CS
clock_gettime(CLOCK_MONOTONIC, &ts);
.CE
A helper function can be used the approximate value of the clock in a
double precision float, in seconds:
.CS
double get_time()
{
struct timespec tv;
if(clock_gettime(CLOCK_MONOTONIC, &tv) != 0)
{
perror("clock_gettime failed");
exit(EXIT_FAILURE);
}
return (double)(ts.tv_sec) +
(double)ts.tv_nsec * 1.0e-9;
}
.CE
The start and end points must be measured after the synchronization of
all the processes and threads, so the complete computation work can be
bounded to fit inside the measured interval. An example for a MPI
program:
.CS
double start, end, delta_time;
MPI_Barrier();
start = get_time();
run_simulation();
MPI_Barrier();
end = get_time();
delta_time = end - start;
.CE
.\" ===================================================================
.NH 2
Format of the execution time
.LP
The measured execution time must be printed to the standard output
(stdout) in scientific notation with at least 7 significative digits.
The following the printf format (or the strict equivalent in other languages)
must be used:
.CS
printf("time %e\\n", delta_time);
.CE
The line must be printed alone and only once: for MPI programs,
only one process shall print the time:
.CS
if(rank == 0) printf("time %e\\n", delta_time);
.CE
Other lines can be printed in the stdout, but without the
.I time
prefix, so that the following pipe can be used to capture the line:
.CS
% ./app | grep "^time"
1.234567e-01
.CE
Ensure that your program follows this convention by testing it with the
above
.I grep
filter; otherwise the results will fail to be parsed when building
the dataset with the execution time.
.\" ===================================================================
.bp
.NH 1
Experimentation