Timing three loops
Place timespec_get calls before, between, and after three C loops. The resulting timestamps delimit each loop’s elapsed time; repeated runs provide samples rather than a single timing. section 15.3
Listing 15.6. Instrumenting three for loops with struct timespec
53 timespec_get(&t[0], TIME_UTC);
54 /* Volatile for i ensures that the loop is effected */
55 for (uint64_t volatile i = 0; i < iterations; ++i) {
56 /* do nothing */
57 }
58 timespec_get(&t[1], TIME_UTC);
59 /* s must be volatile to ensure that the loop is effected */
60 for (uint64_t i = 0; i < iterations; ++i) {
61 s = i;
62 }
63 timespec_get(&t[2], TIME_UTC);
64 /* Opaque computation ensures that the loop is effected */
65 for (uint64_t i = 1; accu0 < upper; i += 2) {
66 accu0 += i;
67 }
68 timespec_get(&t[3], TIME_UTC);
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The listing shows the timestamp calls and the different loop bodies they surround. section 15.3
Instrumentation inserts measurement operations into an execution. A timing call costs time, but its effects can reach further: the compiler may save registers and lose optimization opportunities; a system call may affect scheduling or caches. section 15.3
TAKEAWAY: Instrumentation changes compile-time and runtime properties. Assess measurement bias before interpreting timings. Repeat runs and examine the mean and standard deviation; substantial variation limits what a sample can support. section 15.3
