Timing a loop
Place timespec_get calls before and after a loop, then subtract the timestamps. The calls take time themselves. They may also change how the compiler optimizes the surrounding code and how the operating system schedules it. section 15.3
Measurement bias is distortion introduced by observing a program. A timing call can require register saves, suppress optimizations, trigger a system call, or affect caches. The measured execution may therefore differ from execution without instrumentation. section 15.3
Listing 15.1. Measuring several code snippets repeatedly
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);
69 /* A function call can usually not be optimized out. */
70 for (uint64_t i = 0; i < iterations; ++i) {
71 timespec_get(&tdummy, TIME_UTC);
72 accu1 += tdummy.tv_nsec;
73 }
74 timespec_get(&t[4], TIME_UTC);
75 /* A function call can usually not be optimized out, but
76 an inline function can. */
77 for (uint64_t i = 0; i < iterations; ++i) {
78 timespec_get(&tdummy, TIME_UTC);
79 stats_collect1(&sdummy[1], tdummy.tv_nsec);
80 }
81 timespec_get(&t[5], TIME_UTC);
82 for (uint64_t i = 0; i < iterations; ++i) {
83 timespec_get(&tdummy, TIME_UTC);
84 stats_collect2(&sdummy[2], tdummy.tv_nsec);
85 }
86 timespec_get(&t[6], TIME_UTC);
87 for (uint64_t i = 0; i < iterations; ++i) {
88 timespec_get(&tdummy, TIME_UTC);
89 stats_collect3(&sdummy[3], tdummy.tv_nsec);
90 }
91 timespec_get(&t[7], TIME_UTC);
!@%STYLE%@!
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!@%STYLE%@!
Compare timestamp calls around loops with the loop that calls timespec_get on every iteration: the measurement method is part of what runs. section 15.3
TAKEAWAY: Assess the timing method’s own impact before interpreting a result. Repeat experiments and report statistics such as the mean and standard deviation; repetition helps assess results but does not make instrumentation disappear. section 15.3
