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a4f1f9ac NC |
1 | /** |
2 | * lib/minmax.c: windowed min/max tracker | |
3 | * | |
4 | * Kathleen Nichols' algorithm for tracking the minimum (or maximum) | |
5 | * value of a data stream over some fixed time interval. (E.g., | |
6 | * the minimum RTT over the past five minutes.) It uses constant | |
7 | * space and constant time per update yet almost always delivers | |
8 | * the same minimum as an implementation that has to keep all the | |
9 | * data in the window. | |
10 | * | |
11 | * The algorithm keeps track of the best, 2nd best & 3rd best min | |
12 | * values, maintaining an invariant that the measurement time of | |
13 | * the n'th best >= n-1'th best. It also makes sure that the three | |
14 | * values are widely separated in the time window since that bounds | |
15 | * the worse case error when that data is monotonically increasing | |
16 | * over the window. | |
17 | * | |
18 | * Upon getting a new min, we can forget everything earlier because | |
19 | * it has no value - the new min is <= everything else in the window | |
20 | * by definition and it's the most recent. So we restart fresh on | |
21 | * every new min and overwrites 2nd & 3rd choices. The same property | |
22 | * holds for 2nd & 3rd best. | |
23 | */ | |
24 | #include <linux/module.h> | |
25 | #include <linux/win_minmax.h> | |
26 | ||
27 | /* As time advances, update the 1st, 2nd, and 3rd choices. */ | |
28 | static u32 minmax_subwin_update(struct minmax *m, u32 win, | |
29 | const struct minmax_sample *val) | |
30 | { | |
31 | u32 dt = val->t - m->s[0].t; | |
32 | ||
33 | if (unlikely(dt > win)) { | |
34 | /* | |
35 | * Passed entire window without a new val so make 2nd | |
36 | * choice the new val & 3rd choice the new 2nd choice. | |
37 | * we may have to iterate this since our 2nd choice | |
38 | * may also be outside the window (we checked on entry | |
39 | * that the third choice was in the window). | |
40 | */ | |
41 | m->s[0] = m->s[1]; | |
42 | m->s[1] = m->s[2]; | |
43 | m->s[2] = *val; | |
44 | if (unlikely(val->t - m->s[0].t > win)) { | |
45 | m->s[0] = m->s[1]; | |
46 | m->s[1] = m->s[2]; | |
47 | m->s[2] = *val; | |
48 | } | |
49 | } else if (unlikely(m->s[1].t == m->s[0].t) && dt > win/4) { | |
50 | /* | |
51 | * We've passed a quarter of the window without a new val | |
52 | * so take a 2nd choice from the 2nd quarter of the window. | |
53 | */ | |
54 | m->s[2] = m->s[1] = *val; | |
55 | } else if (unlikely(m->s[2].t == m->s[1].t) && dt > win/2) { | |
56 | /* | |
57 | * We've passed half the window without finding a new val | |
58 | * so take a 3rd choice from the last half of the window | |
59 | */ | |
60 | m->s[2] = *val; | |
61 | } | |
62 | return m->s[0].v; | |
63 | } | |
64 | ||
65 | /* Check if new measurement updates the 1st, 2nd or 3rd choice max. */ | |
66 | u32 minmax_running_max(struct minmax *m, u32 win, u32 t, u32 meas) | |
67 | { | |
68 | struct minmax_sample val = { .t = t, .v = meas }; | |
69 | ||
70 | if (unlikely(val.v >= m->s[0].v) || /* found new max? */ | |
71 | unlikely(val.t - m->s[2].t > win)) /* nothing left in window? */ | |
72 | return minmax_reset(m, t, meas); /* forget earlier samples */ | |
73 | ||
74 | if (unlikely(val.v >= m->s[1].v)) | |
75 | m->s[2] = m->s[1] = val; | |
76 | else if (unlikely(val.v >= m->s[2].v)) | |
77 | m->s[2] = val; | |
78 | ||
79 | return minmax_subwin_update(m, win, &val); | |
80 | } | |
81 | EXPORT_SYMBOL(minmax_running_max); | |
82 | ||
83 | /* Check if new measurement updates the 1st, 2nd or 3rd choice min. */ | |
84 | u32 minmax_running_min(struct minmax *m, u32 win, u32 t, u32 meas) | |
85 | { | |
86 | struct minmax_sample val = { .t = t, .v = meas }; | |
87 | ||
88 | if (unlikely(val.v <= m->s[0].v) || /* found new min? */ | |
89 | unlikely(val.t - m->s[2].t > win)) /* nothing left in window? */ | |
90 | return minmax_reset(m, t, meas); /* forget earlier samples */ | |
91 | ||
92 | if (unlikely(val.v <= m->s[1].v)) | |
93 | m->s[2] = m->s[1] = val; | |
94 | else if (unlikely(val.v <= m->s[2].v)) | |
95 | m->s[2] = val; | |
96 | ||
97 | return minmax_subwin_update(m, win, &val); | |
98 | } |