Version:  2.0.40 2.2.26 2.4.37 3.13 3.14 3.15 3.16 3.17 3.18 3.19 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10

Linux/lib/win_minmax.c

  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 }
 99 

This page was automatically generated by LXR 0.3.1 (source).  •  Linux is a registered trademark of Linus Torvalds  •  Contact us