[rrd-users] Percentile off-by-one

[Joshua Keroes]

How is percentile calculated? We've been calculating this manually, since
before rrdtool offered a PERCENT function. We discovered that our manual
calculation and the one supplied by rrdtool differ by one index. For example
(percentile.pl program attached):

[Thu Aug 13, 19:58:41 | 692] $ percentile.pl -v --cf=AVERAGE --ds=ingress
--dsi=0 --start=1242864000 --end=1245542399 etkfgl624456ubielg01g.rrd

percentile.pl using RRDs-1.2019
etkfgl624456ubielg01g.rrd:
        main::ptile_fetch()
           start = 05/21/09
           end = 06/20/09
           rows = 31
           95% of 31 = 29.45
           95th %-ile row index = 29
           discarded rows of -NaN's = 0
        row 28 = *1126767.40*
        row 29 = *1521699.32*
        row 30 = 4132277.36
   manual  calculation using fetch() = *1521699.32*
   PERCENT calculation using graph() = *1126767.40*
   difference ~ 394931 (26%)


Our manual calculation is grabbing row 29 and PERCENT is grabbing row 28.

>From rrdgraph.c:

case VDEF_PERCENT:{
    rrd_value_t *array;
    int       field;

    if ((array = malloc(steps * sizeof(double))) == NULL) {
        rrd_set_error("malloc VDEV_PERCENT");
        return -1;
    }

    for (step = 0; step < steps; step++) {
        array[step] = data[step * src->ds_cnt];
    }
    qsort(array, step, sizeof(double), vdef_percent_compar);

*    field = (steps - 1) * dst->vf.param / 100; /* <======= array index */*
    dst->vf.val = array[field];
    dst->vf.when = 0;   /* no time component */
    free(array);
}

Should the noted line perhaps read as follows?

  field = steps * dst->vf.param / 100;

If not, what am I missing?

Many thanks,
Joshua
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