ntpd: add a comment, no code changes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

diff --git a/networking/ntpd.c b/networking/ntpd.c
index d4754f8..02761b3 100644 (file)
--- a/networking/ntpd.c
+++ b/networking/ntpd.c
@@ -432,7 +432,7 @@ filter_datapoints(peer_t *p, double t)
                sum += dispersion(&p->filter_datapoint[idx], t) / (2 << i);
 
                if (minoff == p->filter_datapoint[idx].d_offset) {
-                       minoff -= 1;
+                       minoff -= 1; /* so that we don't match it ever again */
                } else
                if (maxoff == p->filter_datapoint[idx].d_offset) {
                        maxoff += 1;
@@ -447,6 +447,13 @@ filter_datapoints(peer_t *p, double t)
        wavg += x; /* add another older6/64 to form older6/32 */
        p->filter_offset = wavg;
        p->filter_dispersion = sum;
+//TODO: fix systematic underestimation with large poll intervals.
+// Imagine that we still have a bit of uncorrected drift,
+// and poll interval is big. Offsets form a progression:
+// 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7, 0.7 is most recent.
+// The algorithm above drops 0.0 and 0.7 as outliers,
+// and then we have this estimation, ~25% off from 0.7:
+// 0.1/32 + 0.2/32 + 0.3/16 + 0.4/8 + 0.5/4 + 0.6/2 = 0.503125
 
        //                       +-----            -----+ ^ 1/2
        //                       |  n-1                 |