ntpd: improve frequency filtering

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

1 files changed, 43 insertions, 21 deletions

diff --git a/networking/ntpd.c b/networking/ntpd.c
index 5431b7499..5cad738c6 100644
--- a/networking/ntpd.c
+++ b/networking/ntpd.c
@@ -390,8 +390,13 @@ static void
 filter_datapoints(peer_t *p, double t)
 {
         int i, idx;
+        int got_newest;
         double minoff, maxoff, wavg, sum, w;
-        double x = x;
+        double x = x; /* for compiler */
+        double oldest_off = oldest_off;
+        double oldest_age = oldest_age;
+        double newest_off = newest_off;
+        double newest_age = newest_age;
 
         minoff = maxoff = p->filter_datapoint[0].d_offset;
         for (i = 1; i < NUM_DATAPOINTS; i++) {
@@ -416,6 +421,7 @@ filter_datapoints(peer_t *p, double t)
         //                      /       (i+1)
         //                     ---     2
         //                     i=0
+        got_newest = 0;
         sum = 0;
         for (i = 0; i < NUM_DATAPOINTS; i++) {
                 VERB4 {
@@ -437,23 +443,36 @@ filter_datapoints(peer_t *p, double t)
                 if (maxoff == p->filter_datapoint[idx].d_offset) {
                         maxoff += 1;
                 } else {
-                        x = p->filter_datapoint[idx].d_offset * w;
+                        oldest_off = p->filter_datapoint[idx].d_offset;
+                        oldest_age = t - p->filter_datapoint[idx].d_recv_time;
+                        if (!got_newest) {
+                                got_newest = 1;
+                                newest_off = oldest_off;
+                                newest_age = oldest_age;
+                        }
+                        x = oldest_off * w;
                         wavg += x;
                         w /= 2;
                 }
 
                 idx = (idx - 1) & (NUM_DATAPOINTS - 1);
         }
+        p->filter_dispersion = sum;
         wavg += x; /* add another older6/64 to form older6/32 */
+        /* Fix systematic underestimation with large poll intervals.
+         * Imagine that we still have a bit of uncorrected drift,
+         * and poll interval is big (say, 100 sec). 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
+         */
+        x = newest_age / (oldest_age - newest_age); /* in above example, 100 / (600 - 100) */
+        if (x < 1) {
+                x = (newest_off - oldest_off) * x; /* 0.5 * 100/500 = 0.1 */
+                wavg += x;
+        }
         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                 |
@@ -472,8 +491,10 @@ filter_datapoints(peer_t *p, double t)
         sum = SQRT(sum) / NUM_DATAPOINTS;
         p->filter_jitter = sum > G_precision_sec ? sum : G_precision_sec;
 
-        VERB3 bb_error_msg("filter offset:%f disp:%f jitter:%f",
-                        p->filter_offset, p->filter_dispersion, p->filter_jitter);
+        VERB3 bb_error_msg("filter offset:%f(corr:%e) disp:%f jitter:%f",
+                        p->filter_offset, x,
+                        p->filter_dispersion,
+                        p->filter_jitter);
 
 }
 
@@ -948,7 +969,7 @@ set_new_values(int disc_state, double offset, double recv_time)
          * of the last clock filter sample, which must be earlier than
          * the current time.
          */
-        VERB3 bb_error_msg("disc_state=%d last_update_offset=%f last_update_recv_time=%f",
+        VERB3 bb_error_msg("disc_state=%d last update offset=%f recv_time=%f",
                         disc_state, offset, recv_time);
         G.discipline_state = disc_state;
         G.last_update_offset = offset;
@@ -1227,26 +1248,27 @@ update_local_clock(peer_t *p, double t)
         tmx.constant = G.poll_exp - 4;
         //tmx.esterror = (u_int32)(clock_jitter * 1e6);
         //tmx.maxerror = (u_int32)((sys_rootdelay / 2 + sys_rootdisp) * 1e6);
-        VERB3 bb_error_msg("b adjtimex freq:%ld offset:%ld constant:%ld status:0x%x",
-                        tmx.freq, tmx.offset, tmx.constant, tmx.status);
         rc = adjtimex(&tmx);
         if (rc < 0)
                 bb_perror_msg_and_die("adjtimex");
-        if (G.kernel_freq_drift != tmx.freq / 65536) {
-                G.kernel_freq_drift = tmx.freq / 65536;
-                VERB2 bb_error_msg("kernel clock drift: %ld ppm", G.kernel_freq_drift);
-        }
-        VERB3 {
-                bb_error_msg("adjtimex:%d freq:%ld offset:%ld constant:%ld status:0x%x",
+        /* NB: here kernel returns constant == G.poll_exp, not == G.poll_exp - 4.
+         * Not sure why. Perhaps it is normal.
+         */
+        VERB3 bb_error_msg("adjtimex:%d freq:%ld offset:%ld constant:%ld status:0x%x",
                                 rc, tmx.freq, tmx.offset, tmx.constant, tmx.status);
 #if 0
+        VERB3 {
                 /* always gives the same output as above msg */
                 memset(&tmx, 0, sizeof(tmx));
                 if (adjtimex(&tmx) < 0)
                         bb_perror_msg_and_die("adjtimex");
                 VERB3 bb_error_msg("c adjtimex freq:%ld offset:%ld constant:%ld status:0x%x",
                                 tmx.freq, tmx.offset, tmx.constant, tmx.status);
+        }
 #endif
+        if (G.kernel_freq_drift != tmx.freq / 65536) {
+                G.kernel_freq_drift = tmx.freq / 65536;
+                VERB2 bb_error_msg("kernel clock drift: %ld ppm", G.kernel_freq_drift);
         }
 // #define STA_MODE 0x4000  /* mode (0 = PLL, 1 = FLL) (ro) */ - ?
 // it appeared after a while: