ntpd: drop offset averaging code

function                                             old     new   delta
filter_datapoints                                    475     174    -301

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

1 files changed, 60 insertions, 23 deletions

diff --git a/networking/ntpd.c b/networking/ntpd.c
index 9c16b2b6b..2bd53f884 100644
--- a/networking/ntpd.c
+++ b/networking/ntpd.c
@@ -212,8 +212,8 @@ typedef struct {
 } msg_t;
 
 typedef struct {
-        double d_recv_time;
         double d_offset;
+        double d_recv_time;
         double d_dispersion;
 } datapoint_t;
 
@@ -517,23 +517,34 @@ static void
 filter_datapoints(peer_t *p)
 {
         int i, idx;
+        double sum, wavg;
+        datapoint_t *fdp;
+
+#if 0
+/* Simulations have shown that use of *averaged* offset for p->filter_offset
+ * is in fact worse than simply using last received one: with large poll intervals
+ * (>= 2048) averaging code uses offset values which are outdated by hours,
+ * and time/frequency correction goes totally wrong when fed essentially bogus offsets.
+ */
         int got_newest;
-        double minoff, maxoff, wavg, sum, w;
+        double minoff, maxoff, w;
         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;
+        fdp = p->filter_datapoint;
+
+        minoff = maxoff = fdp[0].d_offset;
         for (i = 1; i < NUM_DATAPOINTS; i++) {
-                if (minoff > p->filter_datapoint[i].d_offset)
-                        minoff = p->filter_datapoint[i].d_offset;
-                if (maxoff < p->filter_datapoint[i].d_offset)
-                        maxoff = p->filter_datapoint[i].d_offset;
+                if (minoff > fdp[i].d_offset)
+                        minoff = fdp[i].d_offset;
+                if (maxoff < fdp[i].d_offset)
+                        maxoff = fdp[i].d_offset;
         }
 
-        idx = p->datapoint_idx; /* most recent datapoint */
+        idx = p->datapoint_idx; /* most recent datapoint's index */
         /* Average offset:
          * Drop two outliers and take weighted average of the rest:
          * most_recent/2 + older1/4 + older2/8 ... + older5/32 + older6/32
@@ -555,24 +566,24 @@ filter_datapoints(peer_t *p)
                 VERB4 {
                         bb_error_msg("datapoint[%d]: off:%f disp:%f(%f) age:%f%s",
                                 i,
-                                p->filter_datapoint[idx].d_offset,
-                                p->filter_datapoint[idx].d_dispersion, dispersion(&p->filter_datapoint[idx]),
-                                G.cur_time - p->filter_datapoint[idx].d_recv_time,
-                                (minoff == p->filter_datapoint[idx].d_offset || maxoff == p->filter_datapoint[idx].d_offset)
+                                fdp[idx].d_offset,
+                                fdp[idx].d_dispersion, dispersion(&fdp[idx]),
+                                G.cur_time - fdp[idx].d_recv_time,
+                                (minoff == fdp[idx].d_offset || maxoff == fdp[idx].d_offset)
                                         ? " (outlier by offset)" : ""
                         );
                 }
 
-                sum += dispersion(&p->filter_datapoint[idx]) / (2 << i);
+                sum += dispersion(&fdp[idx]) / (2 << i);
 
-                if (minoff == p->filter_datapoint[idx].d_offset) {
+                if (minoff == fdp[idx].d_offset) {
                         minoff -= 1; /* so that we don't match it ever again */
                 } else
-                if (maxoff == p->filter_datapoint[idx].d_offset) {
+                if (maxoff == fdp[idx].d_offset) {
                         maxoff += 1;
                 } else {
-                        oldest_off = p->filter_datapoint[idx].d_offset;
-                        oldest_age = G.cur_time - p->filter_datapoint[idx].d_recv_time;
+                        oldest_off = fdp[idx].d_offset;
+                        oldest_age = G.cur_time - fdp[idx].d_recv_time;
                         if (!got_newest) {
                                 got_newest = 1;
                                 newest_off = oldest_off;
@@ -605,6 +616,32 @@ filter_datapoints(peer_t *p)
         }
         p->filter_offset = wavg;
 
+#else
+
+        fdp = p->filter_datapoint;
+        idx = p->datapoint_idx; /* most recent datapoint's index */
+
+        /* filter_offset: simply use the most recent value */
+        p->filter_offset = fdp[idx].d_offset;
+
+        /*                     n-1
+         *                     ---    dispersion(i)
+         * filter_dispersion =  \     -------------
+         *                      /       (i+1)
+         *                     ---     2
+         *                     i=0
+         */
+        wavg = 0;
+        sum = 0;
+        for (i = 0; i < NUM_DATAPOINTS; i++) {
+                sum += dispersion(&fdp[idx]) / (2 << i);
+                wavg += fdp[idx].d_offset;
+                idx = (idx - 1) & (NUM_DATAPOINTS - 1);
+        }
+        wavg /= NUM_DATAPOINTS;
+        p->filter_dispersion = sum;
+#endif
+
         /*                  +-----                 -----+ ^ 1/2
          *                  |       n-1                 |
          *                  |       ---                 |
@@ -618,13 +655,13 @@ filter_datapoints(peer_t *p)
          */
         sum = 0;
         for (i = 0; i < NUM_DATAPOINTS; i++) {
-                sum += SQUARE(wavg - p->filter_datapoint[i].d_offset);
+                sum += SQUARE(wavg - fdp[i].d_offset);
         }
         sum = SQRT(sum / NUM_DATAPOINTS);
         p->filter_jitter = sum > G_precision_sec ? sum : G_precision_sec;
 
-        VERB3 bb_error_msg("filter offset:%+f(corr:%e) disp:%f jitter:%f",
-                        p->filter_offset, x,
+        VERB3 bb_error_msg("filter offset:%+f disp:%f jitter:%f",
+                        p->filter_offset,
                         p->filter_dispersion,
                         p->filter_jitter);
 }
@@ -1667,15 +1704,15 @@ recv_and_process_peer_pkt(peer_t *p)
 
         p->reachable_bits |= 1;
         if ((MAX_VERBOSE && G.verbose) || (option_mask32 & OPT_w)) {
-                bb_error_msg("reply from:%s reach:0x%02x offset:%+f delay:%f status:0x%02x strat:%d refid:0x%08x rootdelay:%f",
+                bb_error_msg("reply from:%s offset:%+f delay:%f status:0x%02x strat:%d refid:0x%08x rootdelay:%f reach:0x%02x",
                         p->p_dotted,
-                        p->reachable_bits,
                         datapoint->d_offset,
                         p->lastpkt_delay,
                         p->lastpkt_status,
                         p->lastpkt_stratum,
                         p->lastpkt_refid,
-                        p->lastpkt_rootdelay
+                        p->lastpkt_rootdelay,
+                        p->reachable_bits
                         /* not shown: m_ppoll, m_precision_exp, m_rootdisp,
                          * m_reftime, m_orgtime, m_rectime, m_xmttime
                          */