ntpd: drop offset averaging code

function                                             old     new   delta
filter_datapoints                                    475     174    -301

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

diff --git a/networking/ntpd.c b/networking/ntpd.c
index 9c16b2b6b20d86792f35382bd2fa8c20cf3e20d3..2bd53f8846077c7dbd2ac6907793bb0dad57dc2c 100644 (file)
--- 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= 2048) averaging code uses offset values which are outdated by hours,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
                         */