ntpd: experimental code to correct frequency a bit more aggressively

function                                             old     new   delta
update_local_clock                                   730     792     +62
recv_and_process_peer_pkt                            850     835     -15

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

1 files changed, 29 insertions, 23 deletions

diff --git a/networking/ntpd.c b/networking/ntpd.c
index ba666b513..b0bfe440f 100644
--- a/networking/ntpd.c
+++ b/networking/ntpd.c
@@ -138,6 +138,7 @@
  * by staying at smaller poll).
  */
 #define POLLADJ_GATE    4
+#define TIMECONST_HACK_GATE 2
 /* Compromise Allan intercept (sec). doc uses 1500, std ntpd uses 512 */
 #define ALLAN           512
 /* PLL loop gain */
@@ -339,6 +340,7 @@ struct globals {
         double   last_update_offset;    // c.last
         double   last_update_recv_time; // s.t
         double   discipline_jitter;     // c.jitter
+        double   offset_to_jitter_ratio;
         //double   cluster_offset;        // s.offset
         //double   cluster_jitter;        // s.jitter
 #if !USING_KERNEL_PLL_LOOP
@@ -1337,7 +1339,8 @@ update_local_clock(peer_t *p)
                         return 1; /* "ok to increase poll interval" */
                 }
 #endif
-                set_new_values(STATE_SYNC, /*offset:*/ 0, recv_time);
+                offset = 0;
+                set_new_values(STATE_SYNC, offset, recv_time);
 
         } else { /* abs_offset <= STEP_THRESHOLD */
 
@@ -1355,6 +1358,7 @@ update_local_clock(peer_t *p)
                 G.discipline_jitter = SQRT(etemp + (dtemp - etemp) / AVG);
                 if (G.discipline_jitter < G_precision_sec)
                         G.discipline_jitter = G_precision_sec;
+                G.offset_to_jitter_ratio = fabs(offset) / G.discipline_jitter;
                 VERB3 bb_error_msg("discipline jitter=%f", G.discipline_jitter);
 
                 switch (G.discipline_state) {
@@ -1443,7 +1447,7 @@ update_local_clock(peer_t *p)
 
         /* We are in STATE_SYNC now, but did not do adjtimex yet.
          * (Any other state does not reach this, they all return earlier)
-         * By this time, freq_drift and G.last_update_offset are set
+         * By this time, freq_drift and offset are set
          * to values suitable for adjtimex.
          */
 #if !USING_KERNEL_PLL_LOOP
@@ -1482,40 +1486,42 @@ update_local_clock(peer_t *p)
         tmx.modes = ADJ_FREQUENCY | ADJ_OFFSET;
         /* 65536 is one ppm */
         tmx.freq = G.discipline_freq_drift * 65536e6;
-        tmx.offset = G.last_update_offset * 1000000; /* usec */
 #endif
         tmx.modes = ADJ_OFFSET | ADJ_STATUS | ADJ_TIMECONST;// | ADJ_MAXERROR | ADJ_ESTERROR;
-        tmx.offset = (G.last_update_offset * 1000000); /* usec */
-                        /* + (G.last_update_offset < 0 ? -0.5 : 0.5) - too small to bother */
+        tmx.offset = (offset * 1000000); /* usec */
         tmx.status = STA_PLL;
         if (G.ntp_status & LI_PLUSSEC)
                 tmx.status |= STA_INS;
         if (G.ntp_status & LI_MINUSSEC)
                 tmx.status |= STA_DEL;
+
         tmx.constant = G.poll_exp - 4;
-        //tmx.esterror = (u_int32)(clock_jitter * 1e6);
-        //tmx.maxerror = (u_int32)((sys_rootdelay / 2 + sys_rootdisp) * 1e6);
+        /* EXPERIMENTAL.
+         * The below if statement should be unnecessary, but...
+         * It looks like Linux kernel's PLL is far too gentle in changing
+         * tmx.freq in response to clock offset. Offset keeps growing
+         * and eventually we fall back to smaller poll intervals.
+         * We can make correction more agressive (about x2) by supplying
+         * PLL time constant which is one less than the real one.
+         * To be on a safe side, let's do it only if offset is significantly
+         * larger than jitter.
+         */
+        if (tmx.constant > 0 && G.offset_to_jitter_ratio > TIMECONST_HACK_GATE)
+                tmx.constant--;
+
+        //tmx.esterror = (uint32_t)(clock_jitter * 1e6);
+        //tmx.maxerror = (uint32_t)((sys_rootdelay / 2 + sys_rootdisp) * 1e6);
         rc = adjtimex(&tmx);
         if (rc < 0)
                 bb_perror_msg_and_die("adjtimex");
         /* 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
+        VERB3 bb_error_msg("adjtimex:%d freq:%ld offset:%+ld status:0x%x",
+                                rc, tmx.freq, tmx.offset, tmx.status);
         G.kernel_freq_drift = tmx.freq / 65536;
-        VERB2 bb_error_msg("update peer:%s, offset:%+f, jitter:%f, clock drift:%+.3f ppm",
-                        p->p_dotted, G.last_update_offset, G.discipline_jitter, (double)tmx.freq / 65536);
+        VERB2 bb_error_msg("update peer:%s, offset:%+f, jitter:%f, clock drift:%+.3f ppm, tc:%d",
+                        p->p_dotted, offset, G.discipline_jitter, (double)tmx.freq / 65536, (int)tmx.constant);
 
         return 1; /* "ok to increase poll interval" */
 }
@@ -1651,7 +1657,7 @@ recv_and_process_peer_pkt(peer_t *p)
         if (!p->reachable_bits) {
                 /* 1st datapoint ever - replicate offset in every element */
                 int i;
-                for (i = 1; i < NUM_DATAPOINTS; i++) {
+                for (i = 0; i < NUM_DATAPOINTS; i++) {
                         p->filter_datapoint[i].d_offset = datapoint->d_offset;
                 }
         }
@@ -1706,7 +1712,7 @@ recv_and_process_peer_pkt(peer_t *p)
                                         ? "grows" : "falls"
                         );
                 }
-                if (rc > 0 && fabs(q->filter_offset) < POLLADJ_GATE * G.discipline_jitter) {
+                if (rc > 0 && G.offset_to_jitter_ratio < POLLADJ_GATE) {
                         /* was += G.poll_exp but it is a bit
                          * too optimistic for my taste at high poll_exp's */
                         G.polladj_count += MINPOLL;