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ntpd: explain algorithm

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Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
This commit is contained in:
Denys Vlasenko 2010-01-11 02:14:04 +01:00
parent 12628b7797
commit 65d722bb0d
1 changed files with 34 additions and 5 deletions
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39
networking/ntpd.c
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@ -46,8 +46,34 @@
#define MAX_VERBOSE 2
/* High-level description of the algorithm:
*
* We start running with very small poll_exp, BURSTPOLL,
* in order to quickly accumulate INITIAL_SAMLPES datapoints
* for each peer. Then, time is stepped if the offset is larger
* than STEP_THRESHOLD, otherwise it isn't; anyway, we enlarge
* poll_exp to MINPOLL and enter frequency measurement step:
* we collect new datapoints but ignore them for WATCH_THRESHOLD
* seconds. After WATCH_THRESHOLD seconds we look at accumulated
* offset and estimate frequency drift.
*
* After this, we enter "steady state": we collect a datapoint,
* we select the best peer, if this datapoint is not a new one
* (IOW: if this datapoint isn't for selected peer), sleep
* and collect another one; otherwise, use its offset to update
* frequency drift, if offset is somewhat large, reduce poll_exp,
* otherwise increase poll_exp.
*
* If offset is larger than STEP_THRESHOLD, which shouldn't normally
* happen, we assume that something "bad" happened (computer
* was hibernated, someone set totally wrong date, etc),
* then the time is stepped, all datapoints are discarded,
* and we go back to steady state.
*/
#define RETRY_INTERVAL 5 /* on error, retry in N secs */
#define RESPONSE_INTERVAL 15 /* wait for reply up to N secs */
#define INITIAL_SAMLPES 4 /* how many samples do we want for init */
/* Clock discipline parameters and constants */
#define STEP_THRESHOLD 0.128 /* step threshold (s) */
@ -75,8 +101,9 @@
* we grow a counter: += MINPOLL. When it goes over POLLADJ_LIMIT,
* we poll_exp++. If offset isn't small, counter -= poll_exp*2,
* and when it goes below -POLLADJ_LIMIT, we poll_exp--
* (bumped from 30 to 36 since otherwise I often see poll_exp going *2* steps down)
*/
#define POLLADJ_LIMIT 30
#define POLLADJ_LIMIT 36
/* If offset < POLLADJ_GATE * discipline_jitter, then we can increase
* poll interval (we think we can't improve timekeeping
* by staying at smaller poll).
@ -1555,6 +1582,7 @@ recv_and_process_peer_pkt(peer_t *p)
if (q->filter_offset < -POLLDOWN_OFFSET
|| q->filter_offset > POLLDOWN_OFFSET
) {
VERB3 bb_error_msg("offset:%f > POLLDOWN_OFFSET", q->filter_offset);
goto poll_down;
}
}
@ -1856,13 +1884,14 @@ int ntpd_main(int argc UNUSED_PARAM, char **argv)
idx2peer = xzalloc(sizeof(idx2peer[0]) * cnt);
pfd = xzalloc(sizeof(pfd[0]) * cnt);
/* Countdown: we never sync before we sent 5 packets to each peer
/* Countdown: we never sync before we sent INITIAL_SAMLPES+1
* packets to each peer.
* NB: if some peer is not responding, we may end up sending
* fewer packets to it and more to other peers.
* NB2: sync usually happens using 5-1=4 packets, since last reply
* does not come back instantaneously.
* NB2: sync usually happens using INITIAL_SAMLPES packets,
* since last reply does not come back instantaneously.
*/
cnt = G.peer_cnt * 5;
cnt = G.peer_cnt * (INITIAL_SAMLPES + 1);
while (!bb_got_signal) {
llist_t *item;