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ntpd: tweak comments

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Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
This commit is contained in:
Denys Vlasenko 2021-03-02 19:54:09 +01:00
parent 2d6c175d9b
commit 2d48d9b1cc
1 changed files with 9 additions and 23 deletions
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32
networking/ntpd.c
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@ -127,24 +127,15 @@
*/ */
#define MAX_VERBOSE 3 #define MAX_VERBOSE 3
/* High-level description of the algorithm: /* High-level description of the algorithm:
* *
* We start running with very small poll_exp, BURSTPOLL, * We start running with very small poll_exp, BURSTPOLL,
* in order to quickly accumulate INITIAL_SAMPLES datapoints * in order to quickly accumulate INITIAL_SAMPLES datapoints
* for each peer. Then, time is stepped if the offset is larger * for each peer. Then, time is stepped if the offset is larger
* than STEP_THRESHOLD, otherwise it isn't; anyway, we enlarge * than STEP_THRESHOLD, otherwise it isn't stepped.
* 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.
* *
* (frequency measurement step seems to not be strictly needed, * Then poll_exp is set to MINPOLL, and we enter "steady state": we collect
* it is conditionally disabled with USING_INITIAL_FREQ_ESTIMATION * a datapoint, we select the best peer, if this datapoint is not a new one
* define set to 0)
*
* 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 * (IOW: if this datapoint isn't for selected peer), sleep
* and collect another one; otherwise, use its offset to update * and collect another one; otherwise, use its offset to update
* frequency drift, if offset is somewhat large, reduce poll_exp, * frequency drift, if offset is somewhat large, reduce poll_exp,
@ -189,13 +180,10 @@
// ^^^^ used to be 0.125. // ^^^^ used to be 0.125.
// Since Linux 2.6.26 (circa 2006), kernel accepts (-0.5s, +0.5s) range // Since Linux 2.6.26 (circa 2006), kernel accepts (-0.5s, +0.5s) range
/* Stepout threshold (sec). std ntpd uses 900 (11 mins (!)) */
//UNUSED: #define WATCH_THRESHOLD 128
/* NB: set WATCH_THRESHOLD to ~60 when debugging to save time) */
//UNUSED: #define PANIC_THRESHOLD 1000 /* panic threshold (sec) */
/* // #define PANIC_THRESHOLD 1000 /* panic threshold (sec) */
* If we got |offset| > BIGOFF from a peer, cap next query interval
/* If we got |offset| > BIGOFF from a peer, cap next query interval
* for this peer by this many seconds: * for this peer by this many seconds:
*/ */
#define BIGOFF STEP_THRESHOLD #define BIGOFF STEP_THRESHOLD
@ -204,18 +192,16 @@
#define FREQ_TOLERANCE 0.000015 /* frequency tolerance (15 PPM) */ #define FREQ_TOLERANCE 0.000015 /* frequency tolerance (15 PPM) */
#define BURSTPOLL 0 /* initial poll */ #define BURSTPOLL 0 /* initial poll */
#define MINPOLL 5 /* minimum poll interval. std ntpd uses 6 (6: 64 sec) */ #define MINPOLL 5 /* minimum poll interval. std ntpd uses 6 (6: 64 sec) */
/* /* If offset > discipline_jitter * POLLADJ_GATE, and poll interval is > 2^BIGPOLL,
* If offset > discipline_jitter * POLLADJ_GATE, and poll interval is > 2^BIGPOLL,
* then it is decreased _at once_. (If <= 2^BIGPOLL, it will be decreased _eventually_). * then it is decreased _at once_. (If <= 2^BIGPOLL, it will be decreased _eventually_).
*/ */
#define BIGPOLL 9 /* 2^9 sec ~= 8.5 min */ #define BIGPOLL 9 /* 2^9 sec ~= 8.5 min */
#define MAXPOLL 12 /* maximum poll interval (12: 1.1h, 17: 36.4h). std ntpd uses 17 */ #define MAXPOLL 12 /* maximum poll interval (12: 1.1h, 17: 36.4h). std ntpd uses 17 */
/* /* Actively lower poll when we see such big offsets.
* Actively lower poll when we see such big offsets.
* With SLEW_THRESHOLD = 0.125, it means we try to sync more aggressively * With SLEW_THRESHOLD = 0.125, it means we try to sync more aggressively
* if offset increases over ~0.04 sec * if offset increases over ~0.04 sec
*/ */
//#define POLLDOWN_OFFSET (SLEW_THRESHOLD / 3) // #define POLLDOWN_OFFSET (SLEW_THRESHOLD / 3)
#define MINDISP 0.01 /* minimum dispersion (sec) */ #define MINDISP 0.01 /* minimum dispersion (sec) */
#define MAXDISP 16 /* maximum dispersion (sec) */ #define MAXDISP 16 /* maximum dispersion (sec) */
#define MAXSTRAT 16 /* maximum stratum (infinity metric) */ #define MAXSTRAT 16 /* maximum stratum (infinity metric) */