diff options
Diffstat (limited to 'include')
| -rw-r--r-- | include/linux/timex.h | 42 |
1 files changed, 31 insertions, 11 deletions
diff --git a/include/linux/timex.h b/include/linux/timex.h index aa3475fcff64..9910e3bd5b31 100644 --- a/include/linux/timex.h +++ b/include/linux/timex.h @@ -170,17 +170,37 @@ struct timex { #include <asm/timex.h> /* - * SHIFT_KG and SHIFT_KF establish the damping of the PLL and are chosen - * for a slightly underdamped convergence characteristic. SHIFT_KH - * establishes the damping of the FLL and is chosen by wisdom and black - * art. + * SHIFT_PLL is used as a dampening factor to define how much we + * adjust the frequency correction for a given offset in PLL mode. + * It also used in dampening the offset correction, to define how + * much of the current value in time_offset we correct for each + * second. Changing this value changes the stiffness of the ntp + * adjustment code. A lower value makes it more flexible, reducing + * NTP convergence time. A higher value makes it stiffer, increasing + * convergence time, but making the clock more stable. * - * MAXTC establishes the maximum time constant of the PLL. With the - * SHIFT_KG and SHIFT_KF values given and a time constant range from - * zero to MAXTC, the PLL will converge in 15 minutes to 16 hours, - * respectively. + * In David Mills' nanokernel reference implementation SHIFT_PLL is 4. + * However this seems to increase convergence time much too long. + * + * https://lists.ntp.org/pipermail/hackers/2008-January/003487.html + * + * In the above mailing list discussion, it seems the value of 4 + * was appropriate for other Unix systems with HZ=100, and that + * SHIFT_PLL should be decreased as HZ increases. However, Linux's + * clock steering implementation is HZ independent. + * + * Through experimentation, a SHIFT_PLL value of 2 was found to allow + * for fast convergence (very similar to the NTPv3 code used prior to + * v2.6.19), with good clock stability. + * + * + * SHIFT_FLL is used as a dampening factor to define how much we + * adjust the frequency correction for a given offset in FLL mode. + * In David Mills' nanokernel reference implementation SHIFT_FLL is 2. + * + * MAXTC establishes the maximum time constant of the PLL. */ -#define SHIFT_PLL 4 /* PLL frequency factor (shift) */ +#define SHIFT_PLL 2 /* PLL frequency factor (shift) */ #define SHIFT_FLL 2 /* FLL frequency factor (shift) */ #define MAXTC 10 /* maximum time constant (shift) */ @@ -192,10 +212,10 @@ struct timex { #define SHIFT_USEC 16 /* frequency offset scale (shift) */ #define PPM_SCALE ((s64)NSEC_PER_USEC << (NTP_SCALE_SHIFT - SHIFT_USEC)) #define PPM_SCALE_INV_SHIFT 19 -#define PPM_SCALE_INV ((1ll << (PPM_SCALE_INV_SHIFT + NTP_SCALE_SHIFT)) / \ +#define PPM_SCALE_INV ((1LL << (PPM_SCALE_INV_SHIFT + NTP_SCALE_SHIFT)) / \ PPM_SCALE + 1) -#define MAXPHASE 500000000l /* max phase error (ns) */ +#define MAXPHASE 500000000L /* max phase error (ns) */ #define MAXFREQ 500000 /* max frequency error (ns/s) */ #define MAXFREQ_SCALED ((s64)MAXFREQ << NTP_SCALE_SHIFT) #define MINSEC 256 /* min interval between updates (s) */ |