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Merge branch 'timers-for-linus-ntp' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip

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* 'timers-for-linus-ntp' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip:
  ntp: fix comment typos
  ntp: adjust SHIFT_PLL to improve NTP convergence
This commit is contained in:
Linus Torvalds 2009-06-15 09:43:24 -07:00
commit 9aaa630503
1 changed files with 31 additions and 11 deletions
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42
include/linux/timex.h
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@ -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) */