ntp: adjust SHIFT_PLL to improve NTP convergence
authorjohn stultz <johnstul@us.ibm.com>
Wed, 6 May 2009 09:43:57 +0000 (11:43 +0200)
committerIngo Molnar <mingo@elte.hu>
Wed, 6 May 2009 09:44:02 +0000 (11:44 +0200)
commit22cfbbfd9f67b67fe073010f51cb71d3632387d5
tree571aff3fc26da4ae5853fb3cf4d4b924c2893c7b
parent413f81eba35d6ede9289b0c8a920c013a84fac71
ntp: adjust SHIFT_PLL to improve NTP convergence

The conversion to the ntpv4 reference model
f19923937321244e7dc334767eb4b67e0e3d5c74 ("ntp: convert to the NTP4
reference model") in 2.6.19 added nanosecond resolution the adjtimex
interface, but also changed the "stiffness" of the frequency adjustments,
causing NTP convergence time to greatly increase.

SHIFT_PLL, which reduces the stiffness of the freq adjustments, was
designed to be inversely linked to HZ, and the reference value of 4 was
designed for Unix systems using HZ=100.  However Linux's clock steering
code mostly independent of HZ.

So this patch reduces the SHIFT_PLL value from 4 to 2, which causes NTPd
behavior to match kernels prior to 2.6.19, greatly reducing convergence
times, and improving close synchronization through environmental thermal
changes.

The patch also changes some l's to L's in nearby code to avoid misreading
50l as 501.

[ Impact: tweak NTP algorithm for faster convergence ]

Signed-off-by: John Stultz <johnstul@us.ibm.com>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: zippel@linux-m68k.org
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
LKML-Reference: <200905051956.n45JuVo9025575@imap1.linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
include/linux/timex.h