FAQ: http://www.ntp.org/ntpfaq/NTP-s-algo.htm
# ntpq
> as
ind assID status conf reach auth condition last_event cnt
===========================================================
1 943 9614 yes yes none sys.peer reachable 1
2 944 9014 yes yes none reject reachable 1
> rv 944
assID=944 status=9014 reach, conf, 1 event, event_reach,
srcadr=ntpserver_IP, srcport=123, dstadr=clientIP, dstport=123,
leap=00, stratum=1, precision=-20, rootdelay=0.000,
rootdispersion=0.320, refid=GPS, reach=377, unreach=0, hmode=3, pmode=4,
hpoll=6, ppoll=6,
flash=400 peer_dist, keyid=0, ttl=0,
offset=-23289498.129, delay=18.167, dispersion=3.040, jitter=78437.126,
reftime=d9d20ed8.a799ceee Wed, Oct 21 2015 21:17:12.654,
org=d9d20edf.9060a3fb Wed, Oct 21 2015 21:17:19.563,
rec=d9d269f2.580148e4 Thu, Oct 22 2015 3:45:54.343,
xmt=d9d269f2.4ebe53b8 Thu, Oct 22 2015 3:45:54.307,
filtdelay= 36.14 18.17 36.36 31.15 36.72 40.30 18.80 18.83,
filtoffset= -233147 -232894 -232688 -232419 -232155 -231980 -231812 -231704,
filtdisp= 0.00 0.98 1.95 2.90 3.87 4.85 5.79 6.74
flash=400 means: peer distance exceeded
/*
* Peer errors
*/
#define TEST10 0x0200 /* peer bad synch or stratum */
#define TEST11 0x0400 /* peer distance exceeded */
#define TEST12 0x0800 /* peer synchronization loop */
#define TEST13 0x1000 /* peer unreacable */
usually it means etwork blocking, firewall, proxy etc may cause this error
We can try this:
Adding the line
tos maxdist [NUM]
to the /etc/ntp.conf before any lines starting with 'restrict' or 'server'
Default value is 1.5
We can try higher number like 3
# /etc/init.d/ntpd stop
Shutting down ntpd: [ OK ]
# ps -ef |grep ntp |grep -v grep
# rm -rf /var/lib/ntp/drift
# /usr/sbin/ntpdate -u 0.rhel.pool.ntp.org
22 Nov 17:49:01 ntpdate[26709]: adjust time server 212.26.18.41 offset -0.000689 sec
# /usr/sbin/ntpdate -u 1.rhel.pool.ntp.org
22 Nov 17:49:09 ntpdate[26719]: adjust time server 212.26.18.41 offset 0.000809 sec
# /usr/sbin/ntpdate -u 2.rhel.pool.ntp.org
22 Nov 17:49:17 ntpdate[26724]: adjust time server 212.26.18.43 offset -0.000317 sec
# /etc/init.d/ntpd start
Starting ntpd: [ OK ]
# ps -ef |grep ntp |grep -v grep
ntp 26797 1 0 17:50 ? 00:00:00 ntpd -u ntp:ntp -p /var/run/ntpd.pid -g
( wait for 10 - 15 min )
# ntpstat
synchronised to NTP server (212.26.18.41) at stratum 2
time correct to within 968 ms
polling server every 64 s
# ntpq -p
remote refid st t when poll reach delay offset jitter
==============================================================================
*0.rhel.pool.ntp .GPS. 1 u 31 64 277 17.296 -6.289 18.942
+2.rhel.pool.ntp 69.25.96.12 2 u 35 64 377 17.985 -7.786 13.836
# tail -f /var/log/messages
ntpd[8240]: synchronized to 212.26.18.41, stratum 2
Or we can restart this way
# service ntpd stop
# ps -ef |grep ntp |grep -v grep
# rm -rf /var/lib/ntp/drift
# ntpd -qg
# service ntpd start
about the option of -q and -g
-q Exit the ntpd just after the first time the clock is set. This
behavior mimics that of the ntpdate program, which is to be
retired. The -g and -x options can be used with this option.
Note: The kernel time discipline is disabled with this option.
-g Normally, ntpd exits with a message to the system log if the
offset exceeds the panic threshold, which is 1000 s by default.
This option allows the time to be set to any value without
restriction; however, this can happen only once. If the thresh-
old is exceeded after that, ntpd will exit with a message to
the system log. This option can be used with the -q and -x
options. See the tinker command for other options.
Make sure /etc/ntp/step-tickers is correctly populated
# nc -zvnu <IP Address of the NTP server> 123
# tcpdump -s0 port 123 -vvv -i <NIC>
Allow Only Specific Clients
To only allow machines on your own network to synchronize with your NTP server, add the following restrict line to your /etc/ntp.conf file:
restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap
# cat /etc/sysconfig/ntpd
# Drop root to id 'ntp:ntp' by default.
OPTIONS="-u ntp:ntp -p /var/run/ntpd.pid -I eth0"
# Set to 'yes' to sync hw clock after successful ntpdate
SYNC_HWCLOCK=no
# Additional options for ntpdate
NTPDATE_OPTIONS=""
https://docs.oracle.com/cd/E50245_01/E50251/html/vmadm-tshoot-vm-clock.html
Setting the Guest's Clock
PVM
guests may perform their own system clock management, for
example, using the NTPD (Network Time Protocol daemon), or the
hypervisor may
perform system clock management for all guests.
You can set
paravirtualized
guests to manage their own system clocks by setting the
xen.independent_wallclock parameter to
1 in the
/etc/sysctl.conf file. For example:
"xen.independent_wallclock = 1"
If you want to set the hypervisor to manage paravirtualized
guest system clocks, set
xen.independent_wallclock to
0. Any attempts to set or modify the time in
a guest will fail.
You can temporarily override the setting in the
/proc file. For example:
"echo 1 > /proc/sys/xen/independent_wallclock"
http://serverfault.com/questions/245401/xen-hvm-guest-has-severe-clock-drift
Add this line to the beginning of ntpd.conf:
tinker panic 0
To
override the clock source configuration, you should add clocksource= to the
kernel stanza. For example:
kernel /vmlinuz-2.6.18-406.el5 ro
root=/dev/vg0/rootvol elevator=deadline clocksource=acpi_pm
browsing the documentation file
/usr/share/doc/kernel-doc-2.6.32/Documentation/kernel-parameters.txt
clocksource= [GENERIC_TIME] Override the default clocksource
Format: Override the default clocksource and use the clocksource
with the name specified.
Some clocksource names to choose from, depending on
the platform:
[all] jiffies (this is the base, fallback clocksource)
[ACPI] acpi_pm
[ARM] imx_timer1,OSTS,netx_timer,mpu_timer2,
pxa_timer,timer3,32k_counter,timer0_1
[AVR32] avr32
[X86-32] pit,hpet,tsc,vmi-timer;
scx200_hrt on Geode; cyclone on IBM x440
[MIPS] MIPS
[PARISC] cr16
[S390] tod
[SH] SuperH
[SPARC64] tick
[X86-64] hpet,tsc
hpet= [X86-32,HPET] option to control HPET usage
Format: { enable (default) | disable | force |
verbose }
disable: disable HPET and use PIT instead
force: allow force enabled of undocumented chips (ICH4,
VIA, nVidia)
verbose: show contents of HPET registers during setup
notsc [BUGS=X86-32] Disable Time Stamp Counter
An overview on hardware clock and system timer circuits:
When it comes to talk about a system's clock, the hardware sits at
the very bottom. Every typical system has several devices, usually
implemented by clock chips, that provide timing features and can serve
as clocks. So, which hardware is available depends on the particular
architecture. The clock circuits are used both to keep track of the
current time of the day and to make precise time measurements. The timer
circuits are programmed by the kernel, so they issue interrupts at a
fixed, and predefined, frequency. For instance, IA-32 and AMD64 systems
have at least one
programmable interrupt timer (PIT) as a
classical timer circuit, which is usually implemented by an 8254 CMOS
chip. Let's briefly describe the clock and timer circuits that are
usually found with any nearly modern system of those architectures:
- Real Time Clock (RTC)
- The RTC is independent of the system's CPU and any other chips. As
it is energized by a small battery, it continues to tick even when the
system is switched off. The RTC is capable of issuing interrupts at
frequencies ranging between 2 Hz and 8,192 Hz. Linux uses the RTC only
to derive the time and date at boot time.
- Programmable Interrupt Timer (PIT)
- The PIT is a time-measuring device that can be compared to the alarm
clock of a microwave oven: it makes the user aware that the cooking
time interval has elapsed. Instead of ringing a bell, the PIT issues a
special interrupt called timer interrupt, which notifies the kernel that
one more time interval has elapsed. As the time goes by, the PIT goes
on issuing interrupts forever at some fixed (architecture-specific)
frequency established by the kernel.
- Time Stamp Counter (TSC)
- All 80x86 microprocessors include a
CLK input pin,
which receives the clock signal of an external oscillator. Starting with
the Pentium, 80x86 microprocessors sport a counter that is increased at
each clock signal, and is accessible through the TSC register which can
be read by means of the rdtsc assembly instruction. When
using this register the kernel has to take into consideration the
frequency of the clock signal: if, for instance, the clock ticks at 1
GHz, the TSC is increased once every nanosecond. Linux may take
advantage of this register to get much more accurate time measurements.
- CPU Local Timer
- The Local APIC (Advanced Programmable Interrupt Controller) present
in recent 80x86 microprocessors provide yet another time measuring
device, and it is a device, similar to the PIT, which can issue one-shot
or periodic interrupts. There are, however, a few differences:
- The APIC's timer counter is 32 bit long, while the PIT's timer counter is 16 bit long;
- The local APIC timer sends interrupts only to its processor, while
the PIT raises a global interrupt, which may be handled by any CPU in
the system;
- The APIC's timer is based on the bus clock signal, and it can be
programmed in such way to decrease the timer counter every 1, 2, 4, 8,
16, 32, 64, or 128 bus clock signals. Conversely, the PIT, which makes
use of its own clock signals, can be programmed in a more flexible way.
- High Precision Event Timer (HPET)
- The HPET is a timer chip that in some future time is expected to
completely replace the PIT. It provides a number of hardware timers that
can be exploited by the kernel. Basically the chip includes up to eight
32 bit or 64 bit independent counters. Each counter is driven by its
own clock signal, whose frequency must be at least 10 MHz; therefore the
counter is increased at least once in 100 nanoseconds. Any counter is
associated with at most 32 timers, each of which composed by a comparator and a match register.
The HPET registers allow the kernel to read and write the values of the
counters and of the match registers, to program one-shot interrupts,
and to enable or disable periodic interrupts on the timers that support
them.
- ACPI Power Management Timer (ACPI PMT)
- The ACPI PMT is another clock device included in almost all
ACPI-based motherboards. Its clock signal has a fixed frequency of
roughly 3.58 MHz. The device is a simple counter increased at each clock
tick. However the ACPI PMT is preferable to the TSC if the operating
system or the BIOS may dynamically lower the CPU's frequency or voltage.
When this happens, TSC's frequency changes causing time warps and
others side-effects, while the frequency of ACPI PMT does not.
![[Note]](https://lh3.googleusercontent.com/blogger_img_proxy/AEn0k_vRYo8Kyv3BIHDYJ8QQ1MFXu2n4jvs_6UqlJuq_yp8kcorrOh7KwhSHsa6DvFkbDxXdrVSlWfvDC6juiS_XGNbbJiVifaknSvFfYQF0AUFsQxi-g3rnmU_uIFmJ1ryPNs-WykFu8lA0LnyZpK5y5CcVZ3Z3DNmyt3E=s0-d)
