[ntp:questions] Idea to improve ntpd accuracy
Brian.Inglis at SystematicSw.ab.ca
Sat Feb 27 17:11:09 UTC 2016
If you have Windows/Linux on Intel/AMD mobos, try OpenHardwareMonitor on Windows with .NET 2+ and Linux with Mono and Winforms: http://openhardwaremonitor.org/documentation/ and https://github.com/openhardwaremonitor/openhardwaremonitor.git
Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada
On 2016-02-27 06:23, Charles Elliott wrote:
> Here are two other ideas you might want to consider to improve accuracy:
> Feed forward (PID) control on system temperature and significant changes in
> system load. Let me describe the situation. Right now I have only 3
> computers on a home LAN. Two, one of which is my main system for email and
> dictating research notes, process BOINC/Einstein at Home work units and one
> serves only as an external-facing NTP client that distributes time to the
> other two. Due to environmental pressures and the need to conserve energy,
> the two computers processing Einstein at Home WUs stop that at midnight and
> resume at 5 AM automatically. Ambient temperature affects all 3 computers
> equally, thought its effect is most noticeable on the external-facing NTP
> When the two computers shed the load at midnight, the offsets immediately
> decrease to between -1 and -2 ms; then, both machines spend all night
> recovering to zero offset at about 5 AM, whereupon the offsets jump up to
> between +0.5 and +1 ms when the load is resumed. At about 8 AM both
> machines are back to zero offset.
> All during the night, the ambient temperature is falling because
> BOINC/Einstein at Home consumes between 450 and 500 watts of GPU and CPU power
> on each machine on which it runs. Consequently, the frequency offset (in
> ppm) declines all night, reaching a minimum at 5 AM, and then increases
> steadily on all three machines until it reaches a steady state at about 10
> AM. It hovers at a fairly steady state until midnight, whereupon the fall
> and rise pattern is repeated.
> PID control is proportional + integral + derivative. Normally, a control
> signal is proportional to an error signal, such as offset error, in
> proportion to the sum of (offset) errors (integral), and in proportion to
> the change in (offset) error (jitter). However, for temperature, there is no
> concept of error; it is what it is. For control as a function of
> temperature (T), there clearly is a direct functional relationship between
> ppm and T, such as ppm = Kp T. Jitter lags both temperature and system load
> changes by about 2 hours, so I am not sure derivative control is wise or
> even possible. Integral control is what yanks the elevator up to floor
> level when the brakes are slipping. It can be hard to get right because of
> the yo-yo effect. Obviously, more thought has to be put into all this. But
> I have been watching these relationships for several weeks with NTP Plotter,
> and they are obvious and fairly consistent.
> What started me off on this, is that CPUID (www.cpuid.com), makers of CPU-z,
> HardwareMonitor, and HardwareMonitorPro, the latter two of which do an
> excellent job of monitoring all manner of system and peripheral
> temperatures, is offering a "System Monitoring SDK" for evaluation. The
> idea would be to record the relationships between various temperature
> measurements and ppm and then try to adjust the PPM adjustment with
> temperature to see if any improvement is offsets results. However, CPUID
> never replied to my email.
> Charles Elliott
> -----Original Message-----
> From: questions
> [mailto:questions-bounces+elliott.ch=comcast.net at lists.ntp.org] On Behalf Of
> Sent: Thursday, February 25, 2016 4:52 PM
> To: questions at lists.ntp.org
> Subject: [ntp:questions] Idea to improve ntpd accuracy
> This may or may not be worthwhile, but I thought I'd throw it out there and
> see what happens:
> Recent work testing some microsecond-accurate NTP servers lead me to an idea
> that could improve accuracy of measurements made by ntpd. These NTP servers
> have hardware timestamps on receive but that's not possible on transmit w/o
> a custom NIC. I've seen this issue discussed before.
> The next best thing is to generate the transmit timestamp based on a guess
> as to how long it takes the NIC to get on the wire and send the packet. That
> works pretty well as long as there's no other network traffic. In this
> situation, it is possible to make use of microsecond accuracy in an NTP
> Now, add some typical network traffic and the time it takes the NIC to get
> on the wire becomes unpredictable to the tune of 200us or more (for
> 100 base-T Ethernet). The server's microsecond accuracy is largely lost in
> the noise.
> The NIC generates an interrupt after the packet is sent which can result in
> a fairly accurate trailing hardstamp. The problem is...the packet is already
> gone and has the wrong transmit timestamp.
> Here's my idea:
> What if the poll response packet contained a flag or indication of some sort
> which means "this is an approximate transmit timestamp". That packet would
> then be immediately followed by a second response packet with a more
> accurate transmit time. The second packet could be otherwise identical to
> the first, or it could be a new flavor of packet that contained only the
> transmit time (that would save on network bandwidth).
> The ntpd process would need to use the receive time of the first packet (the
> one with an approximate tx timestamp) and merge in the following accurate tx
> timestamp before performing the normal processing associated with a poll
> Here are the pros and cons I can think of:
> * Possible accuracy improvement of 1-2 orders of magnitude. I know ntpd
> already does some work to try and filter out network delay variation so the
> improvement might not be a full 2 orders of magnitude.
> * Could potentially be made compatible backwards compatible with ntp 3/4
> * Increased network traffic
> * Improvement to that level of accuracy might not be of interest to anyone
> * Could be a fair bit of work for at least a couple of folks
> * I may have (or probably) missed some stuff regarding network behavior that
> would reduce the level of improvement that could be realized.
> * Perhaps this is less of an issue on G-bit Ethernet?
> Wondering if anyone thinks this idea is worth pursuing further...?
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