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Re: [gpsd-dev] Clarifications needed for the time-service HOWTO
From: |
Eric S. Raymond |
Subject: |
Re: [gpsd-dev] Clarifications needed for the time-service HOWTO |
Date: |
Wed, 23 Oct 2013 13:33:19 -0400 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
Gary E. Miller <address@hidden>:
> > What sorts of jitter are produced by different parts of the delivery
> > chain? What do typical magnitudes look like?
>
> The jitter from NMEA on a local GPS can be a few hundred mSec. Run a
> traceroute to a server and you see how bad you network can be. Cable
> modems could be up to 500 mSec (and they are wildly asymetric). The
> jitter from a local GPS s defined by the USB transfer, around 1 mSec
> for USB 1.1 The jitter from interrupt processing can be 100uSec and
> sometimes much worse on a loaded system.
I'm not sure how to use those figure for the HOWTO. Can you
recommend a specific way to merge them?
> > Thus, KPPS can only be used with devices passed that way, not with
> > GPSes that are later presented to gpsd by the hotplug system. Those
> > hotplug devices will, however, may be able to use plain, non-kernel
> > PPS. gpsd tries to automatically fall back to this when absence of
> > root permissions makes KPPS unavailable.
>
> Before saying this I would like to see it tested.
Has been. I've seen PPS from a GR601-W using gpsd started as non-root.
> > (Here comes the don't-do-that.)
> >
> > In general, if you start gpsd as other than root, the following
> > things will happen that slightly degrade the accuracy of reported
> > time:
> >
> > 1. Devices passed on the command line will be unable to use KPPS
> > and will fall back to the same plain PPS that all hotplug devices must
> > use, increasing the associated error from ~1 uSec to about ~5 uSec.
>
> Or ptotentially MUCH worse on a single core CPU.
Why is single-core vs. multicore relevant here? And can you give an
estimate of "much worse"?
--
<a href="http://www.catb.org/~esr/">Eric S. Raymond</a>
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- Re: [gpsd-dev] Direct GPS support in ntpd makes me nervous, (continued)
- Re: [gpsd-dev] Direct GPS support in ntpd makes me nervous, Eric S. Raymond, 2013/10/22
- Re: [gpsd-dev] Direct GPS support in ntpd makes me nervous, Harlan Stenn, 2013/10/22
- Re: [gpsd-dev] Direct GPS support in ntpd makes me nervous, Eric S. Raymond, 2013/10/22
- Re: [gpsd-dev] Direct GPS support in ntpd makes me nervous, Harlan Stenn, 2013/10/22
- Re: [gpsd-dev] Clarifications needed for the time-service HOWTO, Eric S. Raymond, 2013/10/22
- Re: [gpsd-dev] Clarifications needed for the time-service HOWTO, Gary E. Miller, 2013/10/21
- Re: [gpsd-dev] Clarifications needed for the time-service HOWTO, Hal Murray, 2013/10/22
- Re: [gpsd-dev] Clarifications needed for the time-service HOWTO,
Eric S. Raymond <=
- Re: [gpsd-dev] Clarifications needed for the time-service HOWTO, Gary E. Miller, 2013/10/23
- Re: [gpsd-dev] Clarifications needed for the time-service HOWTO, Dave Platt, 2013/10/23
- Re: [gpsd-dev] Clarifications needed for the time-service HOWTO, Gary E. Miller, 2013/10/23
- Re: [gpsd-dev] Clarifications needed for the time-service HOWTO, Eric S. Raymond, 2013/10/24
- Re: [gpsd-dev] Clarifications needed for the time-service HOWTO, Hal Murray, 2013/10/23
- Re: [gpsd-dev] Clarifications needed for the time-service HOWTO, Eric S. Raymond, 2013/10/23
- Re: [gpsd-dev] Clarifications needed for the time-service HOWTO, Gary E. Miller, 2013/10/23
- Re: [gpsd-dev] Clarifications needed for the time-service HOWTO, Beat Bolli, 2013/10/23
Re: [gpsd-dev] Clarifications needed for the time-service HOWTO, Miroslav Lichvar, 2013/10/23