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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="";>Eric S. Raymond</a>

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