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Re: [gpsd-dev] register keyword?

From: Gary E. Miller
Subject: Re: [gpsd-dev] register keyword?
Date: Mon, 22 Jan 2018 14:30:52 -0800

Yo Hal!

On Mon, 22 Jan 2018 14:17:43 -0800
Hal Murray <address@hidden> wrote:

> address@hidden said:
> > The mean latency on the one Intel I have tested is instead slightly
> > less than the arm32.  But the Standard Deviation is way worse.  So
> > there is now hard data why a RasPi can measure PPS better than an
> > Intel.   
> What do you mean by latency?  How are you measuring it?

Well, serves me right for opening my mouth.

I just pushed clock_test.c to git head in contrib/.  Basically, the
test runs two back to back clock_getttime(), then waits a bit for the
CPU cache to clear.  The tests below are for 1,000 samples, with 10
milli sec between tests.  Needless to say the data is a bit jumpy, so
I had to show the standard deviation.

> Pure latency isn't bad.  We can correct for it.

Look at the data, then tell me that again.  :-)

Here is one set of VERY preliminary results:

Intel CPU:

# ./a.out -n 10000
samples 10001, delay 10000000 ns
min 13 ns, max 13440 ns, mean 131 ns, median 24 ns, StdDev 964 ns

Arm 32 bit, 32 bit Gentoo, kernel like almost all RasPi distros:

# ./a.out -n 10000
samples 10001, delay 10000000
min 104 ns, max 22969 ns, mean 169 ns, median 156 ns, StdDev 296 ns

Arm 64 bit, 64 bit Gentoo.  Dev kernel.

# ./clock_test -n 10000
samples 10001, delay 10000000
min 156 ns, max 6667 ns, mean 178 ns, median 157 ns, StdDev 113 ns

No wonder people have such a problem breaking the 1 micro second
barrier on PPS!

> There is/was some code in the kernel that would flap an output pin in
> the PPS code.  If that works, we should be able to measure something
> with a scope.

That would be cool.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        address@hidden  Tel:+1 541 382 8588

            Veritas liberabit vos. -- Quid est veritas?
    "If you can’t measure it, you can’t improve it." - Lord Kelvin

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