Yo Eric! On Wed, 19 Oct 2016 21:30:28 -0400 "Eric S. Raymond" <e...@thyrsus.com> wrote:
> Hal Murray <hmur...@megapathdsl.net>:
> > It has a different type of noise. There is no "ready" signal on
> > SHM so the driver polls and the timing on that is just the luck of
> > when ntpd was started.
>
> Same thing with JSON. The client can't predict when JSON will come
> up the wire.
Yes, but it can block until it happens. With SHM it has to stay away
and poll.
> NTP is a very different case. Minimizing jitter in the delivery chain
> is all-important, extensibility and eyeball-friendliness matter barely
> at all. The overhead of formatting the JSON at one end and decoding
> at the other should be avoided if it can be.
I'm not sure why the jitter is important? The timing dependent stuff
is done before the JSON is sent.
> You and others modeled my thought processes incorrectly. I left a
> clue to what I was actually thinking in that communication between
> GPSD and its client libraries can actually have any of three
> transports - JSON, DBUS, or shared memory. I put in the
> shared-memory transport specifically for use by real-time embedded
> systems.
So, a shared memory driver for ntpd?
> From my point of view, the JSON driver in ntpd is a cute stunt that
> doesn't really fit the context very well. I'm mildly flattered that
> somebody thought GPSD JSON was cool enough to want to play with, but
> the same amount of time spent on (say) a POSIX SHM driver would have
> paid off better.
Hmm, I thought I mentioned JSON encoding inside the POSIX SHM driver?
Or you want to invent anotehr wheel?
> > Would pipes work better? Or a network connection passing the same
> > (binary) info as is in the SHM segment?
>
> As Gary just found out in connection with ntpviz, pipes pretty much
> suck for this kind of work due to unpredictable buffering overhead.
> I don't know whether or not that would be true of sockets.
Yeah, major suckage.
> I don't think either is going to match the performance of a
> shared-memory drop.
So, we all vote for JSON in POSIX SHM?
RGDS
GARY
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