Re: [gpsd-dev] Updated docs on NTP segment management

[Gary E. Miller]

Yo Harlan!

On Thu, 26 Feb 2015 20:24:17 +0000
Harlan Stenn <address@hidden> wrote:

> > > One thing to think about is whether or not the SHMID should be
> > > different for each version where there is a change in size of
> > > shmTime.
> > 
> > That also works, but spawns complexity.  I would rather avoid.
> 
> If we start using a non-zero version number in the mode field that
> gets easier.

That would be my vote.
 
> And putting the "version" in the mode field is a case where I wonder
> if that should be in the per-unit data or in the preamble.  There are
> pros/cons with each choice.

Maybe an example would help?  Version needs to be per unit.

> > > I'm also wondering if the V2 SHM interface should include a
> > > preamble structure before the per-unit structures, as that seems
> > > to be a more efficient way to go.  Duplicating data in the
> > > per-unit structures seems wasteful to me.
> > 
> > What is duplicated?  And how to handle multiple senders?
> 
> The version of the SHM protocol/data structure, for example.

Or not.  For example, if linuxptp is send the nSec version and gpsd is
sending the V2 version (at the same time!) there is not one preamble but
two, or more.

> > > I also wonder if there is a good way (and a good reason) to
> > > support:
> > > 
> > > - a mix of V1 and V2 SHM areas on the same shmid
> > 
> > Yes, but not at the same time.  :-)
> 
> Then that would be a "no" :)
> 
> This goes to multiple senders, or even a single sender that wants to
> send V1 and V2 data to clients.

Huh?  Things come 'from ' senders?  Unless the client can tell the
sender what he supports?

> > > - more than 4 units' data structures in an SHM segment
> > 
> > Absolutely.  207 supported now.  With linuxptp and soon more senders
> > talking NTP SHM this could grow large in the lab.
> 
> This is another candidate for a "preamble" data area - "how many units
> are on this SHM segment?"

Hmm, so you are saying, one SHM, that includes a primary header, followed
by one or more unit headers?  Then there could still be multiple simultaeous
SHMs each with multiple sources?

Starting to sound like a tree.  I can't see saving a few bytes helps
anyone and decisions about what is shared/not-shared would be
frozen going forward.  Plus lots of imlications for configuration, unless
there is a way to autoconfigure this.

RGDS
GARY
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