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Re: [gpsd-dev] RC for DRIFT message
From: |
Eric S. Raymond |
Subject: |
Re: [gpsd-dev] RC for DRIFT message |
Date: |
Thu, 26 Feb 2015 06:15:05 -0500 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
Hal Murray <address@hidden>:
>
> > Yes. But the only practical reason to write a HOWTO rather than just saying
> > "Sockets everywhere. Done." is that System V IPC could cut latency by
> > eliminating serialization/deserialization overhead and buffer lag. So speed
> > is an issue, and performance numbers are probably indicated.
>
> Are we confusing two issues here? There is a transport mechanism as in TCP
> or SHM, and there is encoding as in JSON or raw binary. If you are timing
> things, please time sending the same chunk of binary stuff over all transport
> mechanisms and time the JSON encoding/decoding without any transport.
Yeah, sure. There are two latency components here which I'nm *not* confusing:
* serialization/deserialization overhead - zero we're doing memcpy of
a struct, nonzero for JSON encode/decode.
* transport lag - cost of memcpy for SHM, cost of the socket layer for TCP.
> I'm not sure what you mean by "extra computation". Higher link speeds don't
> use more cycles. It's the same data to process, the work just gets done in a
> different timing pattern.
Yes, I phrased that badly. At higher timing speeds the process will have
slightly higher *transient* utilization - same work done in less time.
> Has anybody looked at the data from the multiple-updates-per-second devices?
> Are they actually collecting new data at sub second times, or are they just
> extrapolating using the velocity from the previous second?
I've never seen one myself, and have no basis for a guess.
--
<a href="http://www.catb.org/~esr/">Eric S. Raymond</a>
- Re: [gpsd-dev] RC for DRIFT message, (continued)
- Re: [gpsd-dev] RC for DRIFT message, Gary E. Miller, 2015/02/22
- Re: [gpsd-dev] RC for DRIFT message, Hal Murray, 2015/02/22
- Re: [gpsd-dev] RC for DRIFT message, Eric S. Raymond, 2015/02/23
- Re: [gpsd-dev] RC for DRIFT message, Hal Murray, 2015/02/23
- Re: [gpsd-dev] RC for DRIFT message, Eric S. Raymond, 2015/02/23
- Re: [gpsd-dev] RC for DRIFT message, Hal Murray, 2015/02/24
- Re: [gpsd-dev] RC for DRIFT message, Eric S. Raymond, 2015/02/24
- Re: [gpsd-dev] RC for DRIFT message, Hal Murray, 2015/02/25
- Re: [gpsd-dev] RC for DRIFT message, Gary E. Miller, 2015/02/25
- Re: [gpsd-dev] RC for DRIFT message,
Eric S. Raymond <=
- Re: [gpsd-dev] RC for DRIFT message, Gary E. Miller, 2015/02/23
Re: [gpsd-dev] RC for DRIFT message, Greg Troxel, 2015/02/22