From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from snark.thyrsus.com (static-71-162-243-5.phlapa.fios.verizon.net [71.162.243.5]) by huchra.bufferbloat.net (Postfix) with ESMTP id 4F063200627 for ; Thu, 22 Sep 2011 01:44:51 -0700 (PDT) Received: by snark.thyrsus.com (Postfix, from userid 23) id B27D620C341; Thu, 22 Sep 2011 04:44:50 -0400 (EDT) From: Eric Raymond To: Dave Taht , Jim Gettys , bloat-devel@lists.bufferbloat.net Subject: Revised results on time-service accuracy Message-Id: <20110922084450.B27D620C341@snark.thyrsus.com> Date: Thu, 22 Sep 2011 04:44:50 -0400 (EDT) X-BeenThere: bloat-devel@lists.bufferbloat.net X-Mailman-Version: 2.1.13 Precedence: list List-Id: "Developers working on AQM, device drivers, and networking stacks" List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 22 Sep 2011 08:44:51 -0000 In previous email, I wrote: >Over runs of 20-100 fixes, S-T (the clock-skew figure) ranges from 66 >to 76 milliseconds. Total latency R-T is steady at about 0.38 >seconds. R-S vanishes - it's close to 0.9 milliseconds. Total fix >latency is completely dominated by E-S, the time for gpsd to receive >and process the fix data from the GPS. And I'm not seeing a lot of >bursty variation in these measurements. I've since fixed a minor misunderstanding between my profiler and gnuplot. As previously promised, I can now separate RS232 transmission time from gpsd's analysis time. For the bufferbloat project's purposes, the important news is that reanalysis has confirmed the relatively small (~70msec) skew between GPS time and the NTP-corrected system clock. I'm also still not seeing chaotic variation. In other news: At 9600bps, total fix latency is a little higher than I previously thought (about 0.40sec) and RS232 I/O time does not swamp analysis time as completely as I expected. However, doubling RS232 speed to 19200 cuts typical total latency to about 0.28 sec. -- Eric S. Raymond Where rights secured by the Constitution are involved, there can be no rule making or legislation which would abrogate them. -- Miranda vs. Arizona, 384 US 436 p. 491