On Tue, 2012-02-07 at 20:21 -0500, Andrew Richardson wrote: > On Feb 7, 2012, at 7:30 PM, John Stultz wrote: > > On Wed, 2012-02-08 at 00:16 +0100, Zygmunt Krynicki wrote: > > Hrm. No, that shouldn't be the case. CLOCK_MONOTONIC and > > CLOCK_REALTIME > > are driven by the same accumulation, and are only different by an > > offset. > > > > That said, in the test case you're using CLOCK_MONOTONIC_RAW, which > > I > > don't think you really want, as its not NTP freq corrected. In > > addition > > it is driven by some slightly different accumulation logic. But you > > said CLOCK_REALTIME showed the same issue, so its probably not some > > CLOCK_MONOTONIC_RAW specific bug. > > > > > In general, I don't want the time value moving around on me (in case > something weird is going on and it's changing too much). This seems to > be what most people advise when it comes to profiling something with > sub-second execution, but I might be misunderstanding you slightly.
The difference is "hardware constant" vs "software controlled time constant". And with all things time, its all relative. :) CLOCK_MONOTONIC_RAW is uncorrected, so a second may not really be a second and things like thermal changes can cause fluctuations in your timing intervals. CLOCK_MONOTONIC is NTP corrected, so a second should be a second and thermal drift should be corrected for, but that depends on how much you trust ntpd. That said, CLOCK_MONOTONIC can really only be corrected to 500ppm of CLOCK_MONOTONIC_RAW, so I suspect the difference won't really matter that much unless your measuring longer intervals. Its not like CLOCK_REALTIME, which is more problematic as it may be set back and forth any amount of time. > Seems a bit too high, right? I did get some low values, such as a > 500nanoseconds difference, once. I was expected a harsh lower bound > (e.g. a few ms), but a measurement of 500ns elapsed makes that theory > unlikely. Yea. I suspect something else is at play here. thanks -john _______________________________________________ linaro-dev mailing list linaro-dev@lists.linaro.org http://lists.linaro.org/mailman/listinfo/linaro-dev
