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Date: Sat, 09 May 2020 11:49:27 +0200 From: Thomas Gleixner <tglx@...utronix.de> To: Pali Rohár <pali@...nel.org> Cc: linux-kernel@...r.kernel.org Subject: Re: Missing CLOCK_BOOTTIME_RAW? Pali, Pali Rohár <pali@...nel.org> writes: > On Friday 08 May 2020 22:59:57 Thomas Gleixner wrote: >> Pali Rohár <pali@...nel.org> writes: >> Neither CLOCK_BOOTTIME nor CLOCK_MONOTONIC jump. They are frequency >> corrected when NTP, PTP or PPS are in use. The frequency correction is >> incremental an smothed. They really don't jump and they give you proper >> time in nanoseconds which is as close to the real nanoseconds as it >> gets. > > Hello! I should have been more precise about it. CLOCK_BOOTTIME and > CLOCK_MONOTONIC do not jump but I understood that they are affected by > adjtime(). So these clocks may tick faster or slower than real time. NTP > daemon when see that CLOCK_REALTIME is incorrect, it may speed up or > slow down its ticking. And this is affected also by CLOCK_BOOTTIME and > CLOCK_MONOTONIC, right? Sure, but what's the problem? The adjustemt is done to make the observed time as correct as possible. > You wrote that this clock is subject to drifts. What exactly may happen > with CLOCK_MONOTONIC_RAW? 1) As the frequency of the raw clock is an estimate, resulting time is drifting apart vs. the correct frequency. 2) Depending on the crystal/oszillator there can be temperatur drift as well. Just for clarification. Even with NTP/PTP adjustment the resulting time stamps are never going to be precise vs. an atomic clock, but good enough for 99.9999% of the problems. TBH, I have no idea what real world problem you are trying to solve. Thanks, tglx
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