Date: Fri, 29 Nov 2019 10:10:35 -0700 From: Ian Lepore <ian@freebsd.org> To: Ross Alexander <rwa@athabascau.ca> Cc: "freebsd-arm@freebsd.org" <freebsd-arm@freebsd.org> Subject: Re: rpi3 clock drift Message-ID: <65be6d3628a8d35084f7c98266582090f59b18be.camel@freebsd.org> In-Reply-To: <alpine.BSF.2.21.99999.352.1911282333250.90234@autopsy.pc.athabascau.ca> References: <alpine.BSF.2.21.99999.352.1911271039470.281@autopsy.pc.athabascau.ca> <MWHPR06MB3134EC22EC3148DA800B2B7DAA440@MWHPR06MB3134.namprd06.prod.outlook.com> <alpine.BSF.2.21.99999.352.1911272214050.28592@autopsy.pc.athabascau.ca> <20191129052800.GA37113@server.rulingia.com> <alpine.BSF.2.21.99999.352.1911282333250.90234@autopsy.pc.athabascau.ca>
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On Thu, 2019-11-28 at 23:51 -0700, Ross Alexander wrote: > On Fri, 29 Nov 2019, Peter Jeremy wrote: > > > [...] > > BTW, another *significant* source of jitter is the brand and age of > the sd/mmc card used. As they age, the write speed decreases and > block write latency gets less uniform; this shows up as system clock > jitter in the loopstats. After a few years (3 or 4), the box becomes > a complete falseticker and you need to replace the sd/mmc card. > I'm having a real hard time with this one, conceptually. This is exactly what every one of our products at $work does: precision timing including ntpd and kernel time tracking UTC(GPS) to within a few nanos. We've had products in the field for 15 years still using the original sdcard and still hitting the same on-time performance numbers as the day they were shipped (tracking UTC(GPS) +-10ns RMS). I'm having a hard time picturing how sdcard write performance could impact kernel timekeeping that much. I say this as a person who has spent years working on both the sdcard drivers and the kernel timekeeping code in freebsd. An sdcard driver that used PIO instead of DMA might even cause a bit of jitter in measuring a PPS signal due to interrupt latency (mostly if the interrupt thread priorities weren't right), but even that would be nowhere near enough jitter to become a falseticker, it would be on the order of a few dozen microseconds. -- Ian
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