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Comment by godelski | Hacker News Reader

Despite the resolution being nanoseconds, what is the actual precision of computer clocks? I can't imagine it is actually nanoseconds. Takes me back to teaching physics labs where I had to hound students to remember that the accuracy of their measuring device is not identical to the smallest number it displays...

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For devices clocked above 1Ghz it's perfectly possible for the clock to increment every ns, although that doesn't make it accurate to that level, and multi core systems may have clocks that are not synchronised to that level.

ARMv8 guarantees that it's clock increments at at least 1Ghz, for intel and earlier ARM it's more complicated

Cycle counting is one thing, but it gets tricky when you have frequency scaling in play. Another problem that even without freq scaling, cpu clocks are not designed to be super accurate/exact, and the true frequency might vary significantly even if the nominal freq is fixed

I think on newer x86 chips the ‘cycle’ counter increments at a ~constant rate, rather than changing with variable cpu frequencies. There’s a cpuid flag for this and I think Linux exposes it in procfs somewhere too. Older chips do have the problem you describe, as well as problems with cycle counts diverging (or never being close) between cores/sockets. It still isn’t exact in the way you describe. The most reasonable thing you can do is regularly calibrate based on an actual clock (whose rate also gets calibrated based on ntp…) to have a cycles<->ns conversion factor.

It doesn't matter that much they're not super accurate exact, they do in fact count ticks n tocks j dandy. I wonder if they are counted equally, interesting question if i do say so myself about my own question, i know rising edge is steady w the next rising edge, n falling edge w falling edge, but yeah...no i think that's a specification figure of merit, that they be balanced w respect to each other. N Intel® despite it's many failures, long ago forecast n long overdue due to the sheer difficulty of their business model n how long they kept it going--they were saying it was going to end soon in the late seventies already--n you know what, i'm fine w that. They don't make old chips rot like other software companies i could mention, bits rot but glass is timeless. Which brings me back to the point, in my analysis the problem is not the clocks being inaccurate but rather the jitter, which means a single run will not suffice in describing a repeatable exact clock time taken for eg an inner loop, which is what is worth bothering with. The minimum jitter attainable currently is 1 cycle, n then i guess you run the same code repeatedly n take the minimum with more repeatability as a consequence of that low jitter.

In the early nineties it was not so, you'd get the same number of clock cycles again n again n again.

N then it gets tricky because cycle counts are thresholds set within which, if voltages n frequency are proper, the operation will complete deterministically w a soft error rate no greater than the system as a whole, about one per 30 years of hammering on the chip at sea level. Which is not enough for my tastes, n the jitter is a fucking mess.

I much prefer the GA144, least jitter of any platform bar none, sensible because it is fully asynchronous logic, no clock anywhere in sight until you connect it to an oscillator, n even then the vibration doesn't rock the system like that of a grandfather clock synchs w another grandfather clock with whose pendulum's swing the former clock's pendulum swing is aligned. GA144 it's pretty easy to tell average case complexity of a function down to the tens of picoseconds, at which point you have to check that there's no open window letting a draft in. In fact the time trial will tell you such a draft is coming into the house, it happened to me, while not from a parallel universe in spacial dimensions it is by all means from another universe in the time dimension.

> I can't imagine it is actually nanoseconds

It's nanoseconds.

1? 10? 100? Those are all nanoseconds. When someone is asking about precision it tends to be a good thing to be precise in your answer.

You were similarly not precise in your skepticism. ;)

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