[PATCH] vulkan: Add VK_EXT_calibrated_timestamps extension (radv and anv) [v4]

[Jason Ekstrand]

On Tue, Oct 16, 2018 at 4:07 PM Keith Packard <keithp at keithp.com> wrote:

> Jason Ekstrand <jason at jlekstrand.net> writes:
>
> > I think what Bas is getting at is that there are two problems:
> >
> >  1) We are not sampling at exactly the same time
> >  2) The two clocks may not tick at exactly the same time.
>
> Thanks for the clarification.
>
> > If we want to be conservative, I suspect Bas may be right that adding is
> > the safer thing to do.
>
> Yes, it's certainly safe to increase the value of
> maxDeviation. Currently, the time it takes to sample all of the clocks
> is far larger than the GPU tick, so adding that in would not have a huge
> impact on the value returned to the application.
>
> I'd like to dig in a little further and actually understand if the
> current computation (which is derived directly from the Vulkan spec) is
> wrong, and if so, whether the spec needs to be adjusted.
>
> I think the question is what 'maxDeviation' is supposed to
> represent. All the spec says is:
>
>  * pMaxDeviation is a pointer to a 64-bit unsigned integer value in
>    which the strictly positive maximum deviation, in nanoseconds, of the
>    calibrated timestamp values is returned.
>
> I interpret this as the maximum error in sampling the individual clocks,
> which is to say that the clock values are guaranteed to have been
> sampled within this interval of each other.
>
> So, if we have a monotonic clock and GPU clock:
>
>           0 1 2 3 4 5 6 7 8 9 a b c d e f
> Monotonic -_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-
>
>           0         1         2         3
> GPU       -----_____-----_____-----_____-----_____
>
>
> gpu_period in this case is 5 ticks of the monotonic clock.
>
> Now, I perform three operations:
>
>         start = read(monotonic)
>         gpu   = read(GPU)
>         end   = read(monotonic)
>
> Let's say that:
>
>         start = 2
>         GPU = 1 * 5 = 5 monotonic equivalent ticks
>         end = b
>
> So, the question is only how large the error between GPU and start could
> possibly be. Certainly the GPU clock was sampled some time between
> when monotonic tick 2 started and monotonic tick b ended. But, we have
> no idea what phase the GPU clock was in when sampled.
>
> Let's imagine we manage to sample the GPU clock immediately after the
> first monotonic sample. I'll shift the offset of the monotonic and GPU
> clocks to retain the same values (start = 2, GPU = 1), but now
> the GPU clock is being sampled immediately after monotonic time 2:
>
>                 w x y z 0 1 2 3 4 5 6 7 8 9 a b c d e f
> Monotonic       -_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-
>
>           0         1         2         3
> GPU       -----_____-----_____-----_____-----_____
>
>
> In this case, the GPU tick started at monotonic time y, nearly 5
> monotonic ticks earlier than the measured monotonic time, so the
> deviation between GPU and monotonic would be 5 ticks.
>
> If we sample the GPU clock immediately before the second monotonic
> sample, then that GPU tick either starts earlier than the range, in
> which case the above evaluation holds, or the GPU tick is entirely
> contained within the range:
>
>           0 1 2 3 4 5 6 7 8 9 a b c d e f
> Monotonic -_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-
>
>            z         0         1         2         3
> GPU      __-----_____-----_____-----_____-----_____-----
>
> In this case, the deviation between the first monotonic sample (that
> returned to the application as the monotonic time) and the GPU sample is
> the whole interval of measurement (b - 2).
>
> I think I've just managed to convince myself that Jason's first
> suggestion (max2(sample interval, gpu interval)) is correct, although I
> think we should add '1' to the interval to account for sampling phase
> errors in the monotonic clock. As that's measured in ns, and I'm
> currently getting values in the µs range, that's a small error in
> comparison.
>

You've got me almost convinced as well.  Thanks for the diagrams!  I think
instead of adding 1 perhaps what we want is

max2(sample_interval_ns, gpu_tick_ns + monotonic_tick_ns)

Where monotonic_tick_ns is maybe as low as 1.  Am I following you correctly?

--Jason
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