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Registered Extension Number

185

Revision

1

Extension and Version Dependencies

  • Requires Vulkan 1.0

Other Extension Metadata

Last Modified Date

2018-10-04

IP Status

No known IP claims.

Contributors
  • Matthaeus G. Chajdas, AMD

  • Alan Harrison, AMD

  • Derrick Owens, AMD

  • Daniel Rakos, AMD

  • Jason Ekstrand, Intel

  • Keith Packard, Valve

Description

This extension provides an interface to query calibrated timestamps obtained quasi simultaneously from two time domains.

New Enums

New Enum Constants

  • VK_EXT_CALIBRATED_TIMESTAMPS_EXTENSION_NAME

  • VK_EXT_CALIBRATED_TIMESTAMPS_SPEC_VERSION

  • Extending VkStructureType:

    • VK_STRUCTURE_TYPE_CALIBRATED_TIMESTAMP_INFO_EXT

Issues

1) Is the device timestamp value returned in the same time domain as the timestamp values written by vkCmdWriteTimestamp?

RESOLVED: Yes.

2) What time domain is the host timestamp returned in?

RESOLVED: A query is provided to determine the calibrateable time domains. The expected host time domain used on Windows is that of QueryPerformanceCounter, and on Linux that of CLOCK_MONOTONIC.

3) Should we support other time domain combinations than just one host and the device time domain?

RESOLVED: Supporting that would need the application to query the set of supported time domains, while supporting only one host and the device time domain would only need a query for the host time domain type. The proposed API chooses the general approach for the sake of extensibility.

4) Shouldn’t we use CLOCK_MONOTONIC_RAW instead of CLOCK_MONOTONIC?

RESOLVED: CLOCK_MONOTONIC is usable in a wider set of situations, however, it is subject to NTP adjustments so some use cases may prefer CLOCK_MONOTONIC_RAW. Thus this extension allows both to be exposed.

5) How can the application extrapolate future device timestamp values from the calibrated timestamp value?

RESOLVED: VkPhysicalDeviceLimits::timestampPeriod makes it possible to calculate future device timestamps as follows:

futureTimestamp = calibratedTimestamp + deltaNanoseconds / timestampPeriod

6) Can the host and device timestamp values drift apart over longer periods of time?

RESOLVED: Yes, especially as some time domains by definition allow for that to happen (e.g. CLOCK_MONOTONIC is subject to NTP adjustments). Thus it’s recommended that applications re-calibrate from time to time.

7) Should we add a query for reporting the maximum deviation of the timestamp values returned by calibrated timestamp queries?

RESOLVED: A global query seems inappropriate and difficult to enforce. However, it’s possible to return the maximum deviation any single calibrated timestamp query can have by sampling one of the time domains twice as follows:

timestampX = timestampX_before = SampleTimeDomain(X)
for each time domain Y != X
    timestampY = SampleTimeDomain(Y)
timestampX_after = SampleTimeDomain(X)
maxDeviation = timestampX_after - timestampX_before

8) Can the maximum deviation reported ever be zero?

RESOLVED: Unless the tick of each clock corresponding to the set of time domains coincides and all clocks can literally be sampled simutaneously, there isn’t really a possibility for the maximum deviation to be zero, so by convention the maximum deviation is always at least the maximum of the length of the ticks of the set of time domains calibrated and thus can never be zero.

Version History

  • Revision 1, 2018-10-04 (Daniel Rakos)

    • Internal revisions.

See Also

Document Notes

For more information, see the Vulkan Specification

This page is a generated document. Fixes and changes should be made to the generator scripts, not directly.

Copyright (c) 2014-2020 Khronos Group. This work is licensed under a Creative Commons Attribution 4.0 International License.