Abstract
This specification defines a JavaScript interface that provides the current time
in sub-millisecond resolution and such that it is not subject to system clock skew or adjustments.
Status of this
document
This section describes the status of this document at the time of
its publication. Other documents may supersede this document. A list of
current W3C publications and the latest revision of this technical report
can be found in the W3C technical reports
index at http://www.w3.org/TR/.
This is the First Public Working Draft and Last Call Working Draft for the High Resolution Time Level 2 specification.
Please send comments
to public-web-perf@w3.org
(archived)
with [HighResolutionTime2] at the start of the subject line by 8 January 2014.
This document is produced by
the Web Performance
Working Group. The Web Performance Working Group is part of
the Rich Web Clients
Activity in the
W3C Interaction
Domain.
High Resolution Time Level 2 builds on the first version of High Resolution Time and includes:
Publication as a First Public and Last Call Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
This document was produced by a group operating under
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Table of Contents
- 1
Introduction
- 1.1 Examples
- 2
Conformance requirements
- 3 Terminology
- 4 High Resolution Time
- 4.1 Introduction
- 4.2 Time Origin
- 4.3 The
DOMHighResTimeStamp Type
- 4.4 The
performance.now() method
- 4.5 The
workerStart attribute
- 4.6 Monotonic Clock
- 4.7 Privacy and Security
- 5 References
- Acknowledgements
1 Introduction
This section is non-normative.
The ECMAScript Language Specification defines the Date object as a time value representing time in milliseconds since 01 January, 1970 UTC. For most purposes, this definition of time is sufficient as
these values represent time to millisecond precision for any instant that is within approximately 285,616 years from
01 January, 1970 UTC. The DOMTimeStamp is defined similarly.
In practice, these definitions of time are subject to both clock skew and adjustment of the system clock. The value of time may not always be monotonically increasing and subsequent values may either decrease
or remain the same.
For example, the following script may log a positive number, negative number, or zero.
var mark_start = Date.now();
doTask(); // Some task
if (window.console) window.console.log('Duration of task: ' + (Date.now() - mark_start));
For certain tasks this definition of time may not be sufficient as it does not allow for sub-millisecond resolution and is subject to system clock skew. For example,
-
When attempting to accurately measure the elapsed time of navigating to a Document, fetching of resources or execution of script, a monotonically increasing clock
with sub-millisecond resolution is desired.
-
When calculating the animation state from script, developers will need to accurately know the amount of time that has elapsed in the animation in order to properly update the
next scene of the animation.
-
When calculating the frame rate of a script based animation, developers will need sub-millisecond resolution in order to determine if an animation is drawing at 60 FPS.
Without sub-millisecond resolution, a developer can only determine if an animation is drawing at 58.8 FPS or 62.5 FPS.
-
In order to cue audio to a specific point in an animation or ensure that the audio is synchronized with the animation,
developers will need to accurately know the amount of time elapsed in the animation and audio.
This specification does not propose changing the behavior of Date.now() as it is genuinely useful in determining the current value of the calendar time and has a long history of
usage. The DOMHighResTimeStamp type and the now method of the
Performance interface resolve the issues summarized in this section by providing a monotonically increasing time value in sub-millisecond resolution.
1.1 Examples
This section is non-normative.
A developer may wish to construct a timeline of their entire application, including workers. With a dedicated
worker, this is easy. All
DOMHighResTimeStamps recorded in the worker use the same
time origin as the document.
Worker:
var mark_start = performance.now();
doTaskInWorker(); // Some task
var mark_end = performance.now();
postMessage({'task': 'Some worker task',
'start_time': mark_start,
'end_time': mark_end});
Document:
var mark_start = performance.now();
doTaskInDocument(); // Some other task
var mark_end = performance.now();
var document_entry = {'task': 'Some document task',
'start_time': mark_start,
'end_time': mark_end};
var worker = new Worker('js');
worker.onmessage = function (event) {
var worker_entry = event.data;
plotTimeline([worker_entry, document_entry]);
}
Constructing a timeline with SharedWorkers
is slightly more difficult. The time origin is different,
because a worker may be shared across multiple documents. In that case, a developer must adjust
DOMHighResTimeStamps before displaying them on
the same timeline. This can be done with the
workerStart attribute.
Document:
var mark_start = performance.now();
doTaskInDocument(); // Some other task
var mark_end = performance.now();
var document_entry = {'task': 'Some document task',
'start_time': mark_start,
'end_time': mark_end};
var worker = new SharedWorker('js');
worker.port.onmessage = function (event) {
var worker_entry = event.data;
worker_entry.start_time -= worker.workerStart;
worker_entry.end_time -= worker.workerStart;
plotTimeline([worker_entry, document_entry]);
}
Note that the SharedWorkerGlobalScope
doesn't have access to its workers' workerStart values. If needed, the document can send a message to
the worker containing its workerStart value.
All diagrams, examples, and notes in this specification are non-normative,
as are all sections explicitly marked non-normative. Everything else in this
specification is normative.
The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in the normative parts of this document
are to be interpreted as described in RFC 2119.
For readability, these words do not appear in all uppercase letters in this specification.
Some conformance requirements are phrased as requirements on attributes,
methods or objects. Such requirements are to be interpreted as requirements
on user agents.
The IDL fragments in this specification must be interpreted as
required for conforming IDL fragments, as described in the Web IDL
specification. [Web IDL]
3 Terminology
The construction "a Foo object", where Foo is actually an interface, is sometimes used instead of
the more accurate "an object implementing the interface Foo".
The term "JavaScript" is used to refer to
ECMA-262,
rather than the official term ECMAScript, since the term JavaScript is more widely known.
4 High Resolution Time
4.1 Introduction
This section is non-normative.
This specification defines an interface that provides the current time
in sub-millisecond resolution and such that it is not subject to system clock skew or adjustments.
4.2 Time Origin
The time origin is the time value from which time is measured. The time origin must be equal to the time of the start of
navigation of the current document.
For a dedicated worker,
the time origin must be equal to the time of the start of navigation
of the document where it was created.
For a shared worker,
the time origin must be equal to the time of creation of the shared worker.
4.3 The DOMHighResTimeStamp Type
The DOMHighResTimeStamp type is used to store a time value measured relative from the
time origin or a time value that represents a duration
between two DOMHighResTimeStamps.
-
Type Definition DOMHighResTimeStamp
-
A DOMHighResTimeStamp SHOULD represent a time in milliseconds accurate to a microsecond.
If the User Agent is unable to provide a time value accurate to a microsecond due to hardware or software constraints, the User Agent
can represent a DOMHighResTimeStamp as a time in milliseconds accurate to a millisecond.
IDL Definition-
typedef double DOMHighResTimeStamp;
4.4 The performance.now() method
partial interface Performance {
DOMHighResTimeStamp now();
};
interface WorkerPerformance {
DOMHighResTimeStamp now();
};
partial interface WorkerGlobalScope {
readonly attribute WorkerPerformance performance;
};
now method
The now method MUST
return a DOMHighResTimeStamp representing the time in milliseconds
from the time origin to the occurrence of the call to the now method.
4.5 The workerStart attribute
partial interface SharedWorker {
readonly attribute DOMHighResTimeStamp workerStart;
};
workerStart attribute
The workerStart attribute MUST
return a DOMHighResTimeStamp representing the difference between
the time origin of the
SharedWorkerGlobalScope
associated with the SharedWorker
and the time origin of the current document.
4.6 Monotonic Clock
The time values returned when calling the now method MUST be monotonically increasing and not subject to system clock
adjustments or system clock skew. The difference between any two chronologically recorded time values returned from the
now method MUST never be negative.
4.7 Privacy and Security
Statistical fingerprinting is a privacy concern where a malicious web site may determine whether a user has visited a
third-party web site by measuring the timing of cache hits and misses of resources in the third-party web site.
Though the now method of the Performance interface
returns time data to a greater accuracy than before, it does not make this privacy concern significantly worse than it was already.
5 References
5.1 Normative References
- [IETF RFC 2119]
-
Key words for use in RFCs to Indicate Requirement Levels,
Scott Bradner, Author. Internet Engineering Task Force, March 1997. Available at
http://www.ietf.org/rfc/rfc2119.txt.
- [ECMA-262]
-
ECMAScript Language Specification, 5.1
Edition. ECMA International, Standard
ECMA-262, June 2011. This version of the ECMAScript
Language is available from http://www.ecma-international.org/publications/standards/Ecma-262.htm.
- [High Resolution Time]
- High Resolution Time, Jatinder Mann, Editor. World Wide Web Consortium, December 2012. This version of the High Resolution Time Recommendation is available from http://www.w3.org/TR/2012/REC-hr-time-20121217/. The latest version of High Resolution Time is available at http://www.w3.org/TR/hr-time/.
- [Navigation Timing]
- Navigation Timing, Zhiheng Wang, Editor. World Wide Web Consortium, December 2012. This version of the Navigation Timing Recommendation is available from http://www.w3.org/TR/2012/REC-navigation-timing-20121217/. The latest version of Navigation Timing is available at http://www.w3.org/TR/navigation-timing/.
- [Web IDL]
- Web
IDL, Cameron McCormack, Editor. World Wide Web
Consortium, April 2012. This version of the Web IDL
specification is available from
http://www.w3.org/TR/2012/CR-WebIDL-20120419/.
The latest version of Web
IDL is available at http://www.w3.org/TR/WebIDL/.
- [Web Workers]
- Web Workers, Ian Hickson, Editor. World Wide Web Consortium, May 2012. This version of the Web Workers specification is available from http://www.w3.org/TR/2012/CR-workers-20120501/. The latest version of Web Workers is available at http://www.w3.org/TR/workers/.
- [HTML5]
- HTML5, Robin Berjon et al., Editors. World Wide Web Consortium, August 2013. This version of the HTML5 is available from http://www.w3.org/TR/html5/. The latest editor's draft of HTML5 is available at http://www.w3.org/html/wg/drafts/html/CR/.
- [Page Visibility]
-
Page Visibility, Jatinder Mann and Arvind Jain, Editors. World Wide Web Consortium, October 2013.
This version of the Page Visibility Recommendation is available from http://www.w3.org/TR/2013/REC-page-visibility-20131029/. The latest version of Page Visibility is available at http://www.w3.org/TR/page-visibility/.
Acknowledgements
I would like to sincerely thank Karen Anderson, Nat Duca, Tony Gentilcore, Arvind Jain, and Jason Weber
to acknowledge their contributions to this work.