Frame Timing (original) (raw)

Abstract

This specification defines an interface to help web developers measure the performance of their applications by giving them access to frame performance data to facilitate smoothness (i.e. Frames per Second and Time to Frame) measurements.

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 a work in progress and may change without any notices.

This document was published by the Web Performance Working Group as a First Public Working Draft. This document is intended to become a W3C Recommendation. If you wish to make comments regarding this document, please send them topublic-web-perf@w3.org (subscribe,archives) with [Frame Timing] at the start of your email's subject. All comments are welcome.

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Table of Contents

1. Introduction

This section is non-normative.

Web developers need the ability to assess and understand the performance characteristics of their applications. While JavaScript [ECMA-262] provides a mechanism to approximate application Frames per Second (FPS) and Time to Frame (TTF) (by calling a nearly empty requestAnimationFrame [Animation-Timing] in a loop), the precision of this method has a high variance. Some example situations where this returns misleading results are:

This document defines the PerformanceRenderTiming and PerformanceCompositeTiming interfaces, and extensions to the Performance interface, which expose frame performance data to be used for smoothness measurements.

2. Conformance

As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.

The key words MAY, MUST, and SHOULD are to be interpreted as described in [RFC2119].

Requirements phrased in the imperative as part of algorithms (such as "strip any leading space characters" or "return false and abort these steps") are to be interpreted with the meaning of the key word ("MUST", "SHOULD", "MAY", etc) used in introducing the algorithm.

Some conformance requirements are phrased as requirements on attributes, methods or objects. Such requirements are to be interpreted as requirements on user agents.

Conformance requirements phrased as algorithms or specific steps may be implemented in any manner, so long as the end result is equivalent. (In particular, the algorithms defined in this specification are intended to be easy to follow, and not intended to be performant.)

The IDL fragments in this specification MUST be interpreted as required for conforming IDL fragments, as described in the Web IDL specification. [WebIDL]

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 ECMAScript [ECMA-262], rather than the official term ECMAScript, since the term JavaScript is more widely known.

4.1 Introduction

This section is non-normative.

The PerformanceRenderTiming and PerformanceCompositeTiming interfaces, and extensions to the Performance interface give web developers access to browser smoothness information so they can better measure the performance characteristics of their applications.

4.2 Extensions to the Performance Interface

partial interface Performance { void clearFrameTimings (); void setFrameTimingBufferSize (unsigned long maxSize); attribute EventHandler onframetimingbufferfull; };

4.2.1 Attributes

onframetimingbufferfull of type EventHandler,

The event handler for the frametimingbufferfull event. Immediately after the buffer used to store the list of PerformanceRenderTiming and PerformanceCompositeTiming resources becomes full, the User Agent MUST fire a simple event named frametimingbufferfull that bubbles, isn't cancelable, has no default action, at thePerformance object [Navigation-Timing-2].

4.2.2 Methods

clearFrameTimings

The method clearFrameTimings clears the buffer used to store the current list of PerformanceRenderTiming resources.

No parameters.

Return type: void

setFrameTimingBufferSize

The setFrameTimingBufferSize method, when invoked, MUST set the maximum number of PerformanceRenderTiming resources that may be stored in the buffer to the value of the maxSize parameter.

Parameter Type Nullable Optional Description
maxSize unsigned long ✘ ✘ The maxSize parameter sets the maximum number of PerformanceRenderTiming resources that will be stored in the buffer.

Return type: void

4.3 The PerformanceRenderTiming Interface

The PerformanceRenderTiming interface participates in the Performance Timeline and extends the following attributes of the PerformanceEntry interface [Performance-Timeline]:

name

This attribute MUST return the name of the requesting document.

entryType

This attribute MUST return the DOMString "render".

startTime

This attribute MUST return a DOMHighResTimeStamp with current frame's beginning time value, which is the time that the requestAnimationFrame callback (see [Animation-Timing]) would have fired for the frame. [HR-Time].

duration

This attribute MUST return a DOMHighResTimeStamp with the difference between the end of the frame and the startTime of the frame. The end of the frame is defined as what the beginning time of the next frame's requestAnimationFrame callback (see [Animation-Timing]) would have been, had one been scheduled in the current frame. If a requestAnimationFrame is already requested, its beginning time SHOULD be used.

interface PerformanceRenderTiming : PerformanceEntry { readonly attribute unsigned long sourceFrameNumber; };

4.3.1 Attributes

This attribute MUST return the source frame number for the event. This is used to correlate PerformanceRenderTiming and PerformanceCompositeTiming events to measure time-to-frame delays.

4.4 The PerformanceCompositeTiming Interface

The PerformanceCompositeTiming interface participates in the Performance Timeline and extends the following attributes of the PerformanceEntry interface [Performance-Timeline]:

name

This attribute MUST return the name of the requesting document.

entryType

This attribute MUST return the DOMString "composite".

startTime

This attribute MUST return a DOMHighResTimeStamp with the event's time value [HR-Time].

duration

The duration attribute MUST return a DOMHighResTimeStamp of value 0.

interface PerformanceCompositeTiming : PerformanceEntry { readonly attribute unsigned long sourceFrameNumber; };

4.4.1 Attributes

This attribute MUST return the last known source frame number for the event. This is used to correlate PerformanceRenderTiming and PerformanceCompositeTiming events to measure time-to-frame delays.

5. Monotonic Clock

The time values stored within the interface MUST monotonically increase to ensure they are not affected by adjustments to the system clock. The difference between any two chronologically recorded time values MUST never be negative. The user agent MUST record the system clock at the beginning of the navigation and define subsequent time values in terms of a monotonic clock measuring time elapsed from the beginning of the navigation.

6. Privacy and Security

This section is non-normative.

The interfaces defined in this specification expose potentially sensitive timing information on specific JavaScript activity of a page. However, unlike other interfaces defined in the Performance Timeline, the interfaces defined in this specification do not have any restrictions on sharing timing information through script. This is because the web platform has been designed with the invariant that any script included on a page has the same access as any other script included on that page regardless of the origin of the script.

A. Acknowledgements

Thanks to Enne Walker, Rick Byers and Chris Harrelson for their helpful comments.

B. References

B.1 Normative references

[Animation-Timing]

James Robinson; Cameron McCormack. Timing control for script-based animations. 31 October 2013. W3C Candidate Recommendation. URL: http://www.w3.org/TR/animation-timing/

[HR-Time]

Jatinder Mann. High Resolution Time. 17 December 2012. W3C Recommendation. URL: http://www.w3.org/TR/hr-time/

[Navigation-Timing-2]

Jatinder Mann; Tobin Titus; Arvind Jain. Navigation Timing 2. 20 January 2015. W3C Working Draft. URL: http://www.w3.org/TR/navigation-timing-2/

[Performance-Timeline]

Jatinder Mann; Zhiheng Wang. Performance Timeline. 12 December 2013. W3C Recommendation. URL: http://www.w3.org/TR/performance-timeline/

[RFC2119]

S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119

[WebIDL]

Cameron McCormack. Web IDL. 19 April 2012. W3C Candidate Recommendation. URL: http://www.w3.org/TR/WebIDL/

B.2 Informative references

[ECMA-262]

ECMAScript Language Specification, Edition 5.1. June 2011. URL: http://www.ecma-international.org/publications/standards/Ecma-262.htm