User Timing

Editor's Draft July 11, 2012

This version:
http://dvcs.w3.org/hg/webperf/raw-file/tip/specs/UserTiming/Overview.html
Latest version:
http://dvcs.w3.org/hg/webperf/raw-file/tip/specs/UserTiming/Overview.html
Editors:
Jatinder Mann, Microsoft Corp.,
Zhiheng Wang, Google Inc. (Until July 2013)
Anderson Quach, Microsoft Corp. (Until March 2011)

Abstract

This specification defines an interface to help web developers measure the performance of their applications by giving them access to high precision timestamps.

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.

Please send comments to public-web-perf@w3.org (archived) with [UserTiming] at the start of the subject line.

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.

Publication as a 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 the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

Table of Contents

  1. 1 Introduction
  2. 2 Conformance requirements
  3. 3 Terminology
  4. 4 User Timing
    1. 4.1 Introduction
    2. 4.2 Extensions to the Performance Interface
    3. 4.3 The PerformanceMark Interface
    4. 4.4 The PerformanceMeasure Interface
    5. 4.5 Vendor Prefixes
  5. 5 Monotonic Clock
  6. 6 Privacy and Security
  7. Acknowledgements
  8. References

1 Introduction

This section is non-normative.

Web developers need the ability to assess and understand the performance characteristics of their applications. While JavaScript [ECMA262] provides a mechanism to measure application latency (retrieving the current timestamp from the Date.now() method), the precision of this timestamp varies between user agents.

This document defines the PerformanceMark and PerformanceMeasure interfaces, and extensions to the Performance interface, which expose a high precision timestamp to developers so they can better measure the performance of their applications.

The following script shows how a developer can use the interfaces defined in this document to obtain timing data related to developer scripts.

    <!doctype html>
    <html>
      <head>
        <title>User Timing example</title>
      </head>
      <body onload="init()">
        <script>
           function init() 
           {
                performance.mark("startTask1");
                doTask1(); // Some developer code
                performance.mark("endTask1");
                
                performance.mark("startTask2");
                doTask2(); // Some developer code
                performance.mark("endTask2");

                measurePerf();
           }

           function measurePerf() 
           {
               var perfEntries = performance.getEntriesByType("mark");
               for (var i = 0; i < perfEntries.length; i++)
               {
                     if (window.console) console.log("Name: "        + perfEntries[i].name      + 
                                                     " Entry Type: " + perfEntries[i].entryType +
                                                     " Start Time: " + perfEntries[i].startTime + 
                                                     " Duration: "   + perfEntries[i].duration  + "\n");
               }
           }
        </script>
      </body>
    </html>
    

2 Conformance requirements

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 RFC2119. For readability, these words do not appear in all uppercase letters in this specification. [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. [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 ECMAScript [ECMA262], rather than the official term ECMAScript, since the term JavaScript is more widely known.

Throughout this work, all time values are measured in milliseconds since the start of navigation of the document. For example, the start of navigation of the document occurs at time 0. The term current time refers to the number of milliseconds since the start of navigation of the document until the current moment in time. This definition of time is based on the High Resolution Time specification [High Resolution Time] and is different from the definition of time used in the Navigation Timing specification [Navigation Timing], where time is measured in milliseconds since midnight of January 1, 1970 (UTC).

4 User Timing

4.1 Introduction

This section is non-normative

The PerformanceMark and PerformanceMeasure interfaces, and extensions to the Performance interface, give web developers access to a high precision, monotonically increasing timestamp so they can better measure the performance characteristics of their applications.

4.2 Extensions to the Performance Interface

  partial interface Performance {
    void mark(DOMString markName);
    void clearMarks(optional  DOMString markName);

    void measure(DOMString measureName, optional DOMString startMark, optional DOMString endMark);
    void clearMeasures(optional DOMString measureName);
  };
    

mark method

This method stores a timestamp with the associated name (a "mark").

Parameters

in markName type of DOMString

The name associated with the timestamp.

Mark names may be re-used within the same document. Each call to the mark() method stores a new timestamp under the specified mark name.

The mark name cannot be the same name as any of the attributes in the PerformanceTiming interface [Navigation Timing].

Recommended Mark Names

Developers are encouraged to use the following Recommended Mark Names to mark common interactions. The user agent is responsible for storing a new timestamp under the specified mark name for Recommended Mark Names, just like any user specified mark name. The user agent does not validate that the usage of the Recommended Mark Name is appropriate or consistent with its description.

"mark_fully_loaded"

The time when the page is considered fully loaded as marked by the developer in their application.

"mark_fully_visible"

The time when the page is considered completely visible to an end-user as marked by the developer in their application.

"mark_above_the_fold"

The time when all of the content in the visible viewport has been presented to the end-user as marked by the developer in their application.

"mark_time_to_user_action"

The time of the first user interaction with the page during or after a navigation, such as scroll or click, as marked by the developer in their application.

No Return Value

Exceptions

Throws a SYNTAX_ERR exception if the markName argument is the same name as an attribute in the PerformanceTiming interface.

clearMarks method

If the markName argument is not specified, this method removes all marks and their associated DOMHighResTimeStamp time values.

If the markName argument is specified, this method removes all DOMHighResTimeStamp time values for the given mark name.

If the markName argument is specified but the specified markName does not exist, this method will do nothing.

Parameters

in markName type of DOMString

[optional] The name of the mark whose DOMHighResTimeStamp time values should be cleared. If not specified, all marks will be cleared.

No Return Value

No Exceptions

measure method

This method stores the DOMHighResTimeStamp duration between two marks along with the associated name (a "measure").

The behavior of this method depends on which arguments are specified:

The startMark and endMark arguments may be the name of one of the attributes in the PerformanceTiming interface [Navigation Timing]. In this case, the value of that attribute is used as the DOMHighResTimeStamp time value.

Parameters

in name type of DOMString

The name associated with the measure.

Measure names may be re-used within the same document. Each call to the measure() method stores a new duration under the specified measure name.

Measure names live independently from mark names.

in startMark type of DOMString

[optional] The name of the start mark.

If specified, the most recent DOMHighResTimeStamp time value of the start mark is used.

If not specified, navigationStart is used.

May be the name of one of the attributes in the PerformanceTiming interface [Navigation Timing]. In this case, the value of that attribute is used as the start DOMHighResTimeStamp time value.

in endMark type of DOMString

[optional] The name of the end mark.

If specified, the most recent DOMHighResTimeStamp time value of the end mark is used.

If not specified, the current time as a DOMHighResTimeStamp is used.

May be the name of one of the attributes in the PerformanceTiming interface [Navigation Timing]. In this case, the value of that attribute is used as the end DOMHighResTimeStamp time value.

No Return Value

Exceptions

Throws a SYNTAX_ERR exception if the start mark or end mark does not exist.

Throws an INVALID_ACCESS_ERR exception if either startMark or endMark argument, or both, have the same name as a PerformanceTiming attribute with a time value of 0.

clearMeasures method

If the measureName argument is not specified, this method removes all measures and their associated DOMHighResTimeStamp durations.

If the measureName argument is specified, this method removes all DOMHighResTimeStamp durations for the given measure name.

If the measureName argument is specified but the specified measureName does not exist, this method will do nothing.

Parameters

in measureName type of DOMString

[optional] The name of the measure whose DOMHighResTimeStamp durations should be cleared. If not specified, all measures will be cleared.

No Return Value

No Exceptions

4.3 The PerformanceMark Interface

  interface PerformanceMark : PerformanceEntry {
  };
    

The PerformanceMark interface also exposes marks created via the mark() method to the Performance Timeline [Performance Timeline]. The PerformanceMark interface extends the following attributes of the PerformanceEntry interface:

The name attribute will return the mark's name.

The entryType attribute will return the DOMString mark.

The startTime attribute will return a DOMHighResTimeStamp with the mark's time value [High Resolution Time].

The duration attribute will return a DOMHighResTimeStamp of value 0.

4.4 The PerformanceMeasure Interface

  interface PerformanceMeasure : PerformanceEntry {
  };
    

The PerformanceMeasure interface also exposes measures created via the measure() method to the Performance Timeline [Performance Timeline]. The PerformanceMeasure interface extends the following attributes of the PerformanceEntry interface:

The name attribute will return the measure's name.

The entryType attribute will return the DOMString measure.

The startTime attribute will return a DOMHighResTimeStamp with the measure's start mark [High Resolution Time].

The duration attribute will return a DOMHighResTimeStamp with the duration of the measure.

4.5 Vendor Prefixes

Vendor-specific proprietary user agent extensions are discouraged. If such extensions are needed, e.g. for experimental purposes, vendors must use the following extension mechanisms:

If the extension to be added is a Standard Mark Name, the Standard Mark Name must:

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.

Acknowledgements

Thanks to Karen Anderson, Tony Gentilcore, Nic Jansma, James Simonsen, Steve Souders, Sigbjorn Vik, and Jason Weber for their useful comments that led to changes to this specification and their contributions to this work.

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.
[Navigation Timing]
Navigation Timing, Zhiheng Wang, Editor. World Wide Web Consortium, July 2012. This version of the Navigation Timing specification is available from http://www.w3.org/TR/2012/PR-navigation-timing-20120726/. The latest version of Navigation Timing is available at http://www.w3.org/TR/navigation-timing/.
[Performance Timeline]
Performance Timeline, Jatinder Mann, et al, Editors. World Wide Web Consortium, July 2012. This version of the Performance Timeline specification is available from http://www.w3.org/TR/2012/WD-performance-timeline-20120726/. The latest version of Performance Timeline is available at http://www.w3.org/TR/performance-timeline/.
[High Resolution Time]
High Resolution Time, Jatinder Mann, Editor. World Wide Web Consortium, May 2012. This version of the High Resolution Time specification is available from http://www.w3.org/TR/2012/CR-hr-time-20120522/. The latest version of High Resolution Time is available at http://www.w3.org/TR/hr-time/.
[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/.