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RE: [HighResolutionTime] comments on performance.now

From: Jatinder Mann <jmann@microsoft.com>
Date: Tue, 20 Mar 2012 22:21:47 +0000
To: Simon Pieters <simonp@opera.com>, "public-web-perf@w3.org" <public-web-perf@w3.org>
Message-ID: <EE4C13A1D11CFA49A58343DE361B0B04176CDDA6@TK5EX14MBXC254.redmond.corp.microsoft.com>
> I think it's ridiculous to have a spec that defines a single method. It would make sense to fold this into the spec that defines the Performance
> interface:  http://dvcs.w3.org/hg/webperf/raw-file/tip/specs/NavigationTiming/Overview.html


The High Resolution Time spec defines the DOMHighResTimeStamp type and performance.now() method. As other specifications may be interested in using a DOMHighResTimeStamp type (e.g., requestAnimationFrame) but not interested in Navigation Timing, having a separate spec makes sense. In addition, not only is Navigation Timing in CR (waiting to go to PR), it doesn't even use high resolution time. 

We have already defined extensions to the Performance interface in many specifications.

>The spec says that the timer is accurate to at least a tenth of a millisecond, there's hardware 
>that doesn't support that accuracy. It should say to be as accurate as possible, but is not required 
>to be more accurate than a tenth of a millisecond.

If High Resolution Time doesn't require at least a tenth of a millisecond, it won't be providing any higher resolution than Date.now(), one of the goals of this feature. I believe all latest Windows (since '95 I believe), iOS, and Android hardware support sub-millisecond resolution clocks; what hardware were you thinking of?

If we think there is enough new hardware that won't be able to support sub-millisecond resolution, I am fine changing the MUST clause to a SHOULD clause.

> The spec should call out whether the clock should be ticking while the document is not "fully active" (as defined in HTML).

To be clear, you are bringing this up because you want to know if performance.now() includes the "time of suspension" when a page is in the session history? I don't understand why we wouldn't want to include the time of suspension - performance.now() defines the current time since the start of navigation. Let me know if I misunderstood your intention here.

Thanks,
Jatinder

Received on Tuesday, 20 March 2012 22:22:23 UTC

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