Date.prototype.getTime

Brendan Eich brendan at mozilla.org
Wed Jun 28 14:14:16 PDT 2006


On Jun 28, 2006, at 1:39 PM, P T Withington wrote:

> On 2006-06-28, at 15:47 EDT, Brendan Eich wrote:
>
>>> Date.tick:  getter returning an integer that increases linearly  
>>> with elapsed time and is guaranteed to increase by at least 1  
>>> when accessed successively.  (I.e., can be used to measure the  
>>> elapsed time of a trivial expression.)
>>>
>>> Date.tickScale:  float that is the ratio of Date.tick units to  
>>> nanoseconds.
>>
>> That's not bad, but why not just give 'em nanoseconds since the  
>> epoch so they don't have to normalize.
>
> a) For when nanoseconds is too coarse?  tickScale could be to any  
> convenient unit, seconds would be fine too.

People want finer granularity when benchmarking.  In the interest of  
future-proofing, we went to ns.   To avoid worrying about wrapping  
and races, we opted to make nanotime return the whole enchilada.

> b) So the language implementor can choose a scale that fits their  
> implementation:  fine enough to be able to time the simplest  
> expression accurately, but not so fine as to cause huge overhead in  
> the implementation.

This is a good point.

> I think the applications for Date and 'tick' are different.  Date  
> needs to be relative to an epoch because people use it for absolute  
> times.  'tick' does not.

Usually not, agreed.

>   It is for very accurate measurement of intervals.  It can start  
> with an arbitrary value.  It can wrap.

I continue to think that wrapping is problematic.

>   From the size of integers and the scale, you can determine the  
> maximum interval you can measure, use modular arithmetic, and use  
> Date to compensate if you really need that granularity for a longer  
> interval (I believe such applications will be rare).

There's a race if you don't have an atomic sample of the coarser and  
the finer unit clocks.

> I even think that having an accurate, fine-grained clock as a  
> primitive is important enough that I would give up synchronizing it  
> with Date for greater accuracy.  If computing [ milliseconds,  
> nanoseconds ] takes more time that the expression I am trying to  
> measure, I have a problem.

Agreed already! ;-)

> [In case you haven't guessed, I am writing a Javascript profiler in  
> Javascript and am being thwarted by the lack of granularity in  
> Date.  And, I didn't invent the tick/scale interface.  I stole it  
> from the DEC Alpha which has a control register that increments at  
> the CPU clock frequency and a register that tells you what that  
> frequency is.  This register is readable in user space, so makes a  
> really fine profiling instrument.]

I thought this was familiar :-).  I will take your proposal, noodle  
on it, and wiki something to get TG1 buy-in.  Thanks,

/be




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