[Tinyos-host-mote-wg] Re: [Tinyos-2.0wg] hardware latency

[Philip Levis]

On Wed, 2005-09-07 at 13:56 -0700, Phil Buonadonna wrote:
> I'm still trying to get my head around this:
> 
> 1) what latency are we refering to? Startup latency? Other? From the notes,
> the reference was to WakeupTime which doesn't refer to "How long does it
> take to wake up" but rather "When I need to wakeup".
> 
> 2) From the meeting a couple of weeks ago, there was an action to "come up
> with a simple-use case where hw component could not come up with correct
> statement as to what power state it needs to be in; and app would have to
> dictate."
> A real simple one here would be the radio. The radio component has no idea
> if it needs to remain on to receive packets... the app has to dictate that. 
> 
> Or does this refer to some micro-scale kind of anticipation. 


Startup latency. That is, the time it takes the processor to return to
operation from its sleep mode. Lama's concern was that for some CPU
sleep states, this can be significant (5ms) and therefore disrupt
operation. Components (let's not use the term app, it is really
misleading for this discussion) may have timing requirements that need
to constrain this latency; this is information that is not expressed in
the hardware configuration. Therefore, the PM system needs to have a
mechanism for components in the system to specify their latency
constraints. 

The term "latency constraints" makes it sound like a much bigger deal
than it is. Really, what it comes down to is you have a component that
keeps track of all of the latency requirements (e.g., stores a value for
each client). All it does is give the PM system the tightest requirement
(e.g., smallest latency).

A fictional example:

Clock can tolerate 5 ms latency in its next interrupt
Radio can tolerate 1 ms latency for its TDMA
Closed loop control system can tolerate 80us latency for actuation

So the manager tells PM "don't go into an LP state that would impose a
latency greater than 80us."

Phil

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