Re: [gpsd-dev] RFC 2783

[Hal Murray]

>> USB is polled.  ...
> No, USB 1.1 is scheduled, at least on the PL2303.  1024 slots.  So we see in
> practice about +/1 600 uSec on the GR-601W. 

We are saying the same thing, just using different words.

The key idea is that the polling is using a periodic schedule.  There is no 
interrupt mechanism.  The closest equivalent of interrupt latency is polling 
interval.


> That may seem poor, but is way better than I get with just NTP over my local
> GigE net. 

Most GigE drivers batch interrupts to reduce CPU load.  What's your round 
trip time?  Mine is typically 200 microseconds.

For the Raspberry Pi, the Ethernet is on USB so there is more fuzz there.  I 
assume it's polled more frequently than 1 ms but I don't have numbers handy.


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