[ntp:hackers] FreeBSD and serial port cards

Hal Murray hmurray at megapathdsl.net
Tue Mar 6 02:19:50 PST 2007


[Context was a wild goose chase where I suggested the low latency stuff.]

> I don't believe that FreeBSD has such an option.  At least I couldn't
> find it in the sio sources.  What are you referring to? 

Sorry for dropping the ball on this.

Everybody talks about getting better results from refclocks that are 
connected with "low latency" mode so I assumed the NTP code magically did it 
whenever it can.

Normal serial chips have a small FIFO.  It's typically used to batch 
interrupts - at the cost of increased latency.  I think turning it off is 
just two lines of code in the right place in the kernel: test the bit and set 
the wakeup threshold on the FIFO down to 1.

Linux has a ASYNC_LOW_LATENCY flag.  The parse driver sets it.  
ntp_refclock.c doesn't.

What do FreeBSD geeks do if they want lower latency?


-- 
These are my opinions, not necessarily my employer's.  I hate spam.





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