[ntp:questions] Which version of Linux works best?

unruh unruh at wormhole.physics.ubc.ca
Fri Mar 12 00:14:51 UTC 2010

On 2010-03-11, Hal Murray <hal-usenet at ip-64-139-1-69.sjc.megapath.net> wrote:
>>> Modern Linux kernels don't support PPS in the sense of RFC-whateveritis.
>>> There is support for an ioctl that says "wake me up when a modem signal changes".
>>> gpsd uses that to provide PPS support.  I don't have any data.
>>I believe but am not sure, that that uses an interrupt.
> I think so.  But the point is that with the PPS support, the
> kernel grabs a timestamp in the interrupt routine.  The ioctl

So? The interrupt still takes the same time to be activated. On a GHZ
system, there is enough time in 1usec to run 1000 commands, and it is
hard to imagine that many being used to return the ioctl. I have worried
about that and it would be nice if someone ran the system such that say
one timed when the out pin on the parallel port was activated, and the
time that the serial port ioctl returned. I know on my parallel port
interrupt, the test I ran showed that the time between activating the
pin on the parallel port and the parallel port interrupt service routine
timestamping the interrupt was of the order of 1usec. It would be nice
to see what it is for the serial port. I doubt it is much more than

> stuff just wakes up the user program so it can grab the timestamp.
> On a lightly loaded system, that will probably work OK.  But if
> the system gets busy, there will be more noise in the data.

If the system gets busy, the interrupts themselves will have delays as
well, putting noise in the system. One of the reasons I used the
parallel port was because the parallel interrupt has a higher priority
than the serial (comes earlier in the interrupt chain). 
But measurements would be great. 


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