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

unruh unruh at wormhole.physics.ubc.ca
Fri Mar 12 16:40:53 UTC 2010

On 2010-03-12, David Woolley <david at ex.djwhome.demon.invalid> wrote:
> unruh wrote:
>> 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
> That's 1000 machine cycles, not 1000 instructions.  On modern systems, 

And since most modern processors are pipelined and parallelized it may
mean more than 1000 instructions. 

> I'm not sure that 1000 cycles isn't a typical time for a system call on 
> modern, high level language progammed, bloatware.  (I seem to remember 
> hand coding an ISR in assembler to a budget of 100 instructions (for 
> 68000) and it not being that easy.)

No idea, which is why I would love to see tests to see how long it takes
the serial port to respond. I know the parallel port takes something
like 1 -2 usec between 
raise parallel port out line
get and process interrupt and deliver to kernel interrupt processing
 (The out line is connected to the parallel port interrupt control line)

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