[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 
Timestamp
raise parallel port out line
get and process interrupt and deliver to kernel interrupt processing
module
Timestamp
 (The out line is connected to the parallel port interrupt control line)




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