Timing Ada programs using the DEC UNIX microtimer kernel option

[dccoote@werple.mira.net.au (David Coote)]

I got all excited when I found out about the microtimer kernel option. Timing 
measurements we did with this option installed did seem to indicate improved 
resolution. Interestingly enough, the measured overhead of various POSIX 
timing calls seemed to improve. (Judging from how long the call took using 
other timing probes.)

But I'm not sure how this kernel option works. DEC USA support and what 
documentation exists tells me that you have a tick of 1024Hz on an AS500 and 
1200Hz on an AS4100. With the microtimer kernel option installed (quoting 
from a DEC support email)

"As for micro-timer resolution - the clock resolution remains the same - 
1/1024; however, any call to clock_gettime(3) is guaranteed to be 
monotonically increasing with a granularity of 1 microsecond. (i.e. the clock 
is still ticking at 1/1024, but clock_gettime(3) calls return values that are 
unique and APPEAR [DEC's emphasis] to have 1 microsecond resolution. This is 
useful for critical timestamping.)"

Well, how do they do this monotonic increase? If the appropriate parts of the 
kernel are getting tickled every 1024Hz, how is the kernel returning finer 
resolution between ticks?

Anyone know anything about this?

David