Re: accuracy

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Sun, 11 Mar 2007 18:38:58 +0100, Pascal Obry wrote:

>> I.e. it does this only once. The potential problems with this is that:
> 
> No it is not does once but each time a clock is requested for adjustment
> if needed.

You mean each time Clock is called? That would be a very expensive
implementation. A periodic task would be less demanding. But ideally it
should be something sitting close to the PCI bus.

>> 1. The accuracy of GetSystemTimeAsFileTime is very low.
> 
> Yes but it is not used for final clock returned value.

How so? Ada.Calendar has Split and Time_Of. Any error in
GetSystemTimeAsFileTime will show itself there, if not compensated using
some digital filtering technique.

>> 2. All system calls have non-zero latencies.
>> 
>> 3. There is a chance that the thread will be preempted between querying the
>> counter and calling to a system time query, which would additionally
>> increase the experienced latency up to milliseconds.
> 
> During initialization of the runtime the performance counter and the
> system time are read until this is done during a minimal amount of time.
> This gives the base reference for the performance counter and the os time.

I am lost you here. My interpretation of what you wrote above is that the
factor and offset between the readings of performance counter and
GetSystemTimeAsFileTime is readjusted each time Clock is called.

Anyway there are so many issues when reading from any clock sources, which
could prevent you from getting good measurements at once, like bus load,
interrupts, preempting etc. Also when the source of GetSystemTimeAsFileTime
is ticks, it could have systematic digitization error (1-2ms), which cannot
be compensated quickly.

> If this is still unclear have a look at the corresponding implementation
> in the GNAT sources.

That is what I am trying to avoid... (:-))

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de