Re: Ada.Execution_Time

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

On Tue, 28 Dec 2010 14:14:57 +0000, Simon Wright wrote:

> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
> 
>> And conversely, the catastrophic accuracy of the VxWorks real-time
>> clock service does not hinder its usability for real-time application.
> 
> Catastrophic?
>
> The Radstone PPC7A cards (to take one example) have two facilities: (a)
> the PowerPC decrementer, run off a crystal with some not-too-good quoted
> accuracy (50 ppm, I think), and (b) a "real time clock".
> 
> The RTC would be much better termed a time-of-day clock, since what it
> provides is the date and time to 1 second precision. It also needs
> battery backup, not easy to justify on naval systems (partly because it
> adversely affects the shelf life of the boards, partly because navies
> don't like noxious chemicals in their equipment).
> 
> We never used the RTC.
>
> The decrementer is the facility used by VxWorks, and hence by GNAT under
> VxWorks, to support time; both Ada.Calendar and Ada.Real_Time (we are
> still at Ada 95 so I have no idea about Ada.Execution_Time). We run with
> clock interrupts at 1 ms and (so far as we can tell from using bus
> analysers) the interrupts behave perfectly reliably.

Yes, this thing. In our case it was Pentium VxWorks 6.x. (The PPC we used
prior to it had poor performance) The problem was that Ada.Real_Time.Clock
had the accuracy of the clock interrupts, i.e. 1ms, which is by all
accounts catastrophic for a 1.7GHz processor. You can switch some tasks
forth and back between two clock changes.
 
> For a higher-resolution view of time we've extended Ada.Calendar, using
> the PowerPC's mftb (Move From Time Base) instruction to measure sub-tick
> intervals (down to 40 ns).

So did we in our case. VxWorks has means to access the Pentium high
resolution counter. We took Ada.Real_Time and replaced the Clock function
with one that used the counter multiplied by its frequency.

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