Re: Duration vs. Ada.Real_Time

[Simon Wright <simon@pushface.org>]

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"Atle R�stad" <aer@edh.ericsson.se> writes:

> Hi
> 
> I have some code that has a max of 30 milliseconds to process, and need to
> measure if this is possible. But the requirement for Duration is that
> Duration'Small must be less then 20 milliseconds, and
> Ada.Real_Time.Time_Unit must be less then 20 microseconds.
[...]
> I will run the program on an Solaris 8. How will this affect my time
> measuring? What resolution can I expect?

(1) On Solaris (>=2.6 or so, I think) the default operating system
    tick is 10 mS.

(2) GNAT uses nanosleep() to implement delay (pretty sure of this).

(3) nanosleep() sleeps for *at least* the time you specify, rounded up
    to an integral number of ticks.

(4) So, if you "delay 0.000_001;", you'll delay for at least 10 mS and
    up to 20 mS.

However, you can change the OS tick to 1 mS by writing

  set hires_tick 1

in /etc/system (I may have the exact grammar wrong, mail me at work
for the details). This will give you a maximum repetition frequency of
500 Hz.

NB, that 1 is a boolean meaning 'true', _not_ the number of milliseconds!

Running as root gives you real-time dispatching but doesn't change the
behaviour of nanosleep().

Linux is similar, though there's no easy way to change the tick rate
(you can edit /usr/include/asm/param.h, or figure a way to redefine
HZ, and rebuild the kernel; worked fine for us, but YMMV, though,
since the library won't match). And don't try delays < 2 mS as root,
they're executed as busy loops.

-- 
Simon Wright                       Work Email: simon.j.wright@amsjv.com
Alenia Marconi Systems                        Voice: +44(0)23-9270-1778
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