Re: accuracy

["Randy Brukardt" <randy@rrsoftware.com>]

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
news:p87mtsns4of0.hhld0y03415s.dlg@40tude.net...
...
> As for GetThreadTimes, its absolute precision is 1ms.

No, it is 10ms. The interface offers more precision than is actually
provided.

> Its suggested
> absolute accuracy should be one time quant (which duration depends on the
> system settings). The later does not hold, because the error is in fact
not
> bounded.

I believe that the function was intended for profiling and performance
monitoring, and it surely is not different than any other technique I've
ever seen used for that. All such techniques give you a statistical
approximation to the real behavior. You just have to run them long enough to
make the results statistically significant.

It's theoretically possible for a thread to run in sync so that it never
gets a tick, but I've never seen (or heard of) an instance of that happening
in a real program being profiled. On a real DOS or Windows system, there is
too much asynchronous going on for any "lock-step" to continue for long.

In any case, it is statistical analysis that has to be applied here; it's
clear that the error can be reduced by lengthening the runtime (presuming
that you are willing to assume, as I am, that behavior is essentially random
if looked at over a long enough time period).

My main objection to this data is the gigantic tick rate, which means to get
anything meaningful, you have to run programs for a very long time (at least
a thousand times longer than the tick, and generally a thousand times the
"real value" of a counter before it is sufficiently significant).

OTOH, I don't want to use Ada.Execution_Times to control a program's
behavior. (I think that's a bit dubious, given that a hardware change would
invalidate the assumptions, and typically the important thing is the
response time: which depends on the wall-time, not the CPU time. But a
self-contained embedded system has more control than a program running on
Windows, so it might make sense somewhere.)

                                Randy.