Re: Ada.Execution_Time

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

On Tue, 28 Dec 2010 16:27:18 +0000, (see below) wrote:

> Dmitri reports a modern computer with a timer having a resolution that is
> thousands or millions of times worse than the CPU's logic clock.

You get me wrong, the timer resolution is OK, it is the system service
which does not use it properly. In the case of VxWorks the system time is
incremented from the timer interrupts, e.g. by 1ms. You can set interrupts
to each 1um spending all processor time handling interrupts. It is an OS
architecture problem. System time should have been taken from the real time
counter.

> Why has this aspect of computer architecture degenerated so much, I wonder?
> And why have software people not made more of a push for improvements?

The computer architecture did not degenerate. Modern processor and
motherboard have multiple time sources 3-4. Some of them have a very high
resolution and reliable, e.g. keep on counting in the sleep mode etc. It is
usually the standard OS services to blame for not using these clocks.
Practically in any OS there is a backdoor to get a decent real-time clock.
Then the journey begins. You need to synchronize readings from that clock
(usually a 64-bit counter) with the system time (of miserable accuracy) in
order to get a decent UTC stamp. This is doable using some statistical
method, depending on your needs (monotonic, or not, etc). Shame that the OS
does not do this.

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