[Info-vax] Real time, was: Re: Volatile, was: Re: yet another sys$qiow question

[Johnny Billquist]

On 2015-08-29 20:47, Simon Clubley wrote:
> On 2015-08-28, JF Mezei <jfmezei.spamnot at vaxination.ca> wrote:
>> The reason I had been given for not using ASTs and using a loop for hard
>> real time was that they could ensure timing on a known event loop while
>> AST delivery was unpredictable since something else could stop AST
>> deliveries and delay processing etc.
>>
>> (Just recounting what I was told).
>>
>
> Even if you use a polling loop you still have to account for your code
> being stalled by an interrupt at some random point in time and you need
> to allow for the time it takes for that interrupt to be serviced.

Well, if you really would be doing polling I/O down at the hardware 
level, it would be because you do not use interrupts in the first place, 
so no... There is no time to account for by interrupts. Which would be a 
valid reason for doing polled I/O. However, we are now talking about 
something running on bare metal, and not something where a complex, 
timesharing OS sits between...

>> Similarly, on space station, they used a very old military slow network
>> type that is polled at predictable times because they felt ethernet was
>> unpredictable because you might have a collision. (not considering the
>> fact that with Ethernet 100 times faster than that military protocol
>> would have no problem handling the load generated by those devices).
>>
>
> It doesn't matter how fast Ethernet is; the root problem is that it
> doesn't have deterministic behaviour.

Right.

> It's like the people who say something is "real time" because it goes
> really fast; that isn't what real time is about.

Yes. And that is *very* annoying. Most people do not properly understand 
what hard realtime is.

> Real time is about deterministic behaviour, not speed, and the same is
> true both in software (in a RTOS for example) and in hardware (with the
> networking example above being one relevant example.)

Agreed.

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol