[Info-vax] Baremetal emulators, was: Re: Alpha emulator for OSX

[Johnny Billquist]

On 2016-02-09 00:39, Stephen Hoffman wrote:
> On 2016-02-08 20:40:14 +0000, Johnny Billquist said:
>
>> On 2016-02-08 17:13, Stephen Hoffman wrote:
>>> On 2016-02-08 14:21:55 +0000, lists at openmailbox.org said:
>>>
>>>> Very well said as usual. But is this an issue for OpenVMS users? Is
>>>> OpenVMS used for real-time applications?
>>>
>>> There are many applications with latent dependencies on timing; that
>>> will fail when calls operate too quickly or too slowly.  These
>>> applications are arguably poorly written, but they can and do exist.
>>
>> All very true, but this is a different can of worms, Stephen.
>>
>> Linux is not realtime. In fact, you have no idea how long time might
>> pass between the execution of one instruction and the next in your
>> application. Normally, it will be fast, but there is no bounded limit
>> on it. If you depending on a reaction within a bounded time, you are
>> playing russian roulette using a host OS like Linux.
>> And this of course also affect how long "interrupt" latencies your
>> machine will appear to have. It might be super fast, and it might take
>> a long long time to react to that interrupt. And it varies from time
>> to time.
>> Because with "next instruction", we're now talking about the next
>> instruction in the emulated machine. The actual host will be running
>> and executing instructions normally, just that it might be in the
>> bowels of Linux for a while...
>>
>>     Johnny
>
> I was referring to vanilla, simple, ordinary timing dependencies
> involving incorrectly or unsynchronized operations.  Stuff that goes
> wrong when the underlying timing changes.   About timing-related things
> that should be simple to get right, but — as several folks around here
> are fond of demonstrating with some of the code that gets posted —
> correctly and consistently managing asynch activities isn't always
> something that always gets implemented.  Not even instruction
> dependencies.   (VAX instruction processing was its own special joy to
> deal with, but I digress.)

I totally understood what you said, Hoff. And that is all good points, 
but like I said, it's a different can of worms. It has nothing to do 
with real-time.

> As for Linux Realtime, there are options:  https://www.rtai.org or
> http://xenomai.org or
> https://www.osadl.org/Realtime-Linux.projects-realtime-linux.0.html or
> https://github.com/rtmux depending on specific requirements.
>
> I'm sure there are others.

There are plenty. Most people do not understand what it means to 
actually use them, though, since if you try to use them, you cannot use 
the normal Unix API anymore. You cannot simply take a program written 
for Unix, and think that you can just grab a "realtime Unix", and run 
your program there, and you have realtime. The whole Unix API is the 
problem. If you want realtime, you have to give up Unix.

A name line "Realtime Linux" is deceiving, since it makes people believe 
that they can have Linux and realtime combined, which it is not. Not in 
the way they think anyway.

> But discussions of what is or is not "realtime" — without application
> timing requirements, or at least a set of definitions that can be
> referenced — are rather more like "cloud" or "security" or "bare iron".
>   Either there's a framework for the discussions, or the conversation
> turns into "no true Scotsman".   As this one will continue.

There are some very well established definitions of realtime. I fail to 
see the problem. We have hard realtime and soft realtime. And the 
definition of these are well known. And Unix cannot do hard realtime, 
but it can do acceptable soft realtime (most of the time :-) ).

> With the prices of hardware, doing dedicated processing in a loop —
> either a specific core in a box, or with a dedicated box — or other such
> shenanigans becomes feasible.   Competent computing boards and boxes
> don't cost nearly as much as a PDP-11 or a VAX anymore, after all.

Agreed. And this is why I said that I'm not sure if VMS systems are used 
in realtime today. But if they are, the problem with running emulation 
on a Linux host are real.

	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