Re: How to get Ada to ?cross the chasm??

[Niklas Holsti <niklas.holsti@tidorum.invalid>]

On 18-05-07 23:29 , Paul Rubin wrote:
> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
    ...
>>> So you still get arbitrarily long pauses.
>> No, because I know the structures I deal with and can avoid that
>> choosing proper design.
>
> In other words you are manually managing memory, which is what
> refcounting is supposed to get you out of.

No, Dmitry just knows (by design of the data structure) that there are 
no long/deep chains of references needing to be updated.

>> Millisecond is very long for many applications,
>
> In such a system you better not use a refcount scheme either, without a
> WCET analysis.

Some kind of WCET analysis is very often needed in real-time systems 
whatever kind of memory management is used. This is another of the ways 
in which such systems differ from the typical applications of GC-based 
languages.

However, there is a functional programming language, Hume, designed for 
real-time systems, 
https://en.wikipedia.org/wiki/Hume_(programming_language).

> I'd add as well: most refcount systems I know of don't move objects
> around in memory, so you get memory fragmentation and high cache miss
> rates.

Real-time systems with reference counting often use special "heaps" or 
"pools" with block sizes designed to fit the application and to avoid or 
minimize fragmentation. Caches are often small, and main memories not 
very large either, so the penalty for cache misses is often less than in 
larger server-like systems. For example, in my current project a RAM 
access needs only 2 wait states, and we in fact run with the D-cache 
disabled (this reminds me that we should experiment to see if the 
program is faster or slower with the D-cache enabled...)

The most critical task in this application has a 500 Hz frequency and is 
budgeted at something like 0.5 - 0.8 ms per activation. The application 
uses reference counting for some data structures, but the 500 Hz task is 
designed not to use it directly; a lower-frequency support task supplies 
and receives the reference-counted data to and from the 500 Hz task. 
Just as an example.

I think Dmitry often deals with shorter times and higher task 
frequencies. But then his stuff is ground-based and runs on more 
powerful processors.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
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