Re: Ada Streams usage (was Escape Sequences in Strings)

[Marin David Condic <mcondic.nospam@acm.org>]

Ted Dennison wrote:

> We are using streams for subsystem checkpointing in a hard real time
> system. Its actually working quite well. But I did have to write my own
> stream types to get it to work the way I needed it to.
>

Well, part of my objection to streams is the overhead of the data motion and
the way it degenerates to a (potentially) large number of small procedure
calls. (It wouldn't be so bad if the compiler could figure out when to
generate a really fast block-move instruction instead of a bazillion
procedure calls. But I suppose there may be language issues - or maybe just
that it is really hard to do.) If you find a way to "roll your own" that
eliminates or minimizes this, then it can still work quite well.

I guess some of it depends on how hard is "hard". Its at least a
deterministic amount of overhead if the message sizes don't vary. (Dynamic
things like Unbounded_String, etc. might pose problems in this respect.) But
if you are running a heavily loaded processor, the overhead is unnecessary
work that ought to be eliminated so you can use it for something more
important.

Maybe my usage of the word is not entirely accurate. "Hard-real-time"
usually means that failure to meet deadlines constitutes failure of the
system. If your processor does a gazillion-instructions-per-second and
you're only using 1% of your CPU, it can still be a "hard-real-time" system.
I suppose I was thinking more along the lines of "difficult" real time
systems. :-)

>
> Streams are quite useful any time you need to mainupulate large amounts
> of unstructured data entirely within Ada. But if you need low-level
> control over what is going on you are probably going to end up having to
> write your own streams and stream attribute routines. That can be a lot
> of work compared to some of the old-fashioned alternatives. Plus if you
> are rewriting both ends, often there's no benifit to using streams over
> just making your own custom object with read and write methods.

Yeah. I understand. In most cases, I'd like to just use the standard Ada
streams because it just automagically works for you. You do some fairly
small amount of work at the base level and it just off and does it for
everything you build up on it. Very elegant. Very tidy. Extremely clever.
But when you've got to overcome the representation issues and/or eliminate
the overhead by doing your own Read and Write routines, you end up putting
so much extra work into it that you might just as well pitch streams and
handle all the bytes yourself.

MDC
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