Re: Ada versus Java - Tasking

[bobduff@world.std.com (Robert A Duff)]

In article <OHK.97Jan18194202@ultra.tfdt-o.nta.no>,
Ole-Hjalmar Kristensen FOU.TD/DELAB <ohk@ultra.tfdt-o.nta.no> wrote:
>These task switching times are still pretty horrible though. Using
>Unix processes and pipes or System V message queues, you can do just
>as well. Any idea of why this is so? Does it follow from the
>definition of an Ada task, or is it just this particular
>implementation.

No, it's not hard to make task switching fast, if you're willing to give
up the idea that every Ada task should map onto exactly one OS
task/thread.  But there are some advantages to that one-for-one mapping,
despite the HUGE performance penalty.  E.g., when a task does a blocking
system call, it's nice if some other task gets to run while it's
waiting.  Making that happen is hard (but I don't think impossible!)
unless each Ada task maps to an OS task (thread).  And if the OS threads
are horribly slow (which is often the case), then tough luck.

>With a task switching overhead of this magnitude, tasks become
>unattractive as a way of programs dealing with high-speed IO, for
>example.

Indeed.

- Bob