Re: Embedded languages based on early Ada (from "Re: Preferred OS, processor family for running embedded Ada?")

[Simon Farnsworth <usenet@farnz.org.uk>]

Jonathan Bromley wrote:

> On Sat, 3 Mar 2007 12:00:08 +0100, "Dmitry A. Kazakov"
> <mailbox@dmitry-kazakov.de> wrote:
> 
>>> The compiler should be able to generate code which generates a
>>> reasonable number of threads, depending on the hardware being used.
>>
>>Yes
> 
> For heaven's sake...   You have a statically-determinable number of
> processors.  It's your (or your compiler's) choice whether each of
> those processors runs a single thread, or somehow runs multiple
> threads.   If each processor is entitled to run multiple threads, then
> there's no reason why the number and structure of cooperating
> threads should not be dynamically variable.  If you choose to run
> one thread on each processor, your thread structure is similarly
> static.  Hardware people have been obliged to think about this
> kind of thing for decades.  Software people seem to have a
> pretty good grip on it too, if the textbooks and papers I've read
> are anything to go by.  Why is it suddenly such a big deal?
> 
Not disagreeing with most of what you're saying, but I do feel the need to
point out the existence of systems with hotpluggable CPUs. Sun and IBM have
both sold systems for some years where CPUs can be added and removed at
runtime; software is expected to just cope with this.

Also in the software domain; there is a cost to switching between different
threads. Thus, in software, the aim is to limit the number of runnable
threads to the number of active CPUs. If there are more threads runnable
than CPUs available, some CPU time is wasted switching between threads,
which is normally undesirable behaviour.
-- 
Simon Farnsworth