Re: Tasking performance between Ada83 and Ada95

[dewar@merv.cs.nyu.edu (Robert Dewar)]

<<Are you saying it's impossible to write an Ada run-time system that does
parallelism without using operating system threads (on a Unix-like OS)?
That's not true -- I've done it.  I wrote an Ada 83 RTS that ran on a
parallel shared-memory machine where the OS had no threads support.  So
we created one Unix process per CPU (you need a way to know how many
CPUs there are).  Almost all memory in all the processes was mapped to
the same place.  And then the task scheduler decided which task to run
in which process according to the usual priority rules.  Of course,
non-blocking I/O required writing Text_IO (etc) in terms of system calls
that don't block, but interrupt when the I/O is done.>>

No one said that this is impossible. It is of course possible, but it does
not seem to be a particularly useful approach in practice on modern day
operating systems and hardware. Or put it another way, we have not found
even one customer interested in this kind of simulation. All our customers
using multi-processors definitely want Ada to use the underlying operating
systems threads, since this gives them all kinds of capabilities, like
assigning threads to processors, dealing with thread hierarchies, special
kinbds of inter-thread syncrhonization, debugging tools that know about the
threads, performance analyzers that know about the threads etc.