Re: Two questions

[stachour@umn-cs.CS.UMN.EDU (Paul Stachour)]

In article <796@aber-cs.UUCP> pcg@cs.aber.ac.uk (Piercarlo Grandi) writes:

> ....
>There is another problem with Ada tasking, and it is well known to those

*** Stop. Let's get the problem in the right place.  It is NOT a 
*** problem with Ada, but with the OS that is running Ada.

>who know OS/MVS and IMS. When an Ada task takes a page fault, the entire
>address space is suspended waiting for resolution of the page fault;
>another Ada task is not redispatched, even if it could, because on virtually
>all the Ada implementations I know of (notably the VMS one) the OS does not
>know about Ada tasks at all. 
>  ....
>There are only two solutions, one fairly horrible, having a signal delivered
>by the OS on a page fault to the in-address space scheduler, the second
>and proper one is to have threads in the OS and associated Ada tasks with
>the threads. Unfortunately the second one can be fairly expensive, many
>systems have high overhead threads (e.g. OS/MVS).
***  Stop.  There is at least a third solution.  In ancient, 
***  multi-processor batch-systems that had time-sharing grafted on,
***  such as Honeywell's GCOS3, the time-sharing monitor handled its
***  own sub-threads, such as deciding when things were dispatchable
***  when they were not and when to swap a subtask and ...
***  And it let the OS do the dispatching.
***  Substitute/time-sharing-monitor/Ada RSL/ and it's not too different.
***  In the past, we had a multi-processing CPU that could do the
***  equivalent of dispatching two "ready" Ada tasks within a single OS
***  process.  Today, you tell me that OS/MVS, VAX/VMS, and such ilk
***  cannot even dispatch one such ready task.  ...Sigh
***  ...Ugh   Isn't "progress" wonderful.