NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/sys/amiga/misc-part1/3957

Internet Message Format | 1996-08-05 | 3.3 KB

Path: EU.net!sun4nl!xs4all!usenet
From: [e-mail] (Jeroen T. Vermeulen)
Newsgroups: comp.sys.amiga.misc
Subject: Re: OS features
Date: Fri, 2 Feb 96 03:30:37
Organization: Leiden University, Mathematics & Computer Science, The Netherlands
Message-ID: <[e-mail]>
References: <[e-mail]> <4dh2dm$[e-mail]> <[e-mail]> <4do01q$[e-mail]> <[e-mail]> <4dtg0e$[e-mail]> <[e-mail]> <4e384c$[e-mail]>
NNTP-Posting-Host: asd05-06.dial.xs4all.nl
Mime-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Transfer-Encoding: 8bit
X-NewsSoftware: GRn 2.1 Feb 19, 1994


In article <4e384c$[e-mail]> [e-mail] (Michael van Elst) writes:
>
> >What is probably meant is that deadlocks must be prevented, where memory is
> >swapped out that needs to be present in real mem for the swap mechanism as a
> >whole to work.  Of course the task list etc. must also be kept consistent during
> >Forbid().
>
> And more. There is data used within interrupts. It is rather impossible
> to page in data for interrupt code.

Damn, forgot about those for a moment!


> >It would have to lock, of course.  With `single-threaded' I mean here that only
> >one instance of the code could be active at any time (as if it were a task), but
> >no actual context switch is made (as if it were a library).
>
> As I said this is not possible because the driver may consist of several
> tasks that _do_ require context switches. It also is a waste of time
> to forbid other tasks to run after one task page-faulted.

As we're talking about implementing a special-purpose device driver here anyway,
I don't see a reason (yet) why it can't be programmed so as to run in a single
task.

Once again, this isn't exactly my favourite solution but it could at least be
possible.  As for the wasted time, I've heard it said that with other OS's it's
often better to have the CPU run idle while swapping than to switch tasks and
run head-on into a new page fault (I don't _know_ it though).


> > - Private memory allocated by the VM subsystem
>
> Most of the VM system itself can be in virtual memory. It even
> makes sense as the MMU tables can be pretty big.

Would this work for a single-level VM system (or the top level of a multi-level
system)?

> > - Memory shared between any task and the VM subsystem + VM device driver
>
> This has to be locked during the transfer only.

That depends on implementation, I guess.  There must never be "circular page
fault" on such memory (and excessive copying is probably best avoided).


> >Of course, but it must be supported for existing software.  And with the scheme
> >outlined above I think it can be, even in such multiprocessor configurations.
>
> As I said, Forbid() is not just a semaphore to shared memory.

Of course not.  But what exactly is the observable difference, if any?


>                                 Michael van Elst
> Internet: [e-mail]

--
============================================================================
#  Jeroen T. Vermeulen   \"How are we doing kid?"/   Yes, we use Amigas.   #
#---  [e-mail]    ---\"Oh, same as always."/--         ...          --#
#[e-mail] \ "That bad, huh?"  /  Got a problem with that? #