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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Arguments for single-mutex-exclusion on protected types (Was:
 Does Ada tasking profit from multi-core cpus?)
Date: Sun, 18 Mar 2007 13:57:20 -0400
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Date: 2007-03-18T13:57:20-04:00
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"Randy Brukardt" <randy@rrsoftware.com> writes:

> I don't understand Jeffrey's point either, but isn't it true that ceiling
> locking is essentially irrelevant on multiprocessor systems? That is, the
> point is to get rid of the lock, but you can't do that on a multi-processor
> (one processor can be running a lower priority task without anything being
> wrong, and that task had better be blocked from accessing the protected
> object). So ceiling locking has no advantage on a multi processor, it just
> restricts what you can do. If multiple cores continue to grow in popularity,
> it seems that the whole ceiling locking thing will become essentially
> irrelevant - just another case of premature optimization.
>
> (Can you tell I don't like ceiling locking much??)

On a multiprocessor, you can use ceilings plus spin locks to protect
protected objects.  The point is to avoid any queued waiting to enter a
PO (all the queuing is done with entry queues in this model).  That can
be made quite efficient, and the ceilings still prevent certain forms of
priority inversion.  So, no, I don't think ceilings are entirely
irrelevant on multiprocessors.

Ceilings are largely irrelevant if you're building Ada tasking on top of
some other system (posix or windows threads, for example) that wants to
do things differently.  I mean, if you're using the
one-Ada-task-per-thread model.

Another issue is that Ada's priority model has nothing to say about
processor affinity.

- Bob