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From: bobduff@world.std.com (Robert A Duff)
Subject: Re: Java vs Ada 95 (Was Re: Once again,
 Ada absent from DoD SBIR solicitation)
Date: 1996/11/03
Message-ID: <E0B6CL.84r@world.std.com>#1/1
X-Deja-AN: 194117742
references: <325BC3B3.41C6@hso.link.com>
 <EACHUS.96Nov1103759@spectre.mitre.org> <E07Dwq.5L@world.std.com>
 <55gkch$gg6@fozzie.sun3.iaf.nl>
organization: The World Public Access UNIX, Brookline, MA
newsgroups: comp.lang.ada
Date: 1996-11-03T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <55gkch$gg6@fozzie.sun3.iaf.nl>,
Geert Bosch <geert@fozzie.sun3.iaf.nl> wrote:
>Robert A Duff (bobduff@world.std.com) wrote:
>`` Note that in Ada, or any language with tasking, the reference count
>   updates have to do locking in general, which might be rather
>   expensive. ''
>
>It is possible to have implementations that do not need locking for
>objects that are not aliased or allocated by an allocator (using new).

I don't understand what you mean by that.  I thought we were talking
about unbounded strings, and similar packages, where you allocate stuff
on the heap and use reference counting to know when to reclaim it.  The
stuff on the heap is, of course, aliased.

>In the more general case a very simple spin-lock is enough and the
>overhead should only be one memory-read when the object is not locked.
>String objects are locked almost never and when they are locked they
>are only locked for a short time.

True, but we're talking about an implementation of unbounded strings
where a simple string assignment is just a copy of a pointer.  Adding
locking, plus incr/decr of reference counts, is a substantial overhead
for such a simple operation.  If GC can eliminate that overhead, it may
well be a performance win, despite the fact that there is also some
GC-specific overhead.  (And I've seen some papers giving measurements
that agree with that suspicion.)

- Bob