On Fri, 04 Feb 2011 07:26:22 -0500, bearophile <[email protected]>
wrote:
I have found an interesting post by Scott Johnson in this Lambda the
Ultimate thread:
http://lambda-the-ultimate.org/node/724#comment-6621
He says:
9th circle: Concurrent mutable state. The obnoxious practice of
mutating shared state from multiple threads of control, leading into a
predictable cycle of race conditions, deadlocks, and other assorted
misbehavior from which there is no return. And if a correct solution
(for synchronization) is found for a given program, chances are any
substantial change to the program will make it incorrect again. But you
won't find it, instead your customer will. Despite that, reams of code
(and TONS of middleware) has been written to try and make this
tractable. And don't get me started on a certain programming language
which starts with "J" that saw fit to make EVERY object have its very
own monitor....<
This is just one quotation, but I have found similar comments four or
five other times around the Web.
So is the design choice of copying this part of the Java design inside D
good? I'd like opinions on this topic.
Recently I have suggested an optional @nomonitor annotation for D
classes (to optionally remove a word from class instances and to reduce
class instantiation overhead a bit). Another option is doing the
opposite, and defining a @withmonitor annotation where you want a class
to have a monitor.
Hmm... Well, I'd recommend making @nomonitor the default and then only
annotate certain classes @withmonitor, although I'd prefer a different
keyword, say 'shared'. Oh, wait a second. *sigh* Every since it was
decided that a class couldn't contain both shared and non-shared
methods/fields I've been expecting that the monitor and support for
synchronized methods would be removed from Object or at least from
Object's spec. But this is likely a high-cost/low-gain optimization and a
lot of things regarding shared/immutable classes need to be fixed before
it can happen. I do think it should happen, not so much for the word of
memory, but in order to prevent objects not designed to be shared from
being shared. Which is what the 9th circle is talking about.
One of the cool thing about D's monitors, is that by manually setting
them, you can protect multiple objects with a single monitor. So, for a
set of interwoven objects (i.e. trees, etc) you're not acquiring a new
lock every method call and you are not going to have an internal
deadlock/race. This is actually the essential runtime feature of many
ownership systems. (Although, without an actual ownership-type system you
can't elide synchronization entirely)
