Arrays with volatile elements

On Sat, Dec 27, 2008 at 7:51 AM, Gavin Lowe <gavin [dot] lowe [at] comlab [dot] ox [dot] ac [dot] uk> wrote:

Hi

I have an array that is being acted on simultaneously by several
threads.  I need the entries in the array to be volatile (so updates
in one cache get propagated to other caches).  Is the correct way to
declare the array as follows?

 val x = new Array[Double @volatile](N)

How would this differ from the following?

 @volatile var x = new Array[Double](N)

What you want isn't directly achievable given the design of the JVM. Fortunately it doesn't need to be: All this would achieve in JVM land is a happens before guarantee on reads and writes, which is easy to achieve.

You could write something like the following:

class VolatileDoubleArray(val length : Int){
  val array = new Array[Double](length);
  @volatile var marker = 0;
  def apply(i : Int) = {marker; array(i); }
  def update(i : Int, x : Double) { array(i) = x; marker = 0; }
}

Every write to the array happens-before a write to the volatile marker variable and every read from the array happens-after a read from the volatile marker. This should give you the ordering guarantees you want.

I'd be shocked if the first one actually did anything useful with the volatile annotation.  The second one says the reference to the whole array is volatile, but the elements inside the array are not.

Java 1.5 and up have Atomic*Array in java.util.concurrent.atomic .   With them you can use either "compareAndSet" to get CAS semantics, or just use "set" to get volatile semantics.  Unfortunately, there doesn't seem to be an AtomicDoubleArray, which is a bit surprising since deep in the bowels of the beast it should be the same thing as AtomicLongArray.

See http://java.sun.com/javase/6/docs/api/java/util/concurrent/atomic/package-summary.html for details.

On Sat, Dec 27, 2008 at 4:51 AM, Gavin Lowe <gavin [dot] lowe [at] comlab [dot] ox [dot] ac [dot] uk> wrote:

Hi

I have an array that is being acted on simultaneously by several
threads.  I need the entries in the array to be volatile (so updates
in one cache get propagated to other caches).  Is the correct way to
declare the array as follows?

 val x = new Array[Double @volatile](N)

How would this differ from the following?

 @volatile var x = new Array[Double](N)

Best wishes

Gavin

Does the jvm spec allow for such a thing?

On Dec 27, 2008 4:53 AM, "Gavin Lowe" <gavin [dot] lowe [at] comlab [dot] ox [dot] ac [dot] uk> wrote:

Hi

I have an array that is being acted on simultaneously by several
threads.  I need the entries in the array to be volatile (so updates
in one cache get propagated to other caches).  Is the correct way to
declare the array as follows?

 val x = new Array[Double @volatile](N)

How would this differ from the following?

 @volatile var x = new Array[Double](N)

Best wishes

Gavin