Lgd. Viktor Klang

Systems all the way down

Viktor Klang bio photo

This is the eighth of several posts describing the evolution of scala.concurrent.Future in Scala 2.12.x. For the previous post, click here.

Goodbye, sun.misc.Unsafe

A Future can be seen as a tri-state1 state machine with the following distinct states:

  1. Uncompleted
  2. Completed with a successful result
  3. Completed with a failed result

In order to have an «asynchronous Future» it is vital to be able to register logic to be executed once the Future becomes Completed.

This means that we can encode the state 1 as a «sequence of callbacks & their ExecutionContext», state 2 as a scala.util.Success, and state 3 as a scala.util.Failure.

Given that, we only need to have a single var in our Future-implementation, which will either be a Seq, a Success or a Failure.

Since Future can both be completed and have new callbacks added concurrently we need to be able to access this var atomically, so simply making it @volatile won’t be enough: we need Compare-And-Set semantics.

The first Akka Future which served as the main implementation inspiration for SIP-14, it used an inner field of type AtomicReference.

In the initial scala.concurrent.Future design, instead of taking the cost of having to allocate an extra object for the AtomicReference and take the cost for that indirection, we used what’s known as an AtomicReferenceFieldUpdater (ARFU).

Now, from a Scala perspective there are 2 problems with ARFUs: 1. They require the use of static fields—which Scala does not support, and, 2. They are about 10-20% slower than sun.misc.Unsafe access.

Since performance is always important in the Standard Library, we changed to use sun.misc.Unsafe for the implementation of scala.concurrent.Future and be OK with it requiring us to have a base-class in Java with a static field to hold the memory address offset of the field.

But! For 2.12.x we decided to take a «better» way, which eliminates the need for static fields and sun.misc.Unsafe:

We have now completely replaced the use of sun.misc.Unsafe for DefaultPromise (the internal implementation of the Scala Standard Library Promises) with extending AtomicReference internally, this means that there is no need for a base-class in Java and no need for ARFUs or sun.misc.Unsafe.


  1. The same, excellent, performance as previously
  2. With much better platform compatibility and security

1: In practice it turns out to be a four-state state machine due to the feature of «Promise Linking», we’ll cover that topic in the next post.

Click here for the next part in this blog series.