ConcurrentHashMap parallelismThreshold

Question

The ConcurrentHashMap got a couple new methods. I have two questions regarding them:

1. Why aren't they declared in ConcurrentMap?
2. What exactly does the parallelismThreshold mean or do?

Answer 1

1. These new methods seem to rely on implementation details specific to ConcurrentHashMap, but you would have to get an answer from the Java 8 authors to be sure. (they do browse SO)

2. From the Javadoc of ConcurrentHashMap:

   These bulk operations accept a parallelismThreshold argument. Methods proceed sequentially if the current map size is estimated to be less than the given threshold. Using a value of Long.MAX_VALUE suppresses all parallelism. Using a value of 1 results in maximal parallelism by partitioning into enough subtasks to fully utilize the ForkJoinPool.commonPool() that is used for all parallel computations. Normally, you would initially choose one of these extreme values, and then measure performance of using in-between values that trade off overhead versus throughput.

Answer 2

The parallelismThreshold determines whether bulk operations would be executed sequentially or in parallel. Running in parallel has some overhead, so it becomes useful only above some map size threshold.

ConcurrentHashMaps support a set of sequential and parallel bulk operations that, unlike most Stream methods, are designed to be safely, and often sensibly, applied even with maps that are being concurrently updated by other threads; for example, when computing a snapshot summary of the values in a shared registry. There are three kinds of operation, each with four forms, accepting functions with Keys, Values, Entries, and (Key, Value) arguments and/or return values. Because the elements of a ConcurrentHashMap are not ordered in any particular way, and may be processed in different orders in different parallel executions, the correctness of supplied functions should not depend on any ordering, or on any other objects or values that may transiently change while computation is in progress; and except for forEach actions, should ideally be side-effect-free. Bulk operations on Map.Entry objects do not support method setValue.

- forEach: Perform a given action on each element. A variant form applies a given
    transformation on each element before performing the action.
- search: Return the first available non-null result of applying a given function
    on each element; skipping further search when a result is found.
- reduce: Accumulate each element. The supplied reduction function cannot rely on
    ordering (more formally, it should be both associative and commutative).
    There are five variants:
    - Plain reductions. (There is not a form of this method for (key, value)
        function arguments since there is no corresponding return type.)
    - Mapped reductions that accumulate the results of a given function applied
        to each element.
    - Reductions to scalar doubles, longs, and ints, using a given basis value.

These bulk operations accept a parallelismThreshold argument. Methods proceed sequentially if the current map size is estimated to be less than the given threshold. Using a value of Long.MAX_VALUE suppresses all parallelism. Using a value of 1 results in maximal parallelism by partitioning into enough subtasks to fully utilize the ForkJoinPool.commonPool() that is used for all parallel computations. Normally, you would initially choose one of these extreme values, and then measure performance of using in-between values that trade off overhead versus throughput.

(Source)