Parallel Collections

Concrete Parallel Collection Classes


Parallel Array

A ParArray sequence holds a linear, contiguous array of elements. This means that the elements can be accessed and updated efficiently by modifying the underlying array. Traversing the elements is also very efficient for this reason. Parallel arrays are like arrays in the sense that their size is constant.

scala> val pa = scala.collection.parallel.mutable.ParArray.tabulate(1000)(x => 2 * x + 1)
pa: scala.collection.parallel.mutable.ParArray[Int] = ParArray(1, 3, 5, 7, 9, 11, 13,...

scala> pa reduce (_ + _)
res0: Int = 1000000

scala> pa map (x => (x - 1) / 2)
res1: scala.collection.parallel.mutable.ParArray[Int] = ParArray(0, 1, 2, 3, 4, 5, 6, 7,...

Internally, splitting a parallel array splitter amounts to creating two new splitters with their iteration indices updated. Combiners are slightly more involved.Since for most transformer methods (e.g. flatMap, filter, takeWhile, etc.) we don’t know the number of elements (and hence, the array size) in advance, each combiner is essentially a variant of an array buffer with an amortized constant time += operation. Different processors add elements to separate parallel array combiners, which are then combined by chaining their internal arrays. The underlying array is only allocated and filled in parallel after the total number of elements becomes known. For this reason, transformer methods are slightly more expensive than accessor methods. Also, note that the final array allocation proceeds sequentially on the JVM, so this can prove to be a sequential bottleneck if the mapping operation itself is very cheap.

By calling the seq method, parallel arrays are converted to ArraySeq collections, which are their sequential counterparts. This conversion is efficient, and the ArraySeq is backed by the same underlying array as the parallel array it was obtained from.

Parallel Vector

A ParVector is an immutable sequence with a low-constant factor logarithmic access and update time.

scala> val pv = scala.collection.parallel.immutable.ParVector.tabulate(1000)(x => x)
pv: scala.collection.parallel.immutable.ParVector[Int] = ParVector(0, 1, 2, 3, 4, 5, 6, 7, 8, 9,...

scala> pv filter (_ % 2 == 0)
res0: scala.collection.parallel.immutable.ParVector[Int] = ParVector(0, 2, 4, 6, 8, 10, 12, 14, 16, 18,...

Immutable vectors are represented by 32-way trees, so splitters are split by assigning subtrees to each splitter. Combiners currently keep a vector of elements and are combined by lazily copying the elements. For this reason, transformer methods are less scalable than those of a parallel array. Once the vector concatenation operation becomes available in a future Scala release, combiners will be combined using concatenation and transformer methods will become much more efficient.

Parallel vector is a parallel counterpart of the sequential Vector, so conversion between the two takes constant time.

Parallel Range

A ParRange is an ordered sequence of elements equally spaced apart. A parallel range is created in a similar way as the sequential Range:

scala> (1 to 3).par
res0: scala.collection.parallel.immutable.ParRange = ParRange(1, 2, 3)

scala> (15 to 5 by -2).par
res1: scala.collection.parallel.immutable.ParRange = ParRange(15, 13, 11, 9, 7, 5)

Just as sequential ranges have no builders, parallel ranges have no combiners. Mapping the elements of a parallel range produces a parallel vector. Sequential ranges and parallel ranges can be converted efficiently one from another using the seq and par methods.

Parallel Hash Tables

Parallel hash tables store their elements in an underlying array and place them in the position determined by the hash code of the respective element. Parallel mutable hash sets (mutable.ParHashSet) and parallel mutable hash maps (mutable.ParHashMap) are based on hash tables.

scala> val phs = scala.collection.parallel.mutable.ParHashSet(1 until 2000: _*)
phs: scala.collection.parallel.mutable.ParHashSet[Int] = ParHashSet(18, 327, 736, 1045, 773, 1082,...

scala> phs map (x => x * x)
res0: scala.collection.parallel.mutable.ParHashSet[Int] = ParHashSet(2181529, 2446096, 99225, 2585664,...

Parallel hash table combiners sort elements into buckets according to their hashcode prefix. They are combined by simply concatenating these buckets together. Once the final hash table is to be constructed (i.e. combiner result method is called), the underlying array is allocated and the elements from different buckets are copied in parallel to different contiguous segments of the hash table array.

Sequential hash maps and hash sets can be converted to their parallel variants using the par method. Parallel hash tables internally require a size map which tracks the number of elements in different chunks of the hash table. What this means is that the first time that a sequential hash table is converted into a parallel hash table, the table is traversed and the size map is created - for this reason, the first call to par takes time linear in the size of the hash table. Further modifications to the hash table maintain the state of the size map, so subsequent conversions using par and seq have constant complexity. Maintenance of the size map can be turned on and off using the useSizeMap method of the hash table. Importantly, modifications in the sequential hash table are visible in the parallel hash table, and vice versa.

Parallel Hash Tries

Parallel hash tries are a parallel counterpart of the immutable hash tries, which are used to represent immutable sets and maps efficiently. They are supported by classes immutable.ParHashSet and immutable.ParHashMap.

scala> val phs = scala.collection.parallel.immutable.ParHashSet(1 until 1000: _*)
phs: scala.collection.parallel.immutable.ParHashSet[Int] = ParSet(645, 892, 69, 809, 629, 365, 138, 760, 101, 479,...

scala> => x * x).sum
res0: Int = 332833500

Similar to parallel hash tables, parallel hash trie combiners pre-sort the elements into buckets and construct the resulting hash trie in parallel by assigning different buckets to different processors, which construct the subtries independently.

Parallel hash tries can be converted back and forth to sequential hash tries by using the seq and par method in constant time.

Parallel Concurrent Tries

A concurrent.TrieMap is a concurrent thread-safe map, whereas a mutable.ParTrieMap is its parallel counterpart. While most concurrent data structures do not guarantee consistent traversal if the data structure is modified during traversal, Ctries guarantee that updates are only visible in the next iteration. This means that you can mutate the concurrent trie while traversing it, like in the following example which outputs square roots of number from 1 to 99:

scala> val numbers = scala.collection.parallel.mutable.ParTrieMap((1 until 100) zip (1 until 100): _*) map { case (k, v) => (k.toDouble, v.toDouble) }
numbers: scala.collection.parallel.mutable.ParTrieMap[Double,Double] = ParTrieMap(0.0 -> 0.0, 42.0 -> 42.0, 70.0 -> 70.0, 2.0 -> 2.0,...

scala> while (numbers.nonEmpty) {
     |   numbers foreach { case (num, sqrt) =>
	 |     val nsqrt = 0.5 * (sqrt + num / sqrt)
	 |     numbers(num) = nsqrt
	 |     if (math.abs(nsqrt - sqrt) < 0.01) {
	 |       println(num, nsqrt)
	 |		 numbers.remove(num)
	 |	   }
	 |   }
	 | }

Combiners are implemented as TrieMaps under the hood– since this is a concurrent data structure, only one combiner is constructed for the entire transformer method invocation and shared by all the processors.

As with all parallel mutable collections, TrieMaps and parallel ParTrieMaps obtained by calling seq or par methods are backed by the same store, so modifications in one are visible in the other. Conversions happen in constant time.

Performance characteristics

Performance characteristics of sequence types:

  head tail apply update prepend append insert
ParArray C L C C L L L
ParVector eC eC eC eC eC eC -
ParRange C C C - - - -

Performance characteristics of set and map types:

  lookup add remove
ParHashSet/ParHashMap eC eC eC
ParHashSet/ParHashMap C C C
ParTrieMap eC eC eC


The entries in the above two tables are explained as follows:

C The operation takes (fast) constant time.
eC The operation takes effectively constant time, but this might depend on some assumptions such as maximum length of a vector or distribution of hash keys.
aC The operation takes amortized constant time. Some invocations of the operation might take longer, but if many operations are performed on average only constant time per operation is taken.
Log The operation takes time proportional to the logarithm of the collection size.
L The operation is linear, that is it takes time proportional to the collection size.
- The operation is not supported.

The first table treats sequence types–both immutable and mutable–with the following operations:

head Selecting the first element of the sequence.
tail Producing a new sequence that consists of all elements except the first one.
apply Indexing.
update Functional update (with updated) for immutable sequences, side-effecting update (with update for mutable sequences.
prepend Adding an element to the front of the sequence. For immutable sequences, this produces a new sequence. For mutable sequences it modified the existing sequence.
append Adding an element and the end of the sequence. For immutable sequences, this produces a new sequence. For mutable sequences it modified the existing sequence.
insert Inserting an element at an arbitrary position in the sequence. This is only supported directly for mutable sequences.

The second table treats mutable and immutable sets and maps with the following operations:

lookup Testing whether an element is contained in set, or selecting a value associated with a key.
add Adding a new element to a set or key/value pair to a map.
remove Removing an element from a set or a key from a map.
min The smallest element of the set, or the smallest key of a map.

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