Header file: <Kokkos_UnorderedMap.hpp>

Kokkos’s unordered map is designed to efficiently handle tens of thousands of concurrent insertions. Consequently, the API is significantly different from the standard unordered_map. The two key differences are fixed capacity and index based.

  • Fixed capacity: The capacity of the unordered_map is fixed when inside a parallel algorithm. This means that an insert can fail when the capacity of the map is exceeded. The capacity of the map can be changed (rehash) from the host.

  • Index based: Instead of returning pointers or iterators (which would not work when moving between memory spaces) the map uses integer indexes. This also allows the map to store data in cache friendly ways. The availability of indexes is managed by an internal atomic bitset based on uint32_t.


template<typename Key, typename Value, typename Device = Kokkos::DefaultExecutionSpace>
class UnorderedMap#
Template Parameters:
  • Key – Must be a POD (Plain Old Data type)

  • Valuevoid indicates an unordered set, otherwise must be trivially copyable

  • Device – Device is any class or struct with the following public typedefs or type aliases: execution_space, memory_space, and device_type


UnorderedMap(uint32_t capacity_hint);#

Create map with enough space for at least capacity_hint number of objects


Host Only

Public Member Functions


Clear the map


Host Only

bool rehash(uint32_t requested_capacity);#

Rehash map to given capacity, the current size is used as a lower bound O(capacity)


Host Only

uint32_t size() const;#

Current size of the map, O(capacity)


Host Only

KOKKOS_INLINE_FUNCTION uint32_t capacity() const;#

Capacity of the map, O(1)

KOKKOS_INLINE_FUNCTION UnorderedMapInsertResult insert(key) const;#

Insert the given key into the map with a default constructed value

KOKKOS_INLINE_FUNCTION UnorderedMapInsertResult insert(Key key, Value value, Insert op = NoOp) const;#

Insert the given key/value pair into the map and optionally specify the operator, op, used for combining values if key already exists

KOKKOS_INLINE_FUNCTION uint32_t find(Key key) const#

Return the index of the key if it exist, otherwise return invalid_index

KOKKOS_INLINE_FUNCTION bool exists(Key key) const;#

Does the key exist in the map

KOKKOS_INLINE_FUNCTION bool valid_at(uint32_t index) const;#

Is the current index a valid key/value pair

KOKKOS_INLINE_FUNCTION Key key_at(uint32_t index) const;#

Return the current key at the index

KOKKOS_INLINE_FUNCTION Value value_at(uint32_t index) const;#

Return the current value at the index

KOKKOS_INLINE_FUNCTION constexpr bool is_allocated() const;#

Return true if the internal views (keys, values, hashmap) are allocated

class UnorderedMapInsertResult#

Public Methods

KOKKOS_INLINE_FUNCTION bool success() const;#

Was the key/value pair successfully inserted into the map

KOKKOS_INLINE_FUNCTION bool existing() const;#

Is the key already present in the map

KOKKOS_INLINE_FUNCTION bool failed() const;#

Did the insert fail?

KOKKOS_INLINE_FUNCTION uint32_t index() const;#

Index where the key exists in the map as long as failed() == false

template<class ValueTypeView, class ValuesIdxType>
struct UnorderedMapInsertOpTypes#
Template Parameters:
  • ValueTypeView – The UnorderedMap value array type.

  • ValuesIdxType – The index type for lookups in the value array.

Public Insertion Operator Types

struct NoOp#

Insert the given key/value pair into the map

struct AtomicAdd#

Duplicate key insertions sum values together.

Insertion using default UnorderedMapInsertOpTypes::NoOp#

There are 3 potential states for every insertion which are reported by the UnorderedMapInsertResult:

  • success: implies that the current thread has successfully inserted its key/value pair

  • existing: implies that the key is already in the map and its current value is unchanged

  • failed means that either the capacity of the map was exhausted or that a free index was not found with a bounded search of the internal atomic bitset. A failed insertion requires the user to increase the capacity (rehash) and restart the algoritm.

// use the default NoOp insert operation
using map_op_type = Kokkos::UnorderedMapInsertOpTypes<value_view_type, size_type>;
using noop_type   = typename map_op_type::NoOp;
noop_type noop;
parallel_for(N, KOKKOS_LAMBDA (uint32_t i) {
  map.insert(i, values(i), noop);
// OR;
parallel_for(N, KOKKOS_LAMBDA (uint32_t i) {
  map.insert(i, values(i));

Insertion using UnorderedMapInsertOpTypes::AtomicAdd#

The behavior from Insertion using default UnorderedMapInsertOpTypes::NoOp holds true with the exception that the UnorderedMapInsertResult:

  • existing implies that the key is already in the map and the existing value at key was summed with the new value being inserted.

// use the AtomicAdd insert operation
using map_op_type     = Kokkos::UnorderedMapInsertOpTypes<value_view_type, size_type>;
using atomic_add_type = typename map_op_type::AtomicAdd;
atomic_add_type atomic_add;
parallel_for(N, KOKKOS_LAMBDA (uint32_t i) {
  map.insert(i, values(i), atomic_add);


Iterating over Kokkos’ UnorderedMap is different from iterating over a standard container. The pattern is to iterate over the capacity of the map and check if the current index is valid.


// assume umap is an existing Kokkos::UnorderedMap
parallel_for(umap.capacity(), KOKKOS_LAMBDA (uint32_t i) {
    if( umap.valid_at(i) ) {
        auto key   = umap.key_at(i);
        auto value = umap.value_at(i);