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This is a header only library offering a variety of dynamically growing concurrent hash tables. That all work by dynamically migrating the current table once it gets too full.
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growt (GrowTable) is a header only library implementing a concurrent growing hash table. There are different variants depending on the use case. See explanation below.

Using the supplied cmake build system you can compile an example file (example/example.cpp) and a multitude of different tests/benchmarks. These were also used to generate the plots displayed in our scientific publication => (slightly outdated).

Hash table interface

To remain fast while storing some per thread data, we use handles. These help us to remain fast when accessing the hash table. Even though this is a change from the traditional interface, we try to remain as close to std::unordered_map as possible (within the handle). To achieve this we implemented an iterator based interface. Our iterators remain secure even when the table is migrated in the background. This is necessary because in concurrent scenarios one can never be certain, when a table migration occurs.

All our hash tables support the following functionality:

called on global object

  • HashTable(size_t n) - constructor allocating a table large enough to hold n elements.
  • HashTable::Handle getHandle() - create handles that can be used to access the table (alternatively HashTable::Handle(HashTable global_table_obj))

called through handles within the handles, we try to keep the interface as close to std::unordered_map as possible:

  • std::pair(iterator, bool) insert(uint64_t k, uint64_t d) - if no element with key k was present, insert stores the data element d at key k and returns an iterator (and true). Otherwise it returns an iterator to the already present element and false.
  • std::pair(iterator, bool) update(uint64_t k, UpdateFunction f, <parameters for f>...) - looks for an element with key k. If present, the stored data is changed atomically using the supplied update function (interface/instruction below). The returned iterator points to the found element (or end()), and the bool shows if an element was found.
  • std::pair(iterator, bool) insertOrUpdate(uint64_t k, uint64_t d, UpdateFunction f) - combines the two functions above. If no data was present with key k, then d is inserted, otherwise the stored data is updated. The bool can be used to find out which happened (true -> insertion, false -> update).
  • size_t erase(uint64_t k) - removes the stored key value pair, returns the number of erased entries (at most one). The memory is reclaimed once the table is migrated.
  • iterator find(uint64_t k) - finds the data element stored at key k and returns an iterator (end() if unfound).
  • const_iterator find(uint64_t k) const - same as find

Using handles is not necessary for our non-growing tables.

called through iterators

  • dereferencing/reading the values will return the values that were in the table when the iterator was created.
  • refresh will update the iterator to the current state of the table. This might make the iterator end() (cannot be dereferenced) if the element was erased in the meantime.

called through a reference

  • reading the values will return the values that were in the table when the iterator was created (that was dereferenced to become this reference).
About UpdateFunctions

Our update interface uses a user implemented update function. Any object given as an update function should have an operator()(uint64_t& cur_data, uint64_t key, uint64_t new_data) returning a uint64_t. This function will usually be called by our update operations to change a stored elements data field. This operations does not have to be atomic: it is made atomic using CAS operations. When an atomic version of the method exists, then it can be implemented as an additional function called .atomic(uint64_t& cur_data, uint64_t key, uint64_t new_data), which is only called iff atomics can be called safely (mostly in synchronized growing variants usGrow and psGrow). Presensce of the atomic variant is detected automatically. An example for a fully implemented UpdateFunction might look like this:

struct Increment
    uint64_t operator()(uint64_t& lhs, const uint64_t, const uint64_t rhs) const
        return lhs += rhs;

    // an atomic implementation can improve the performance of updates in .sGrow
    // this will be detected automatically
    uint64_t atomic    (uint64_t& lhs, const uint64_t, const uint64_t rhs) const
        __sync_fetch_and_add(&lhs, rhs);



This package contains many different hash table variants. You can find some example instanciations in data-structures/definitions.h (alternatively look at tests/selection.h, which is used to select a hash table at compile time using compile time definitions).

All our data structures and connected classes are declared in the growt namespace.

Non-growing hash tables
  • folklore (or Circular<SimpleElement, HASHFUNCTION, ALLOCATOR>) is a simple linear probing hash table using atomic operations, to change cell contents.

  • xfolklore (or TSXCircular<SimpleElement, HASHFUNCTION, ALLOCATOR>) similar to Folklore, but uses Intel TSX transactional memory extensions to ensure atomicity instead of atomics. Your machine has to support Intel TSX transactions for this to work (check your cpu-flags and try compiling with -mrtm)

Growing hash tables

Our growing variants use the above non-growing tables. They grow by migrating the entire hash table once it gets too full for the current size. Migration is done in the background without the user knowing about it. During the migration hash table accesses may be delayed until the table is migrated (usually the waiting thread will help with the migration).

Threads can only access our growing hash tables by creating a thread specific handle. These handles cannot be shared between threads.

  • uaGrow (or GrowTable<Circular<MarkableElement, HASHFUNCTION,ALLOCATOR>, WStratUser, EStratAsync>), is a growing table, where threads that access the table are responsible for eventual migrations. These will be performed automatically and asynchronously. Migrated cells are marked to ensure atomicity (this reduces the available key space by one bit. Keys >=2^63 cannot be inserted).

  • usGrow (or GrowTable<Circular<SimpleElement, HASHFUNCTION,ALLOCATOR>, WStratUser, EStratSync>), similar to uaGrow but growing steps are somewhat synchronized (ensures automatically that no updates run during growing phases) eliminating the need for marking.

  • paGrow (or GrowTable<Circular<MarkableElement, HASHFUNCTION,ALLOCATOR>, WStratPool, EStratAsync>), where growing is done by a dedicated pool of growing threads. Similar to uaGrow marking is used to ensure atomicity of the hash table migration.

  • psGrow (or GrowTable<Circular<SimpleElement, HASHFUNCTION,ALLOCATOR>, WStratPool, EStratSync>), combining the thread pool of paGrow with the synchronized growing approach of usGrow.

All growing variants can also be built using the TSXCircular table instead of Circular. For a more in-depth description of our growing variants, check out our paper (

Our tests and Benchmarks

All generated tests (make recipes) have the same name structure.

<test_abbrv>_<growing_indicator>_<hash_table_name> => e.g. ins_full_uaGrow

  • ins - insertion and find test (seperate)
  • mix - mixed inserts and finds
  • agg - aggregation using insertOrUpdate on a skewed key sequence
  • con - updates and finds on a skewed key sequence
  • del - alternating inserts and deletions (approx. constant table size)
full list of hash tables

Some of the following tables have to be activated through cmake options.

  • sequential - our sequential table (use only one thread!)
  • folklore - our non growing tables
  • uaGrow, usGrow, paGrow, psGrow - our main growing tables
  • usnGrow, psnGrow - two alternate growing variants should behave similar to usGrow and psGrow
  • xfolklore, uaxGrow, usxGrow, paxGrow, psxGrow, usnxGrow, psnxGrow - tsx variants of previous tables
  • junction_linear, junction_grampa, junction_leap, folly, cuckoo, tbb_hm, tbb_um - third party tables

Note: in the paper we have some additional third party hash tables. These depend on some additional wrappers and are not reproduced here. Wrappers for their libraries can be found in a branch called legacy_wrappers.

Usage in your own projects

Including our project

To make it easy, you can include the header data-structures/definitions.h, which includes all necessary files and offers the nice type definitions used above. Of course, this may increase the size of your compilation unit significantly -- eventually slowing down compile times. If this is a concern you can include files directly and define the specific hash table. See tests/selection.h for examples of this technique. This header is used to choose a specific hash table and the corresponding includes depending on a compile time constant (e.g. -D UAGROW).


#include "data-structures/markableelement.h"
#include "data-structures/circular.h"
#include "data-structures/strategy/wstrat_user.h"
#include "data-structures/strategy/estrat_async.h"
#include "data-structures/growtable.h"
using uaGrow = GrowTable<Circular<MarkableElement, HASHFCT, ALLOCATOR<MarkableElement> >,
                         WStratUser, EStratAsync>
About our utility functions

utils/alignedallocator.h - a very simple allocator returning only aligned data elements.

utils/poolallocator.h - in many of our growing tests, mapping virtual to physical memory has been a bottleneck. Therefore, we use this allocator it starts by allocating a big amount of memory and uses it as a memory pool for future allocations. Memory mapping is forced in the beginning by writing into the buffer. Different variants of this allocator are available using different malloc variants to allocate the buffer (malloc, libnuma interleaved allocation, huge TLB page allocator).

utils/hashfct.h - some different hash functions the correct implementation is chosen at compile time according to a compile time constant.

utils/counting_wait.h, utils/test_coordination.h, utils/thread_basics.h - all implement some threading capabilities mostly used to simplify writing tests/benchmarks. But also necessary for our thread pool growing variants.

utils/keygen.h - create numbers with a zipf distribution (used to create contentious test instances)

utils/commandline.h - simple command line parser, to read user inputs (used for our tests).


This package can be used all on its own (see example.cpp and ...test.cpp). However third party codes are used for additional functionality/tests. Most of the third party libraries are either searched on your machine (TBB, pthread), or they are placed in submodules (downloaded through git).

We use the following libraries:
for utility:
  • TBB - to implement a fixed memory pool allocator
  • xxHash - usable hash function
as third party hash tables (for benchmarks):
  • TBB - tbb::concurrent_hash_map and tbb::concurrent_unordered_map
  • LibCuckoo - cuckoohash_map
  • Junction - junction::ConcurrentMap_Linear ..._Grampa ..._Leapfrog
  • Folly - folly::AtomicHashMap


Tested using current versions of g++.

Easy build without third party code
git clone
cd growt
mkdir build
cd build
cmake ..
Building with third party libraries

Third party libraries are either installed using your package manager or they are downloaded into the misc/submodules folder.

git clone
cd growt
git submodule init
mkdir build
cd build

note that folly needs quite a lot of extern libraries (zstd, glog, ...) those have to be installed, to compile any test using folly (checkout their github

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