Table of Contents

This benchmark has been adapted from attractivechaos’ Revisiting hash table performance code. It basically contains this main part:

Map<int, int> map;
for (size_t i = 0; i < 50'000'000; ++i) {
    checksum += ++map[rng(max_rng)];

Here rng(max_rng) creates a random number in the range [0, max_rng(. If max_rng is small, not many elements will be inserted but most of them will be accessed. If max_rng is large, mostly new elements will be inserted.

In fact the benchmark is run 4 times, with different max_rng settings:

  • 5% distinct: max_rng = 50M * 5% = 250k, so many values will be duplicates. After 50 million iterations, all 250k elements are inserted. So most map operations will be accesses.
  • 25% distinct: max_rng = 50M * 25% = 12.5M. More inserts, less modifications.
  • 50% distinct: max_rng = 50M * 50% = 25M. Note that due to randomness not all numbers from 0-25M will be inserted. Here the final map’s size contains 21.6M entries. So actually its about 43% of the value range instead of 50%.
  • 100% distinct: Here we make use of the full range of int, so 2^32 numbers are available. Practically all operations are new insertions.



Again, robin_hood::hash is the clear winner. The second fastest hash absl::Hash is in fact quite a bit slower in this benchmark. E.g. the fastest hashmap tsl::robin_map is about 10% faster when it uses robin_hood::hash. libstdc++-v3 hash is still the fastest for many hashes, but again it is a dangerous choice: Absl maps and phmap simply timeout again. The reason libstdc++-v3 hash works so well for some hashes is that the numbers are small, and change in the lower bits. If we would generate random numbers e.g. this way: rng(max_rng) << 10 the trival hash from libstdc++-v3 would have much worse performance due to many collisions.

folly::hasher is again dangerous as well. It does not time out, but the runtime is extremely bad for some maps. I belive the reason is this: the hash uses a native crc32 instruction. While the instruction is quite fast, it only generates a 32bit hash. Some hashmaps rely on 64bit hash data though.


Again, tsl::robin_map is the performance winner. 5% distinct accesses take 0.98 seconds, 100% distinct takes 3.44 seconds with robin_hood::hash. Unfortunately the peak memory usage is also quite high: 2293MB.

robin_hood::unordered_flat_map is the next on the pareto front: It only requires 853 MB peak memory, 2.7 times less, while being only about 10% slower.

folly::F14ValueMap is already 2.7 times slower than tsl::robin_map again with a win of about 10% less memory. Very similar performance has phmap::parallel_flat_hash_map with even less memory. It is remarkable how fast this map is with so little peak memory.

If low memory usage is important to you, the best choice is tsl::sparse_map. It is more than twice as fast as std::unordered_map.


Each entry is total runtime for creating a new map, 50M iterations, then destructing the map.

  1. blue: 5% distinct
  2. orange: 25% distinct
  3. green: 50% distinct
  4. red: 100% distinct