Miserable unordered_map insertion performance / hash function?

Colls is not measuring collisions. If you want to measure collisions, then for each bucket be in the range 0, bucket_count()) get bucket_size(b) . That will tell you how many items are in each bucket.

If there are 2 or more items in the bucket, then you have bucket_size(b) - 1 collisions for bucket be .

Your hash space is 24bits in size. To have 0 collisions, you'd need a hash table the size of your data if your has is perfect, larger by 25-50% if not. My guess is you've made your hash table much, much smaller than this, hence the container is remapping over your data and causing the collisions.

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