unordered_map with memory_pool using fixed_block_allocator

[cohdan]

Hi,

I'm trying to define an unordered_map which pre-allocated all the memory needed for its work to avoid memory operations during runtime (for a performance sensitive app).

I have the following code:

using memory_pool = foonathan::memory::memory_pool<memory::node_pool, memory::fixed_block_allocator<>>;

class something {
    something() : _map(nullptr), _map_object_pool(nullptr) {}

    void init() {
        size_t size_to_allocate = read_some_size_from_conf_file();
        _map_object_pool = make_unique<memory_pool>(memory::unordered_map_node_size<memory::unordered_map<int, int, memory_pool>::value_type>::value, size_to_allocate);
        _map = make_unique<memory::unordered_map<int, int, memory_pool>>(*_map_object_pool);
    }

private:
    unique_ptr<memory::unordered_map<int, int, memory_pool>> _map;
    unique_ptr<memory_pool> _map_object_pool;
}

I'm expecting the pool to allow me to insert (size_to_allocate / memory::unordered_map_node_size<memory::unordered_map<int, int, memory_pool>::value_type>::value) items into the map before the buffer runs out of memory but in fact I see that I can insert much less items (for example with 1MB buffer I'm expecting 32767 items but my buffer is exhausted after ~27000). I can also see that some items insertion reduces the capacity in the free list of the memory_arena by more than 1 (sometimes 2, sometimes 3).

Important to note - I do not have repeating keys and do not expect key hash collisions (if it has any relevance).

I do not understand what is going on there. I guess I'm probably missing something but I can't figure out what.
Can anyone please help?

Thank you in advance!

[cohdan]

I've watched this excellent talk and I understand my issue now - the "unexpected" (by me..) allocations came from bucket management of the unordered_map container and they should be taken into account when planning my pool.