Using boost unordered map

Question

Guys, I am using dynamic programming approach to solve a problem. Here is a brief overview of the approach

1. Each value generated is identified using 25 unique keys.
2. I use the boost::hash_combine to generate the seed for the hash table using these 25 keys.

3. I store the values in a hash table declared as

   boost::unordered_map<Key_Object, Data_Object, HashFunction> hashState;

4. I did a time profiling on my algorithm and found that nearly 95% of the run time is spent towards retrieving/inserting data into the hash table.

5. These were the details of my hash table

   hashState.size() 1880

   hashState.load_factor() 0.610588

   hashState.bucket_count() 3079

   hashState.max_size() 805306456

   hashState.max_load_factor() 1

   hashState.max_bucket_count() 805306457

I have the following two questions

1. Is there anything which I can do to improve the performance of the Hash Table's insert/retrieve operations?

2. C++ STL has hash_multimap which would also suit my requirement. How does boost libraries unordered_map compare with hash_multimap in terms of insert/retrieve performance.

Answer 1

If your hash function is not the culprit, the best you can do is probably using a different map implementation. Since your keys are quite large, using unordered_map from Boost.Intrusive library might be the best option. Alternatively, you could try closed hashing: Google SparseHash or MCT, though profiling is certainly needed because closed hashing is recommended when elements are small enough. (SparseHash is more established and well tested, but MCT doesn't need those set_empty()/set_deleted() methods).

EDIT:

I just noticed there is no intrusive map in the Boost library I mentioned, only set and multiset. Still, you can try the two closed hashing libraries.

EDIT 2:

STL hash_map is not standard, it is probably some extension and not portable across compilers.

Answer 2

Are you sure that the hash function you are using is not the bottleneck? Which time percentage takes the hash function? Can you do the same test and replace the insert/retrievals by a simple call to the hash.