Extending simplified PQ functions

[michael-popov]

• Does this PR have a descriptive title that could go in our release notes?
• [-] Does this PR add any new dependencies?
• [ x] Does this PR modify any existing APIs?
  • Is the change to the API backwards compatible?
• [-] Should this result in any changes to our documentation, either updating existing docs or adding new ones?

Extending functions for in-memory PQ calculations:

• added support for generating and using centroids.
• removed constrain dim % num_pq_chunks == 0.

For testing I modified (but not added to the PR) generate_pq utility. Code for testing loads pivot and centroids data produced by original functions and uses these data to calculate PQ with a new function. Then it compares results with PQ data produced by the original function.

    . . .
    diskann::generate_pq_data_from_pivots<float>(data_path, (uint32_t)num_pq_centers, (uint32_t)num_pq_chunks,
                                             pq_pivots_path, pq_compressed_vectors_path, opq);

    const size_t num_centers = 256;
    std::vector<float> pivot_data;
    std::vector<float> centroids;
    std::vector<float> data;
    std::vector<uint8_t> pq;
    std::vector<uint8_t> control_pq;
    int ret;

    ret = generate_pq_pivots_simplified(train_data, train_size, train_dim, num_pq_chunks,
                                       pivot_data, centroids);
    assert(ret == 0);
    assert(pivot_data.size() == num_centers * train_dim);
    assert(centroids.size() == train_dim);

    load_pivots(pq_pivots_path, pivot_data, centroids);
    load_data(data_path, data);
    load_data(pq_compressed_vectors_path, control_pq);

    ret = generate_pq_data_from_pivots_simplified(&data[0], data.size() / train_dim,
                                                 &pivot_data[0], pivot_data.size(),
                                                 train_dim, num_pq_chunks,
                                                 centroids,
                                                 pq);
    assert(ret == 0);
    assert(pq.size() == control_pq.size());
    for (size_t i = 0; i < pq.size(); i++)
    {
        assert(pq[i] == control_pq[i]);
    }

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[rakri]

why if < 2 we return -1?

[michael-popov]

Return value -1 means functions failed for some reason.

We check input parameters:

• num_pq_chunks == 0 => there is a division by num_pq_chunks a bit later, which will crash the system
• num_pq_chunks == 1 => originally I had offsets.size() == num_pq_chunks, and then I use offsets[1] as the largest possible size for a chunk. If num_pq_chunks == 1, then access offsets[1] will crash the system. Eventually I accepted the wisdom of the original function and added one more element to offsets so offsets.size() == num_pq_chunks+1 but the check of num_pq_chunks value was left the same.

In general, num_pq_chunks==1 does not make sense anyway, right? In this case the whole vector is squashed into a single number 0..255. Too much information is lost. I can't imagine a practical case when num_pq_chunks==1 is useful.

On the other hand, there are many things that I cannot imagine but they are still valid. :)
I can change the check to if (num_pq_chunks == 0) return -1; if required.

[rakri]

I'd say lets do what the last option is, we can allow num_pq_chunks =1.