Using elkan K-Means to improve IVF (#2180)

Signed-off-by: Nshengjun.li <shengjun.li@zilliz.com>

CHANGELOG.md

@@ -6,7 +6,6 @@ Please mark all change in change log and use the issue from GitHub
 
 ## Bug
 -   \#1705 Limit the insert data batch size
--   \#1796 Too much low performance of building index on ubuntu-mysql-version
 -   \#1929 Skip MySQL meta schema field width check
 -   \#1997 Index file missed after compact
 -   \#2073 Fix CheckDBConfigBackendUrl error message
@@ -28,10 +27,12 @@ Please mark all change in change log and use the issue from GitHub
 
 ## Improvement
 -   \#221 Refactor LOG macro
+-   \#1796 Compile Openblas with source code to improve the performance
 -   \#2039 Support Milvus run on SSE CPUs
 -   \#2149 Merge server_cpu_config.template and server_gpu_config.template
 -   \#2153 Upgrade thirdparty oatpp to v1.0.0
 -   \#2167 Merge log_config.conf with server_config.yaml
+-   \#2178 Using elkan K-Means to improve IVF
 
 ## Task
 

core/src/index/thirdparty/faiss/utils/distances.cpp

@@ -51,7 +51,6 @@ int sgemv_(const char *trans, FINTEGER *m, FINTEGER *n, float *alpha,
 namespace faiss {
 
 
-
 /***************************************************************************
  * Matrix/vector ops
  ***************************************************************************/
@@ -353,6 +352,67 @@ static void knn_L2sqr_sse (
     */
 }
 
+static void elkan_L2_sse (
+        const float * x,
+        const float * y,
+        size_t d, size_t nx, size_t ny,
+        float_maxheap_array_t * res) {
+
+    if (nx == 0 || ny == 0) {
+        return;
+    }
+
+    const size_t bs_y = 1024;
+    float *data = (float *) malloc((bs_y * (bs_y - 1) / 2) * sizeof (float));
+
+    for (size_t j0 = 0; j0 < ny; j0 += bs_y) {
+        size_t j1 = j0 + bs_y;
+        if (j1 > ny) j1 = ny;
+
+        auto Y = [&](size_t i, size_t j) -> float& {
+            assert(i != j);
+            i -= j0, j -= j0;
+            return (i > j) ? data[j + i * (i - 1) / 2] : data[i + j * (j - 1) / 2];
+        };
+
+#pragma omp parallel for
+        for (size_t i = j0 + 1; i < j1; i++) {
+            const float *y_i = y + i * d;
+            for (size_t j = j0; j < i; j++) {
+                const float *y_j = y + j * d;
+                Y(i, j) = sqrt(fvec_L2sqr(y_i, y_j, d));
+            }
+        }
+
+#pragma omp parallel for
+        for (size_t i = 0; i < nx; i++) {
+            const float *x_i = x + i * d;
+
+            int64_t ids_i = j0;
+            float val_i = sqrt(fvec_L2sqr(x_i, y + j0 * d, d));
+            float val_i_2 = val_i * 2;
+            for (size_t j = j0 + 1; j < j1; j++) {
+                if (val_i_2 <= Y(ids_i, j)) {
+                    continue;
+                }
+                const float *y_j = y + j * d;
+                float disij = sqrt(fvec_L2sqr(x_i, y_j, d));
+                if (disij < val_i) {
+                    ids_i = j;
+                    val_i = disij;
+                    val_i_2 = val_i * 2;
+                }
+            }
+
+            if (j0 == 0 || res->val[i] > val_i) {
+                res->val[i] = val_i;
+                res->ids[i] = ids_i;
+            }
+        }
+    }
+
+    free(data);
+}
 
 /** Find the nearest neighbors for nx queries in a set of ny vectors */
 static void knn_inner_product_blas (
@@ -608,7 +668,11 @@ void knn_L2sqr (const float * x,
                 float_maxheap_array_t * res,
                 ConcurrentBitsetPtr bitset)
 {
-    if (d % 4 == 0 && nx < distance_compute_blas_threshold) {
+    if (bitset == nullptr && res->k == 1 && nx >= ny * 2) {
+        // Note: L2 but not L2sqr
+        // usually used in IVF::train
+        elkan_L2_sse(x, y, d, nx, ny, res);
+    } else if (d % 4 == 0 && nx < distance_compute_blas_threshold) {
         knn_L2sqr_sse (x, y, d, nx, ny, res, bitset);
     } else {
         NopDistanceCorrection nop;