diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1206657d44..77cf2b37c6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -22,7 +22,7 @@ Please mark all change in change log and use the issue from GitHub
 -   \#1548 Move store/Directory to storage/Operation and add FSHandler
 -   \#1619 Improve compact performance
 -   \#1649 Fix Milvus crash on old CPU 
--   \#1653 IndexFlat performance improvement for NQ less than thread_number
+-   \#1653 IndexFlat (SSE) and IndexBinaryFlat performance improvement for small NQ
 
 ## Task
 
diff --git a/core/src/index/thirdparty/faiss/utils/BinaryDistance.cpp b/core/src/index/thirdparty/faiss/utils/BinaryDistance.cpp
index e89d6aa7ee..ff20a21277 100644
--- a/core/src/index/thirdparty/faiss/utils/BinaryDistance.cpp
+++ b/core/src/index/thirdparty/faiss/utils/BinaryDistance.cpp
@@ -15,175 +15,196 @@
 
 namespace faiss {
 
-    size_t batch_size = 65536;
+static const size_t size_1M = 1 * 1024 * 1024;
+static const size_t batch_size = 65536;
 
-    template <class T>
-    static
-    void binary_distence_knn_hc(
-            int bytes_per_code,
-            float_maxheap_array_t * ha,
-            const uint8_t * bs1,
-            const uint8_t * bs2,
-            size_t n2,
-            bool order = true,
-            bool init_heap = true,
-            ConcurrentBitsetPtr bitset = nullptr)
-    {
-        size_t k = ha->k;
+template <class T>
+static
+void binary_distence_knn_hc(
+        int bytes_per_code,
+        float_maxheap_array_t * ha,
+        const uint8_t * bs1,
+        const uint8_t * bs2,
+        size_t n2,
+        bool order = true,
+        bool init_heap = true,
+        ConcurrentBitsetPtr bitset = nullptr)
+{
+    size_t k = ha->k;
+
+    if ((bytes_per_code + k * (sizeof(float) + sizeof(int64_t))) * ha->nh < size_1M) {
+        int thread_max_num = omp_get_max_threads();
+        // init hash
+        size_t thread_hash_size = ha->nh * k;
+        size_t all_hash_size = thread_hash_size * thread_max_num;
+        float *value = new float[all_hash_size];
+        int64_t *labels = new int64_t[all_hash_size];
+        for (int i = 0; i < all_hash_size; i++) {
+            value[i] = 1.0 / 0.0;
+            labels[i] = -1;
+        }
+
+        T *hc = new T[ha->nh];
+        for (size_t i = 0; i < ha->nh; i++) {
+            hc[i].set(bs1 + i * bytes_per_code, bytes_per_code);
+        }
+
+#pragma omp parallel for
+        for (size_t j = 0; j < n2; j++) {
+            if(!bitset || !bitset->test(j)) {
+                int thread_no = omp_get_thread_num();
+
+                const uint8_t * bs2_ = bs2 + j * bytes_per_code;
+                for (size_t i = 0; i < ha->nh; i++) {
+                    tadis_t dis = hc[i].compute (bs2_);
+
+                    float * val_ = value + thread_no * thread_hash_size + i * k;
+                    int64_t * ids_ = labels + thread_no * thread_hash_size + i * k;
+                    if (dis < val_[0]) {
+                        faiss::maxheap_pop<tadis_t> (k, val_, ids_);
+                        faiss::maxheap_push<tadis_t> (k, val_, ids_, dis, j);
+                    }
+                }
+            }
+        }
+
+        for (size_t t = 1; t < thread_max_num; t++) {
+            // merge hash
+            for (size_t i = 0; i < ha->nh; i++) {
+                float * __restrict value_x = value + i * k;
+                int64_t * __restrict labels_x = labels + i * k;
+                float *value_x_t = value_x + t * thread_hash_size;
+                int64_t *labels_x_t = labels_x + t * thread_hash_size;
+                for (size_t j = 0; j < k; j++) {
+                    if (value_x_t[j] < value_x[0]) {
+                        faiss::maxheap_pop<tadis_t> (k, value_x, labels_x);
+                        faiss::maxheap_push<tadis_t> (k, value_x, labels_x, value_x_t[j], labels_x_t[j]);
+                    }
+                }
+            }
+        }
+
+        // copy result
+        memcpy(ha->val, value, thread_hash_size * sizeof(float));
+        memcpy(ha->ids, labels, thread_hash_size * sizeof(int64_t));
+
+        delete[] hc;
+        delete[] value;
+        delete[] labels;
+
+    } else {
         if (init_heap) ha->heapify ();
 
-        int thread_max_num = omp_get_max_threads();
-        if (ha->nh < 4) {
-            // omp for n2
-            int all_hash_size = thread_max_num * k;
-            float *value = new float[all_hash_size];
-            int64_t *labels = new int64_t[all_hash_size];
-
-            for (int i = 0; i < ha->nh; i++) {
-                T hc (bs1 + i * bytes_per_code, bytes_per_code);
-                // init hash
-                for (int i = 0; i < all_hash_size; i++) {
-                    value[i] = 1.0 / 0.0;
-                }
+        const size_t block_size = batch_size;
+        for (size_t j0 = 0; j0 < n2; j0 += block_size) {
+            const size_t j1 = std::min(j0 + block_size, n2);
 #pragma omp parallel for
-                for (size_t j = 0; j < n2; j++) {
-                    if(!bitset || !bitset->test(j)) {
-                        const uint8_t * bs2_ = bs2 + j * bytes_per_code;
-                        tadis_t dis = hc.compute (bs2_);
+            for (size_t i = 0; i < ha->nh; i++) {
+                T hc (bs1 + i * bytes_per_code, bytes_per_code);
 
-                        int thread_no = omp_get_thread_num();
-                        float * __restrict val_ = value + thread_no * k;
-                        int64_t * __restrict ids_ = labels + thread_no * k;
-                        if (dis < val_[0]) {
-                            faiss::maxheap_pop<tadis_t> (k, val_, ids_);
-                            faiss::maxheap_push<tadis_t> (k, val_, ids_, dis, j);
-                        }
-                    }
-                }
-                // merge hash
+                const uint8_t * bs2_ = bs2 + j0 * bytes_per_code;
+                tadis_t dis;
                 tadis_t * __restrict bh_val_ = ha->val + i * k;
                 int64_t * __restrict bh_ids_ = ha->ids + i * k;
-                for (int i = 0; i < all_hash_size; i++) {
-                    if (value[i] < bh_val_[0]) {
-                        faiss::maxheap_pop<tadis_t> (k, bh_val_, bh_ids_);
-                        faiss::maxheap_push<tadis_t> (k, bh_val_, bh_ids_, value[i], labels[i]);
-                    }
-                }
-            }
-            delete[] value;
-            delete[] labels;
-
-        } else {
-            const size_t block_size = batch_size;
-            for (size_t j0 = 0; j0 < n2; j0 += block_size) {
-                const size_t j1 = std::min(j0 + block_size, n2);
-#pragma omp parallel for
-                for (size_t i = 0; i < ha->nh; i++) {
-                    T hc (bs1 + i * bytes_per_code, bytes_per_code);
-
-                    const uint8_t * bs2_ = bs2 + j0 * bytes_per_code;
-                    tadis_t dis;
-                    tadis_t * __restrict bh_val_ = ha->val + i * k;
-                    int64_t * __restrict bh_ids_ = ha->ids + i * k;
-                    size_t j;
-                    for (j = j0; j < j1; j++, bs2_+= bytes_per_code) {
-                        if(!bitset || !bitset->test(j)){
-                            dis = hc.compute (bs2_);
-                            if (dis < bh_val_[0]) {
-                                faiss::maxheap_pop<tadis_t> (k, bh_val_, bh_ids_);
-                                faiss::maxheap_push<tadis_t> (k, bh_val_, bh_ids_, dis, j);
-                            }
+                size_t j;
+                for (j = j0; j < j1; j++, bs2_+= bytes_per_code) {
+                    if(!bitset || !bitset->test(j)){
+                        dis = hc.compute (bs2_);
+                        if (dis < bh_val_[0]) {
+                            faiss::maxheap_pop<tadis_t> (k, bh_val_, bh_ids_);
+                            faiss::maxheap_push<tadis_t> (k, bh_val_, bh_ids_, dis, j);
                         }
                     }
-
                 }
+
             }
         }
-
-        if (order) ha->reorder ();
     }
 
-    void binary_distence_knn_hc (
-            MetricType metric_type,
-            float_maxheap_array_t * ha,
-            const uint8_t * a,
-            const uint8_t * b,
-            size_t nb,
-            size_t ncodes,
-            int order,
-            ConcurrentBitsetPtr bitset)
-    {
-        switch (metric_type) {
-        case METRIC_Jaccard:
-        case METRIC_Tanimoto:
-            switch (ncodes) {
+    if (order) ha->reorder ();
+}
+
+void binary_distence_knn_hc (
+        MetricType metric_type,
+        float_maxheap_array_t * ha,
+        const uint8_t * a,
+        const uint8_t * b,
+        size_t nb,
+        size_t ncodes,
+        int order,
+        ConcurrentBitsetPtr bitset)
+{
+    switch (metric_type) {
+    case METRIC_Jaccard:
+    case METRIC_Tanimoto:
+        switch (ncodes) {
 #define binary_distence_knn_hc_jaccard(ncodes) \
-            case ncodes: \
-                binary_distence_knn_hc<faiss::JaccardComputer ## ncodes> \
-                    (ncodes, ha, a, b, nb, order, true, bitset); \
-            break;
-            binary_distence_knn_hc_jaccard(8);
-            binary_distence_knn_hc_jaccard(16);
-            binary_distence_knn_hc_jaccard(32);
-            binary_distence_knn_hc_jaccard(64);
-            binary_distence_knn_hc_jaccard(128);
-            binary_distence_knn_hc_jaccard(256);
-            binary_distence_knn_hc_jaccard(512);
+        case ncodes: \
+            binary_distence_knn_hc<faiss::JaccardComputer ## ncodes> \
+                (ncodes, ha, a, b, nb, order, true, bitset); \
+        break;
+        binary_distence_knn_hc_jaccard(8);
+        binary_distence_knn_hc_jaccard(16);
+        binary_distence_knn_hc_jaccard(32);
+        binary_distence_knn_hc_jaccard(64);
+        binary_distence_knn_hc_jaccard(128);
+        binary_distence_knn_hc_jaccard(256);
+        binary_distence_knn_hc_jaccard(512);
 #undef binary_distence_knn_hc_jaccard
-            default:
-                binary_distence_knn_hc<faiss::JaccardComputerDefault>
-                        (ncodes, ha, a, b, nb, order, true, bitset);
-                break;
-            }
-            break;
-
-        case METRIC_Substructure:
-            switch (ncodes) {
-#define binary_distence_knn_hc_Substructure(ncodes) \
-            case ncodes: \
-                binary_distence_knn_hc<faiss::SubstructureComputer ## ncodes> \
-                    (ncodes, ha, a, b, nb, order, true, bitset); \
-            break;
-            binary_distence_knn_hc_Substructure(8);
-            binary_distence_knn_hc_Substructure(16);
-            binary_distence_knn_hc_Substructure(32);
-            binary_distence_knn_hc_Substructure(64);
-            binary_distence_knn_hc_Substructure(128);
-            binary_distence_knn_hc_Substructure(256);
-            binary_distence_knn_hc_Substructure(512);
-#undef binary_distence_knn_hc_Substructure
-            default:
-                binary_distence_knn_hc<faiss::SubstructureComputerDefault>
-                        (ncodes, ha, a, b, nb, order, true, bitset);
-                break;
-            }
-            break;
-
-        case METRIC_Superstructure:
-            switch (ncodes) {
-#define binary_distence_knn_hc_Superstructure(ncodes) \
-            case ncodes: \
-                binary_distence_knn_hc<faiss::SuperstructureComputer ## ncodes> \
-                    (ncodes, ha, a, b, nb, order, true, bitset); \
-            break;
-            binary_distence_knn_hc_Superstructure(8);
-            binary_distence_knn_hc_Superstructure(16);
-            binary_distence_knn_hc_Superstructure(32);
-            binary_distence_knn_hc_Superstructure(64);
-            binary_distence_knn_hc_Superstructure(128);
-            binary_distence_knn_hc_Superstructure(256);
-            binary_distence_knn_hc_Superstructure(512);
-#undef binary_distence_knn_hc_Superstructure
-            default:
-                binary_distence_knn_hc<faiss::SuperstructureComputerDefault>
-                        (ncodes, ha, a, b, nb, order, true, bitset);
-                break;
-            }
-            break;
-
         default:
+            binary_distence_knn_hc<faiss::JaccardComputerDefault>
+                    (ncodes, ha, a, b, nb, order, true, bitset);
             break;
         }
+        break;
+
+    case METRIC_Substructure:
+        switch (ncodes) {
+#define binary_distence_knn_hc_Substructure(ncodes) \
+        case ncodes: \
+            binary_distence_knn_hc<faiss::SubstructureComputer ## ncodes> \
+                (ncodes, ha, a, b, nb, order, true, bitset); \
+        break;
+        binary_distence_knn_hc_Substructure(8);
+        binary_distence_knn_hc_Substructure(16);
+        binary_distence_knn_hc_Substructure(32);
+        binary_distence_knn_hc_Substructure(64);
+        binary_distence_knn_hc_Substructure(128);
+        binary_distence_knn_hc_Substructure(256);
+        binary_distence_knn_hc_Substructure(512);
+#undef binary_distence_knn_hc_Substructure
+        default:
+            binary_distence_knn_hc<faiss::SubstructureComputerDefault>
+                    (ncodes, ha, a, b, nb, order, true, bitset);
+            break;
+        }
+        break;
+
+    case METRIC_Superstructure:
+        switch (ncodes) {
+#define binary_distence_knn_hc_Superstructure(ncodes) \
+        case ncodes: \
+            binary_distence_knn_hc<faiss::SuperstructureComputer ## ncodes> \
+                (ncodes, ha, a, b, nb, order, true, bitset); \
+        break;
+        binary_distence_knn_hc_Superstructure(8);
+        binary_distence_knn_hc_Superstructure(16);
+        binary_distence_knn_hc_Superstructure(32);
+        binary_distence_knn_hc_Superstructure(64);
+        binary_distence_knn_hc_Superstructure(128);
+        binary_distence_knn_hc_Superstructure(256);
+        binary_distence_knn_hc_Superstructure(512);
+#undef binary_distence_knn_hc_Superstructure
+        default:
+            binary_distence_knn_hc<faiss::SuperstructureComputerDefault>
+                    (ncodes, ha, a, b, nb, order, true, bitset);
+            break;
+        }
+        break;
+
+    default:
+        break;
     }
+}
 
 }
diff --git a/core/src/index/thirdparty/faiss/utils/distances.cpp b/core/src/index/thirdparty/faiss/utils/distances.cpp
index 68bc6d9b82..ac0e3b8321 100644
--- a/core/src/index/thirdparty/faiss/utils/distances.cpp
+++ b/core/src/index/thirdparty/faiss/utils/distances.cpp
@@ -154,50 +154,68 @@ static void knn_inner_product_sse (const float * x,
     size_t k = res->k;
 
     size_t thread_max_num = omp_get_max_threads();
-    if (nx < 4) {
-        // omp for ny
-        size_t all_hash_size = thread_max_num * k;
-        float *value = new float[all_hash_size];
-        int64_t *labels = new int64_t[all_hash_size];
+    
+    size_t thread_hash_size = nx * k;
+    size_t all_hash_size = thread_hash_size * thread_max_num;
+    float *value = new float[all_hash_size];
+    int64_t *labels = new int64_t[all_hash_size];
+
+    // init hash
+    for (size_t i = 0; i < all_hash_size; i++) {
+        value[i] = -1.0 / 0.0;
+        labels[i] = -1;
+    }
 
-        for (size_t i = 0; i < nx; i++) {
-            // init hash
-            for (size_t i = 0; i < all_hash_size; i++) {
-                value[i] = -1.0 / 0.0;
-            }
-            const float *x_i = x + i * d;
 #pragma omp parallel for
-            for (size_t j = 0; j < ny; j++) {
-                if(!bitset || !bitset->test(j)) {
-                    const float *y_j = y + j * d;
-                    float ip = fvec_inner_product (x_i, y_j, d);
+    for (size_t j = 0; j < ny; j++) {
+        if(!bitset || !bitset->test(j)) {
+            size_t thread_no = omp_get_thread_num();
+            const float *y_j = y + j * d;
+            for (size_t i = 0; i < nx; i++) {
+                const float *x_i = x + i * d;
+                float ip = fvec_inner_product (x_i, y_j, d);
 
-                    size_t thread_no = omp_get_thread_num();
-                    float * __restrict val_ = value + thread_no * k;
-                    int64_t * __restrict ids_ = labels + thread_no * k;
-                    if (ip > val_[0]) {
-                        minheap_pop (k, val_, ids_);
-                        minheap_push (k, val_, ids_, ip, j);
-                    }
+                float * val_ = value + thread_no * thread_hash_size + i * k;
+                int64_t * ids_ = labels + thread_no * thread_hash_size + i * k;
+                if (ip > val_[0]) {
+                    minheap_pop (k, val_, ids_);
+                    minheap_push (k, val_, ids_, ip, j);
                 }
             }
-
-            // merge hash
-            float * __restrict simi = res->get_val(i);
-            int64_t * __restrict idxi = res->get_ids (i);
-            minheap_heapify (k, simi, idxi);
-            for (size_t i = 0; i < all_hash_size; i++) {
-                if (value[i] > simi[0]) {
-                    minheap_pop (k, simi, idxi);
-                    minheap_push (k, simi, idxi, value[i], labels[i]);
-                }
-            }
-            minheap_reorder (k, simi, idxi);
         }
-        delete[] value;
-        delete[] labels;
+    }
 
-    } else {
+    for (size_t t = 1; t < thread_max_num; t++) {
+        // merge hash
+        for (size_t i = 0; i < nx; i++) {
+            float * __restrict value_x = value + i * k;
+            int64_t * __restrict labels_x = labels + i * k;
+            float *value_x_t = value_x + t * thread_hash_size;
+            int64_t *labels_x_t = labels_x + t * thread_hash_size;
+            for (size_t j = 0; j < k; j++) {
+                if (value_x_t[j] > value_x[0]) {
+                    minheap_pop (k, value_x, labels_x);
+                    minheap_push (k, value_x, labels_x, value_x_t[j], labels_x_t[j]);
+                }
+            }
+        }
+    }
+
+    for (size_t i = 0; i < nx; i++) {
+        float * value_x = value + i * k;
+        int64_t * labels_x = labels + i * k;
+        minheap_reorder (k, value_x, labels_x);
+    }
+
+    // copy result
+    memcpy(res->val, value, thread_hash_size * sizeof(float));
+    memcpy(res->ids, labels, thread_hash_size * sizeof(int64_t));
+
+    delete[] value;
+    delete[] labels;
+
+/*
+    else {
         size_t check_period = InterruptCallback::get_period_hint (ny * d);
         check_period *= thread_max_num;
 
@@ -230,6 +248,7 @@ static void knn_inner_product_sse (const float * x,
             InterruptCallback::check ();
         }
     }
+    */
 }
 
 static void knn_L2sqr_sse (
@@ -242,55 +261,68 @@ static void knn_L2sqr_sse (
     size_t k = res->k;
 
     size_t thread_max_num = omp_get_max_threads();
-    if (nx < 4) {
-        // omp for ny
-        size_t all_hash_size = thread_max_num * k;
-        float *value = new float[all_hash_size];
-        int64_t *labels = new int64_t[all_hash_size];
 
-        for (size_t i = 0; i < nx; i++) {
-            // init hash
-            for (size_t i = 0; i < all_hash_size; i++) {
-                value[i] = 1.0 / 0.0;
-            }
-            for (size_t i = 0; i < k; i++) {
-                labels[i] = -1;
-            }
-            const float *x_i = x + i * d;
+    size_t thread_hash_size = nx * k;
+    size_t all_hash_size = thread_hash_size * thread_max_num;
+    float *value = new float[all_hash_size];
+    int64_t *labels = new int64_t[all_hash_size];
+
+    // init hash
+    for (size_t i = 0; i < all_hash_size; i++) {
+        value[i] = 1.0 / 0.0;
+        labels[i] = -1;
+    }
+
 #pragma omp parallel for
-            for (size_t j = 0; j < ny; j++) {
-                if(!bitset || !bitset->test(j)) {
-                    const float *y_j = y + j * d;
-                    float disij = fvec_L2sqr (x_i, y_j, d);
+    for (size_t j = 0; j < ny; j++) {
+        if(!bitset || !bitset->test(j)) {
+            size_t thread_no = omp_get_thread_num();
+            const float *y_j = y + j * d;
+            for (size_t i = 0; i < nx; i++) {
+                const float *x_i = x + i * d;
+                float disij = fvec_L2sqr (x_i, y_j, d);
 
-                    size_t thread_no = omp_get_thread_num();
-                    float * __restrict val_ = value + thread_no * k;
-                    int64_t * __restrict ids_ = labels + thread_no * k;
-                    if (disij < val_[0]) {
-                        maxheap_pop (k, val_, ids_);
-                        maxheap_push (k, val_, ids_, disij, j);
-                    }
+                float * val_ = value + thread_no * thread_hash_size + i * k;
+                int64_t * ids_ = labels + thread_no * thread_hash_size + i * k;
+                if (disij < val_[0]) {
+                    maxheap_pop (k, val_, ids_);
+                    maxheap_push (k, val_, ids_, disij, j);
                 }
             }
-
-            // merge hash
-            float * __restrict simi = res->get_val(i);
-            int64_t * __restrict idxi = res->get_ids (i);
-            memcpy(simi, value, k * sizeof(float));
-            memcpy(idxi, labels, k * sizeof(int64_t));
-            maxheap_heapify (k, simi, idxi, value, labels, k);
-            for (size_t i = k; i < all_hash_size; i++) {
-                if (value[i] < simi[0]) {
-                    maxheap_pop (k, simi, idxi);
-                    maxheap_push (k, simi, idxi, value[i], labels[i]);
-                }
-            }
-            maxheap_reorder (k, simi, idxi);
         }
-        delete[] value;
-        delete[] labels;
+    }
 
-    } else {
+    for (size_t t = 1; t < thread_max_num; t++) {
+        // merge hash
+        for (size_t i = 0; i < nx; i++) {
+            float * __restrict value_x = value + i * k;
+            int64_t * __restrict labels_x = labels + i * k;
+            float *value_x_t = value_x + t * thread_hash_size;
+            int64_t *labels_x_t = labels_x + t * thread_hash_size;
+            for (size_t j = 0; j < k; j++) {
+                if (value_x_t[j] < value_x[0]) {
+                    maxheap_pop (k, value_x, labels_x);
+                    maxheap_push (k, value_x, labels_x, value_x_t[j], labels_x_t[j]);
+                }
+            }
+        }
+    }
+
+    for (size_t i = 0; i < nx; i++) {
+        float * value_x = value + i * k;
+        int64_t * labels_x = labels + i * k;
+        maxheap_reorder (k, value_x, labels_x);
+    }
+
+    // copy result
+    memcpy(res->val, value, thread_hash_size * sizeof(float));
+    memcpy(res->ids, labels, thread_hash_size * sizeof(int64_t));
+
+    delete[] value;
+    delete[] labels;
+
+    /*
+    else {
         size_t check_period = InterruptCallback::get_period_hint (ny * d);
         check_period *= thread_max_num;
 
@@ -322,6 +354,7 @@ static void knn_L2sqr_sse (
             InterruptCallback::check ();
         }
     }
+    */
 }
 
 
diff --git a/core/src/index/thirdparty/faiss/utils/hamming.cpp b/core/src/index/thirdparty/faiss/utils/hamming.cpp
index 6fb4c95d56..0760bc0dd3 100644
--- a/core/src/index/thirdparty/faiss/utils/hamming.cpp
+++ b/core/src/index/thirdparty/faiss/utils/hamming.cpp
@@ -40,7 +40,7 @@
 #include <faiss/utils/utils.h>
 
 static const size_t BLOCKSIZE_QUERY = 8192;
-
+static const size_t size_1M = 1 * 1024 * 1024;
 
 namespace faiss {
 
@@ -278,50 +278,69 @@ void hammings_knn_hc (
         ConcurrentBitsetPtr bitset = nullptr)
 {
     size_t k = ha->k;
-    if (init_heap) ha->heapify ();
 
-    int thread_max_num = omp_get_max_threads();
-    if (ha->nh < 4) {
-        // omp for n2
-        int all_hash_size = thread_max_num * k;
+    if ((bytes_per_code + k * (sizeof(hamdis_t) + sizeof(int64_t))) * ha->nh < size_1M) {
+        int thread_max_num = omp_get_max_threads();
+        // init hash
+        size_t thread_hash_size = ha->nh * k;
+        size_t all_hash_size = thread_hash_size * thread_max_num;
         hamdis_t *value = new hamdis_t[all_hash_size];
         int64_t *labels = new int64_t[all_hash_size];
+        for (int i = 0; i < all_hash_size; i++) {
+            value[i] = 0x7fffffff;
+            labels[i] = -1;
+        }
+
+        HammingComputer *hc = new HammingComputer[ha->nh];
+        for (size_t i = 0; i < ha->nh; i++) {
+            hc[i].set(bs1 + i * bytes_per_code, bytes_per_code);
+        }
 
-        for (int i = 0; i < ha->nh; i++) {
-            HammingComputer hc (bs1 + i * bytes_per_code, bytes_per_code);
-            // init hash
-            for (int i = 0; i < all_hash_size; i++) {
-                value[i] = 0x7fffffff;
-            }
 #pragma omp parallel for
-            for (size_t j = 0; j < n2; j++) {
-                if(!bitset || !bitset->test(j)) {
-                    const uint8_t * bs2_ = bs2 + j * bytes_per_code;
-                    hamdis_t dis = hc.hamming (bs2_);
+        for (size_t j = 0; j < n2; j++) {
+            if(!bitset || !bitset->test(j)) {
+                int thread_no = omp_get_thread_num();
 
-                    int thread_no = omp_get_thread_num();
-                    hamdis_t * __restrict val_ = value + thread_no * k;
-                    int64_t * __restrict ids_ = labels + thread_no * k;
+                const uint8_t * bs2_ = bs2 + j * bytes_per_code;
+                for (size_t i = 0; i < ha->nh; i++) {
+                    hamdis_t dis = hc[i].hamming (bs2_);
+
+                    hamdis_t * val_ = value + thread_no * thread_hash_size + i * k;
+                    int64_t * ids_ = labels + thread_no * thread_hash_size + i * k;
                     if (dis < val_[0]) {
                         faiss::maxheap_pop<hamdis_t> (k, val_, ids_);
                         faiss::maxheap_push<hamdis_t> (k, val_, ids_, dis, j);
                     }
                 }
             }
+        }
+
+        for (size_t t = 1; t < thread_max_num; t++) {
             // merge hash
-            hamdis_t * __restrict bh_val_ = ha->val + i * k;
-            int64_t * __restrict bh_ids_ = ha->ids + i * k;
-            for (int i = 0; i < all_hash_size; i++) {
-                if (value[i] < bh_val_[0]) {
-                    faiss::maxheap_pop<hamdis_t> (k, bh_val_, bh_ids_);
-                    faiss::maxheap_push<hamdis_t> (k, bh_val_, bh_ids_, value[i], labels[i]);
+            for (size_t i = 0; i < ha->nh; i++) {
+                hamdis_t * __restrict value_x = value + i * k;
+                int64_t * __restrict labels_x = labels + i * k;
+                hamdis_t *value_x_t = value_x + t * thread_hash_size;
+                int64_t *labels_x_t = labels_x + t * thread_hash_size;
+                for (size_t j = 0; j < k; j++) {
+                    if (value_x_t[j] < value_x[0]) {
+                        faiss::maxheap_pop<hamdis_t> (k, value_x, labels_x);
+                        faiss::maxheap_push<hamdis_t> (k, value_x, labels_x, value_x_t[j], labels_x_t[j]);
+                    }
                 }
             }
         }
+
+        // copy result
+        memcpy(ha->val, value, thread_hash_size * sizeof(hamdis_t));
+        memcpy(ha->ids, labels, thread_hash_size * sizeof(int64_t));
+
+        delete[] hc;
         delete[] value;
         delete[] labels;
 
     } else {
+        if (init_heap) ha->heapify ();
         const size_t block_size = hamming_batch_size;
         for (size_t j0 = 0; j0 < n2; j0 += block_size) {
         const size_t j1 = std::min(j0 + block_size, n2);
@@ -426,48 +445,46 @@ void hammings_knn_hc_1 (
     const size_t nwords = 1;
     size_t k = ha->k;
 
-
     if (init_heap) {
         ha->heapify ();
     }
 
     int thread_max_num = omp_get_max_threads();
-    if (ha->nh < 4) {
+    if (ha->nh == 1) {
         // omp for n2
         int all_hash_size = thread_max_num * k;
         hamdis_t *value = new hamdis_t[all_hash_size];
         int64_t *labels = new int64_t[all_hash_size];
 
-        for (int i = 0; i < ha->nh; i++) {
-            // init hash
-            for (int i = 0; i < all_hash_size; i++) {
-                value[i] = 0x7fffffff;
-            }
-            const uint64_t bs1_ = bs1 [i];
+        // init hash
+        for (int i = 0; i < all_hash_size; i++) {
+            value[i] = 0x7fffffff;
+        }
+        const uint64_t bs1_ = bs1[0];
 #pragma omp parallel for
-            for (size_t j = 0; j < n2; j++) {
-                if(!bitset || !bitset->test(j)) {
-                    hamdis_t dis = popcount64 (bs1_ ^ bs2[j]);
+        for (size_t j = 0; j < n2; j++) {
+            if(!bitset || !bitset->test(j)) {
+                hamdis_t dis = popcount64 (bs1_ ^ bs2[j]);
 
-                    int thread_no = omp_get_thread_num();
-                    hamdis_t * __restrict val_ = value + thread_no * k;
-                    int64_t * __restrict ids_ = labels + thread_no * k;
-                    if (dis < val_[0]) {
-                        faiss::maxheap_pop<hamdis_t> (k, val_, ids_);
-                        faiss::maxheap_push<hamdis_t> (k, val_, ids_, dis, j);
-                    }
-                }
-            }
-            // merge hash
-            hamdis_t * __restrict bh_val_ = ha->val + i * k;
-            int64_t * __restrict bh_ids_ = ha->ids + i * k;
-            for (int i = 0; i < all_hash_size; i++) {
-                if (value[i] < bh_val_[0]) {
-                    faiss::maxheap_pop<hamdis_t> (k, bh_val_, bh_ids_);
-                    faiss::maxheap_push<hamdis_t> (k, bh_val_, bh_ids_, value[i], labels[i]);
+                int thread_no = omp_get_thread_num();
+                hamdis_t * __restrict val_ = value + thread_no * k;
+                int64_t * __restrict ids_ = labels + thread_no * k;
+                if (dis < val_[0]) {
+                    faiss::maxheap_pop<hamdis_t> (k, val_, ids_);
+                    faiss::maxheap_push<hamdis_t> (k, val_, ids_, dis, j);
                 }
             }
         }
+        // merge hash
+        hamdis_t * __restrict bh_val_ = ha->val;
+        int64_t * __restrict bh_ids_ = ha->ids;
+        for (int i = 0; i < all_hash_size; i++) {
+            if (value[i] < bh_val_[0]) {
+                faiss::maxheap_pop<hamdis_t> (k, bh_val_, bh_ids_);
+                faiss::maxheap_push<hamdis_t> (k, bh_val_, bh_ids_, value[i], labels[i]);
+            }
+        }
+
         delete[] value;
         delete[] labels;
 
diff --git a/core/src/index/thirdparty/faiss/utils/superstructure-inl.h b/core/src/index/thirdparty/faiss/utils/superstructure-inl.h
index 8df39992ae..1ebf8946a5 100644
--- a/core/src/index/thirdparty/faiss/utils/superstructure-inl.h
+++ b/core/src/index/thirdparty/faiss/utils/superstructure-inl.h
@@ -8,13 +8,13 @@ namespace faiss {
 
         SuperstructureComputer8 (const uint8_t *a8, int code_size) {
             set (a8, code_size);
-            accu_den = (float)(popcount64 (a0));
         }
 
         void set (const uint8_t *a8, int code_size) {
             assert (code_size == 8);
             const uint64_t *a = (uint64_t *)a8;
             a0 = a[0];
+            accu_den = (float)(popcount64 (a0));
         }
 
         inline float compute (const uint8_t *b8) const {
@@ -35,13 +35,13 @@ namespace faiss {
 
         SuperstructureComputer16 (const uint8_t *a8, int code_size) {
             set (a8, code_size);
-            accu_den = (float)(popcount64 (a0) + popcount64 (a1));
         }
 
         void set (const uint8_t *a8, int code_size) {
             assert (code_size == 16);
             const uint64_t *a = (uint64_t *)a8;
             a0 = a[0]; a1 = a[1];
+            accu_den = (float)(popcount64 (a0) + popcount64 (a1));
         }
 
         inline float compute (const uint8_t *b8) const {