Add warning that python filter works slow in multi-threaded mode

Author: Dmitry Yashunin

python_bindings/bindings.cpp

@@ -611,6 +611,10 @@ class Index {
         if (num_threads <= 0)
             num_threads = num_threads_default;
 
+        if ((filter != nullptr) && (num_threads != 1)) {
+            std::cout << "Warning: search with python filter works slow in multi-threaded mode. For best performance set num_threads=1\n";
+        }
+
         {
             py::gil_scoped_release l;
             get_input_array_shapes(buffer, &rows, &features);

tests/python/bindings_test_filter.py

@@ -47,7 +47,8 @@ def testRandomSelf(self):
         print("Querying only even elements")
         # Query the even elements for themselves and measure recall:
         filter_function = lambda id: id%2 == 0
-        labels, distances = hnsw_index.knn_query(data, k=1, filter=filter_function)
+        # Search with python filter works slow in multi-threaded mode, therefore we set num_threads=1
+        labels, distances = hnsw_index.knn_query(data, k=1, filter=filter_function, num_threads=1)
         self.assertAlmostEqual(np.mean(labels.reshape(-1) == np.arange(len(data))), .5, 3)
         # Verify that there are only even elements:
         self.assertTrue(np.max(np.mod(labels, 2)) == 0)