add some documentation, define number of threads per GPU as a constant

cpp/include/cugraph/mtmg/resource_manager.hpp

@@ -121,6 +121,8 @@ class resource_manager_t {
    * @param instance_manager_id a ncclUniqueId that is shared by all processes participating
    *   in this instance.  All processes must use the same ID in this call, it is up
    *   to the calling code to share this ID properly before the call.
+   * @param n_streams           The number of streams to create in a stream pool for
+   *   each GPU.  Defaults to 16.
    *
    * @return unique pointer to instance manager
    */

cpp/tests/mtmg/threaded_test.cu

@@ -94,8 +94,9 @@ class Tests_Multithreaded
     size_t device_buffer_size{64 * 1024 * 1024};
     size_t thread_buffer_size{4 * 1024 * 1024};
 
+    const int num_threads_per_gpu{4};
     int num_gpus    = gpu_list.size();
-    int num_threads = num_gpus * 4;
+    int num_threads = num_gpus * num_threads_per_gpu;
 
     cugraph::mtmg::resource_manager_t resource_manager;
 
@@ -106,8 +107,10 @@ class Tests_Multithreaded
     ncclUniqueId instance_manager_id;
     ncclGetUniqueId(&instance_manager_id);
 
+    // Currently the only uses for multiple streams for each CPU threads
+    // associated with a particular GPU, which is a constant set above
     auto instance_manager = resource_manager.create_instance_manager(
-      resource_manager.registered_ranks(), instance_manager_id, 4);
+      resource_manager.registered_ranks(), instance_manager_id, num_threads_per_gpu);
 
     cugraph::mtmg::edgelist_t<vertex_t, weight_t, edge_t, edge_type_t> edgelist;
     cugraph::mtmg::graph_t<vertex_t, edge_t, true, multi_gpu> graph;