.. _Flow_Graph_Single_Vs_Broadcast:

Flow Graph Basics: Single-push vs. Broadcast-push

Nodes in the |full_name| flow graph communicate by pushing and pulling messages. Two policies for pushing messages are used, depending on the type of the node:

• single-push: No matter how many successors to the node exist and are able to accept a message, each message will be only sent to one successor.
• broadcast-push: A message will be pushed to every successor which is connected to the node by an edge in push mode, and which accepts the message.

The following code demonstrates this difference:

::

using namespace oneapi::tbb::flow;

std::atomic<size_t> g_cnt;

struct fn_body1 { std::atomic<size_t> &body_cnt; fn_body1(std::atomic<size_t> &b_cnt) : body_cnt(b_cnt) {} continue_msg operator()( continue_msg /dont_care/) { ++g_cnt; ++body_cnt; return continue_msg(); } };

void run_example1() { // example for Flow_Graph_Single_Vs_Broadcast.xml graph g; std::atomic<size_t> b1; // local counts std::atomic<size_t> b2; // for each function _node body std::atomic<size_t> b3; // function_node<continue_msg> f1(g,serial,fn_body1(b1)); function_node<continue_msg> f2(g,serial,fn_body1(b2)); function_node<continue_msg> f3(g,serial,fn_body1(b3)); buffer_node<continue_msg> buf1(g); // // single-push policy // g_cnt = b1 = b2 = b3 = 0; make_edge(buf1,f1); make_edge(buf1,f2); make_edge(buf1,f3); buf1.try_put(continue_msg()); buf1.try_put(continue_msg()); buf1.try_put(continue_msg()); g.wait_for_all(); printf( "after single-push test, g_cnt == %d, b1==%d, b2==%d, b3==%d\n", (int)g_cnt, (int)b1, (int)b2, (int)b3); remove_edge(buf1,f1); remove_edge(buf1,f2); remove_edge(buf1,f3); // // broadcast-push policy // broadcast_node<continue_msg> bn(g); g_cnt = b1 = b2 = b3 = 0; make_edge(bn,f1); make_edge(bn,f2); make_edge(bn,f3); bn.try_put(continue_msg()); bn.try_put(continue_msg()); bn.try_put(continue_msg()); g.wait_for_all(); printf( "after broadcast-push test, g_cnt == %d, b1==%d, b2==%d, b3==%d\n", (int)g_cnt, (int)b1, (int)b2, (int)b3); }

The output of this code is

::

after single-push test, g_cnt == 3, b1==3, b2==0, b3==0 after broadcast-push test, g_cnt == 9, b1==3, b2==3, b3==3

The single-push test uses a buffer_node, which has a "single-push" policy for forwarding messages. Putting three messages to the buffer_node results in three messages being pushed. Notice also only the first function_node is sent to; in general there is no policy for which node is pushed to if more than one successor can accept.

The broadcast-push test uses a broadcast_node, which will push any message it receives to all accepting successors. Putting three messages to the broadcast_node results in a total of nine messages pushed to the function_nodes.

Only nodes designed to buffer (hold and forward received messages) have a "single-push" policy; all other nodes have a "broadcast-push" policy.

Please see the :ref:broadcast_or_send section of :ref:Flow_Graph_Tips, and :ref:Flow_Graph_Buffering_in_Nodes for more information.