• 2021.4
  • 09/27/2021
  • Public Content
Contents

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

Nodes in the Intel® oneAPI Threading Building Blocks (oneTBB) 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.

Product and Performance Information

1

Performance varies by use, configuration and other factors. Learn more at www.Intel.com/PerformanceIndex.