Flow Graph Basics: Single-push vs. Broadcast-push¶
Nodes in the 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.
Please see the Sending to One or Multiple Successors section of Flow Graph Tips and Tricks, and Flow Graph Basics: Buffering and Forwarding for more information.