.. _use_concurrency_limits:
Use Concurrency Limits
======================
To control the number of instances of a single node, you can use the
concurrency limit on the node. To cause it to reject messages after it
reaches its concurrency limit, you construct it as a "rejecting" node.
A function node is constructed with one or more template arguments. The
third argument controls the buffer policy used by the node, and is by
default queueing. With a queueing policy, a ``function_node`` that has
reached its concurrency limit still accepts incoming messages, but
buffers them internally. If the policy is set to rejecting the node will
instead reject the incoming messages.
::
template < typename Input,
typename Output = continue_msg,
graph_buffer_policy = queueing >
class function_node;
For example, you can control the number of big objects in flight in a
graph by placing a rejecting function_node downstream of an ``input_node``,
as is done below:
::
graph g;
int src_count = 0;
int number_of_objects = 0;
int max_objects = 3;
input_node< big_object * > s( g, [&]( oneapi::tbb::flow_control& fc ) -> big_object* {
if ( src_count < M ) {
big_object* v = new big_object();
++src_count;
return v;
} else {
fc.stop();
return nullptr;
}
} );
s.activate();
function_node< big_object *, continue_msg, rejecting > f( g, 3,
[]( big_object *v ) -> continue_msg {
spin_for(1);
delete v;
return continue_msg();
} );
make_edge( s, f );
g.wait_for_all();
The ``function_node`` will operate on at most three big objects
concurrently. The node's concurrency threshold that limits the node to
three concurrent invocations. When the ``function_node`` is running three
instances concurrently, it will start rejecting incoming messages from
the ``input_node``, causing the ``input_node`` to buffer its last created
object and temporarily stop invoking its body object. Whenever the
``function_node`` drops below its concurrency limit, it will pull new
messages from the ``input_node``. At most four big objects will exist
simultaneously, three in the ``function_node`` and one buffered in the
``input_node``.
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