# Predefined Concurrency Limits

[flow_graph.concurrency_limits]

Predefined constants that can be used as `function_node`, `multifunction_node`, and `async_node` constructors arguments to define concurrency limit.

    // Defined in header <oneapi/tbb/flow_graph.h>

    namespace oneapi {
    namespace tbb {
    namespace flow {

        std::size_t unlimited = /*implementation-defined*/;
        std::size_t serial = /*implementation-defined*/;

    } // namespace flow
    } // namespace tbb
    } // namespace oneapi

`unlimited` concurrency allows an unlimited number of invocations of the body to execute concurrently.

`serial` concurrency allows only a single call of body to execute concurrently.