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1987 lines (1553 loc) · 52.9 KB
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#pragma once
#include "observer.hpp"
#include "taskflow.hpp"
/**
@file executor.hpp
@brief executor include file
*/
namespace tf {
// ----------------------------------------------------------------------------
// Executor Definition
// ----------------------------------------------------------------------------
/** @class Executor
@brief class to create an executor for running a taskflow graph
An executor manages a set of worker threads to run one or multiple taskflows
using an efficient work-stealing scheduling algorithm.
@code{.cpp}
// Declare an executor and a taskflow
tf::Executor executor;
tf::Taskflow taskflow;
// Add three tasks into the taskflow
tf::Task A = taskflow.emplace([] () { std::cout << "This is TaskA\n"; });
tf::Task B = taskflow.emplace([] () { std::cout << "This is TaskB\n"; });
tf::Task C = taskflow.emplace([] () { std::cout << "This is TaskC\n"; });
// Build precedence between tasks
A.precede(B, C);
tf::Future<void> fu = executor.run(taskflow);
fu.wait(); // block until the execution completes
executor.run(taskflow, [](){ std::cout << "end of 1 run"; }).wait();
executor.run_n(taskflow, 4);
executor.wait_for_all(); // block until all associated executions finish
executor.run_n(taskflow, 4, [](){ std::cout << "end of 4 runs"; }).wait();
executor.run_until(taskflow, [cnt=0] () mutable { return ++cnt == 10; });
@endcode
All the @c run methods are @em thread-safe. You can submit multiple
taskflows at the same time to an executor from different threads.
*/
class Executor {
friend class FlowBuilder;
friend class Subflow;
friend class Runtime;
public:
/**
@brief constructs the executor with @c N worker threads
The constructor spawns @c N worker threads to run tasks in a
work-stealing loop. The number of workers must be greater than zero
or an exception will be thrown.
By default, the number of worker threads is equal to the maximum
hardware concurrency returned by std::thread::hardware_concurrency.
*/
explicit Executor(size_t N = std::thread::hardware_concurrency());
/**
@brief destructs the executor
The destructor calls Executor::wait_for_all to wait for all submitted
taskflows to complete and then notifies all worker threads to stop
and join these threads.
*/
~Executor();
/**
@brief runs a taskflow once
@param taskflow a tf::Taskflow object
@return a tf::Future that holds the result of the execution
This member function executes the given taskflow once and returns a tf::Future
object that eventually holds the result of the execution.
@code{.cpp}
tf::Future<void> future = executor.run(taskflow);
// do something else
future.wait();
@endcode
This member function is thread-safe.
@attention
The executor does not own the given taskflow. It is your responsibility to
ensure the taskflow remains alive during its execution.
*/
tf::Future<void> run(Taskflow& taskflow);
/**
@brief runs a moved taskflow once
@param taskflow a moved tf::Taskflow object
@return a tf::Future that holds the result of the execution
This member function executes a moved taskflow once and returns a tf::Future
object that eventually holds the result of the execution.
The executor will take care of the lifetime of the moved taskflow.
@code{.cpp}
tf::Future<void> future = executor.run(std::move(taskflow));
// do something else
future.wait();
@endcode
This member function is thread-safe.
*/
tf::Future<void> run(Taskflow&& taskflow);
/**
@brief runs a taskflow once and invoke a callback upon completion
@param taskflow a tf::Taskflow object
@param callable a callable object to be invoked after this run
@return a tf::Future that holds the result of the execution
This member function executes the given taskflow once and invokes the given
callable when the execution completes.
This member function returns a tf::Future object that
eventually holds the result of the execution.
@code{.cpp}
tf::Future<void> future = executor.run(taskflow, [](){ std::cout << "done"; });
// do something else
future.wait();
@endcode
This member function is thread-safe.
@attention
The executor does not own the given taskflow. It is your responsibility to
ensure the taskflow remains alive during its execution.
*/
template<typename C>
tf::Future<void> run(Taskflow& taskflow, C&& callable);
/**
@brief runs a moved taskflow once and invoke a callback upon completion
@param taskflow a moved tf::Taskflow object
@param callable a callable object to be invoked after this run
@return a tf::Future that holds the result of the execution
This member function executes a moved taskflow once and invokes the given
callable when the execution completes.
This member function returns a tf::Future object that
eventually holds the result of the execution.
The executor will take care of the lifetime of the moved taskflow.
@code{.cpp}
tf::Future<void> future = executor.run(
std::move(taskflow), [](){ std::cout << "done"; }
);
// do something else
future.wait();
@endcode
This member function is thread-safe.
*/
template<typename C>
tf::Future<void> run(Taskflow&& taskflow, C&& callable);
/**
@brief runs a taskflow for @c N times
@param taskflow a tf::Taskflow object
@param N number of runs
@return a tf::Future that holds the result of the execution
This member function executes the given taskflow @c N times and returns a tf::Future
object that eventually holds the result of the execution.
@code{.cpp}
tf::Future<void> future = executor.run_n(taskflow, 2); // run taskflow 2 times
// do something else
future.wait();
@endcode
This member function is thread-safe.
@attention
The executor does not own the given taskflow. It is your responsibility to
ensure the taskflow remains alive during its execution.
*/
tf::Future<void> run_n(Taskflow& taskflow, size_t N);
/**
@brief runs a moved taskflow for @c N times
@param taskflow a moved tf::Taskflow object
@param N number of runs
@return a tf::Future that holds the result of the execution
This member function executes a moved taskflow @c N times and returns a tf::Future
object that eventually holds the result of the execution.
The executor will take care of the lifetime of the moved taskflow.
@code{.cpp}
tf::Future<void> future = executor.run_n(
std::move(taskflow), 2 // run the moved taskflow 2 times
);
// do something else
future.wait();
@endcode
This member function is thread-safe.
*/
tf::Future<void> run_n(Taskflow&& taskflow, size_t N);
/**
@brief runs a taskflow for @c N times and then invokes a callback
@param taskflow a tf::Taskflow
@param N number of runs
@param callable a callable object to be invoked after this run
@return a tf::Future that holds the result of the execution
This member function executes the given taskflow @c N times and invokes the given
callable when the execution completes.
This member function returns a tf::Future object that
eventually holds the result of the execution.
@code{.cpp}
tf::Future<void> future = executor.run(
taskflow, 2, [](){ std::cout << "done"; } // runs taskflow 2 times and invoke
// the lambda to print "done"
);
// do something else
future.wait();
@endcode
This member function is thread-safe.
@attention
The executor does not own the given taskflow. It is your responsibility to
ensure the taskflow remains alive during its execution.
*/
template<typename C>
tf::Future<void> run_n(Taskflow& taskflow, size_t N, C&& callable);
/**
@brief runs a moved taskflow for @c N times and then invokes a callback
@param taskflow a moved tf::Taskflow
@param N number of runs
@param callable a callable object to be invoked after this run
@return a tf::Future that holds the result of the execution
This member function executes a moved taskflow @c N times and invokes the given
callable when the execution completes.
This member function returns a tf::Future object that
eventually holds the result of the execution.
@code{.cpp}
tf::Future<void> future = executor.run(
// run the moved taskflow 2 times and invoke the lambda to print "done"
std::move(taskflow), 2, [](){ std::cout << "done"; }
);
// do something else
future.wait();
@endcode
This member function is thread-safe.
*/
template<typename C>
tf::Future<void> run_n(Taskflow&& taskflow, size_t N, C&& callable);
/**
@brief runs a taskflow multiple times until the predicate becomes true
@param taskflow a tf::Taskflow
@param pred a boolean predicate to return @c true for stop
@return a tf::Future that holds the result of the execution
This member function executes the given taskflow multiple times until
the predicate returns @c true.
This member function returns a tf::Future object that
eventually holds the result of the execution.
@code{.cpp}
tf::Future<void> future = executor.run(
taskflow, [](){ return rand()%10 == 0 }
);
// do something else
future.wait();
@endcode
This member function is thread-safe.
@attention
The executor does not own the given taskflow. It is your responsibility to
ensure the taskflow remains alive during its execution.
*/
template<typename P>
tf::Future<void> run_until(Taskflow& taskflow, P&& pred);
/**
@brief runs a moved taskflow and keeps running it
until the predicate becomes true
@param taskflow a moved tf::Taskflow object
@param pred a boolean predicate to return @c true for stop
@return a tf::Future that holds the result of the execution
This member function executes a moved taskflow multiple times until
the predicate returns @c true.
This member function returns a tf::Future object that
eventually holds the result of the execution.
The executor will take care of the lifetime of the moved taskflow.
@code{.cpp}
tf::Future<void> future = executor.run(
std::move(taskflow), [](){ return rand()%10 == 0 }
);
// do something else
future.wait();
@endcode
This member function is thread-safe.
*/
template<typename P>
tf::Future<void> run_until(Taskflow&& taskflow, P&& pred);
/**
@brief runs a taskflow multiple times until the predicate becomes true and
then invokes the callback
@param taskflow a tf::Taskflow
@param pred a boolean predicate to return @c true for stop
@param callable a callable object to be invoked after this run completes
@return a tf::Future that holds the result of the execution
This member function executes the given taskflow multiple times until
the predicate returns @c true and then invokes the given callable when
the execution completes.
This member function returns a tf::Future object that
eventually holds the result of the execution.
@code{.cpp}
tf::Future<void> future = executor.run(
taskflow, [](){ return rand()%10 == 0 }, [](){ std::cout << "done"; }
);
// do something else
future.wait();
@endcode
This member function is thread-safe.
@attention
The executor does not own the given taskflow. It is your responsibility to
ensure the taskflow remains alive during its execution.
*/
template<typename P, typename C>
tf::Future<void> run_until(Taskflow& taskflow, P&& pred, C&& callable);
/**
@brief runs a moved taskflow and keeps running
it until the predicate becomes true and then invokes the callback
@param taskflow a moved tf::Taskflow
@param pred a boolean predicate to return @c true for stop
@param callable a callable object to be invoked after this run completes
@return a tf::Future that holds the result of the execution
This member function executes a moved taskflow multiple times until
the predicate returns @c true and then invokes the given callable when
the execution completes.
This member function returns a tf::Future object that
eventually holds the result of the execution.
The executor will take care of the lifetime of the moved taskflow.
@code{.cpp}
tf::Future<void> future = executor.run(
std::move(taskflow),
[](){ return rand()%10 == 0 }, [](){ std::cout << "done"; }
);
// do something else
future.wait();
@endcode
This member function is thread-safe.
*/
template<typename P, typename C>
tf::Future<void> run_until(Taskflow&& taskflow, P&& pred, C&& callable);
/**
@brief wait for all tasks to complete
This member function waits until all submitted tasks
(e.g., taskflows, asynchronous tasks) to finish.
@code{.cpp}
executor.run(taskflow1);
executor.run_n(taskflow2, 10);
executor.run_n(taskflow3, 100);
executor.wait_for_all(); // wait until the above submitted taskflows finish
@endcode
*/
void wait_for_all();
/**
@brief queries the number of worker threads
Each worker represents one unique thread spawned by an executor
upon its construction time.
@code{.cpp}
tf::Executor executor(4);
std::cout << executor.num_workers(); // 4
@endcode
*/
size_t num_workers() const noexcept;
/**
@brief queries the number of running topologies at the time of this call
When a taskflow is submitted to an executor, a topology is created to store
runtime metadata of the running taskflow.
When the execution of the submitted taskflow finishes,
its corresponding topology will be removed from the executor.
@code{.cpp}
executor.run(taskflow);
std::cout << executor.num_topologies(); // 0 or 1 (taskflow still running)
@endcode
*/
size_t num_topologies() const;
/**
@brief queries the number of running taskflows with moved ownership
@code{.cpp}
executor.run(std::move(taskflow));
std::cout << executor.num_taskflows(); // 0 or 1 (taskflow still running)
@endcode
*/
size_t num_taskflows() const;
/**
@brief queries the id of the caller thread in this executor
Each worker has an unique id in the range of @c 0 to @c N-1 associated with
its parent executor.
If the caller thread does not belong to the executor, @c -1 is returned.
@code{.cpp}
tf::Executor executor(4); // 4 workers in the executor
executor.this_worker_id(); // -1 (main thread is not a worker)
taskflow.emplace([&](){
std::cout << executor.this_worker_id(); // 0, 1, 2, or 3
});
executor.run(taskflow);
@endcode
*/
int this_worker_id() const;
/**
@brief runs a given function asynchronously
@tparam F callable type
@tparam ArgsT parameter types
@param f callable object to call
@param args parameters to pass to the callable
@return a tf::Future that will holds the result of the execution
The method creates an asynchronous task to launch the given
function on the given arguments.
Unlike std::async, the return here is a @em tf::Future that holds
an optional object to the result.
If the asynchronous task is cancelled before it runs, the return is
a @c std::nullopt, or the value returned by the callable.
@code{.cpp}
tf::Fugure<std::optional<int>> future = executor.async([](){
std::cout << "create an asynchronous task and returns 1\n";
return 1;
});
@endcode
This member function is thread-safe.
*/
template <typename F, typename... ArgsT>
auto async(F&& f, ArgsT&&... args);
/**
@brief runs a given function asynchronously and gives a name to this task
@tparam F callable type
@tparam ArgsT parameter types
@param name name of the asynchronous task
@param f callable object to call
@param args parameters to pass to the callable
@return a tf::Future that will holds the result of the execution
The method creates a named asynchronous task to launch the given
function on the given arguments.
Naming an asynchronous task is primarily used for profiling and visualizing
the task execution timeline.
Unlike std::async, the return here is a tf::Future that holds
an optional object to the result.
If the asynchronous task is cancelled before it runs, the return is
a @c std::nullopt, or the value returned by the callable.
@code{.cpp}
tf::Fugure<std::optional<int>> future = executor.named_async("name", [](){
std::cout << "create an asynchronous task with a name and returns 1\n";
return 1;
});
@endcode
This member function is thread-safe.
*/
template <typename F, typename... ArgsT>
auto named_async(const std::string& name, F&& f, ArgsT&&... args);
/**
@brief similar to tf::Executor::async but does not return a future object
This member function is more efficient than tf::Executor::async
and is encouraged to use when there is no data returned.
@code{.cpp}
executor.silent_async([](){
std::cout << "create an asynchronous task with no return\n";
});
@endcode
This member function is thread-safe.
*/
template <typename F, typename... ArgsT>
void silent_async(F&& f, ArgsT&&... args);
/**
@brief similar to tf::Executor::named_async but does not return a future object
This member function is more efficient than tf::Executor::named_async
and is encouraged to use when there is no data returned.
@code{.cpp}
executor.named_silent_async("name", [](){
std::cout << "create an asynchronous task with a name and no return\n";
});
@endcode
This member function is thread-safe.
*/
template <typename F, typename... ArgsT>
void named_silent_async(const std::string& name, F&& f, ArgsT&&... args);
/**
@brief constructs an observer to inspect the activities of worker threads
@tparam Observer observer type derived from tf::ObserverInterface
@tparam ArgsT argument parameter pack
@param args arguments to forward to the constructor of the observer
@return a shared pointer to the created observer
Each executor manages a list of observers with shared ownership with callers.
For each of these observers, the two member functions,
tf::ObserverInterface::on_entry and tf::ObserverInterface::on_exit
will be called before and after the execution of a task.
This member function is not thread-safe.
*/
template <typename Observer, typename... ArgsT>
std::shared_ptr<Observer> make_observer(ArgsT&&... args);
/**
@brief removes an observer from the executor
This member function is not thread-safe.
*/
template <typename Observer>
void remove_observer(std::shared_ptr<Observer> observer);
/**
@brief queries the number of observers
*/
size_t num_observers() const noexcept;
private:
std::condition_variable _topology_cv;
std::mutex _taskflow_mutex;
std::mutex _topology_mutex;
std::mutex _wsq_mutex;
size_t _num_topologies {0};
std::unordered_map<std::thread::id, size_t> _wids;
std::vector<Worker> _workers;
std::vector<std::thread> _threads;
std::list<Taskflow> _taskflows;
Notifier _notifier;
TaskQueue<Node*> _wsq;
std::atomic<size_t> _num_actives {0};
std::atomic<size_t> _num_thieves {0};
std::atomic<bool> _done {0};
std::unordered_set<std::shared_ptr<ObserverInterface>> _observers;
Worker* _this_worker();
bool _wait_for_task(Worker&, Node*&);
void _observer_prologue(Worker&, Node*);
void _observer_epilogue(Worker&, Node*);
void _spawn(size_t);
void _worker_loop(Worker&);
void _exploit_task(Worker&, Node*&);
void _explore_task(Worker&, Node*&);
void _consume_task(Worker&, Node*);
void _schedule(Worker&, Node*);
void _schedule(Node*);
void _schedule(Worker&, const SmallVector<Node*>&);
void _schedule(const SmallVector<Node*>&);
void _set_up_topology(Worker*, Topology*);
void _tear_down_topology(Worker&, Topology*);
void _tear_down_async(Node*);
void _tear_down_invoke(Worker&, Node*);
void _cancel_invoke(Worker&, Node*);
void _increment_topology();
void _decrement_topology();
void _decrement_topology_and_notify();
void _invoke(Worker&, Node*);
void _invoke_static_task(Worker&, Node*);
void _invoke_dynamic_task(Worker&, Node*);
void _join_dynamic_task_external(Worker&, Node*, Graph&);
void _join_dynamic_task_internal(Worker&, Node*, Graph&);
void _detach_dynamic_task(Worker&, Node*, Graph&);
void _invoke_condition_task(Worker&, Node*, SmallVector<int>&);
void _invoke_multi_condition_task(Worker&, Node*, SmallVector<int>&);
void _invoke_module_task(Worker&, Node*);
void _invoke_async_task(Worker&, Node*);
void _invoke_silent_async_task(Worker&, Node*);
void _invoke_cudaflow_task(Worker&, Node*);
void _invoke_syclflow_task(Worker&, Node*);
void _invoke_runtime_task(Worker&, Node*);
template <typename C,
std::enable_if_t<is_cudaflow_task_v<C>, void>* = nullptr
>
void _invoke_cudaflow_task_entry(Node*, C&&);
template <typename C, typename Q,
std::enable_if_t<is_syclflow_task_v<C>, void>* = nullptr
>
void _invoke_syclflow_task_entry(Node*, C&&, Q&);
};
// Constructor
inline Executor::Executor(size_t N) :
_workers {N},
_notifier {N} {
if(N == 0) {
TF_THROW("no cpu workers to execute taskflows");
}
_spawn(N);
// instantite the default observer if requested
if(has_env(TF_ENABLE_PROFILER)) {
TFProfManager::get()._manage(make_observer<TFProfObserver>());
}
}
// Destructor
inline Executor::~Executor() {
// wait for all topologies to complete
wait_for_all();
// shut down the scheduler
_done = true;
_notifier.notify(true);
for(auto& t : _threads){
t.join();
}
}
// Function: num_workers
inline size_t Executor::num_workers() const noexcept {
return _workers.size();
}
// Function: num_topologies
inline size_t Executor::num_topologies() const {
return _num_topologies;
}
// Function: num_taskflows
inline size_t Executor::num_taskflows() const {
return _taskflows.size();
}
// Function: _this_worker
inline Worker* Executor::_this_worker() {
auto itr = _wids.find(std::this_thread::get_id());
return itr == _wids.end() ? nullptr : &_workers[itr->second];
}
// Function: named_async
template <typename F, typename... ArgsT>
auto Executor::named_async(const std::string& name, F&& f, ArgsT&&... args) {
_increment_topology();
using T = std::invoke_result_t<F, ArgsT...>;
using R = std::conditional_t<std::is_same_v<T, void>, void, std::optional<T>>;
std::promise<R> p;
auto tpg = std::make_shared<AsyncTopology>();
Future<R> fu(p.get_future(), tpg);
auto node = node_pool.animate(
std::in_place_type_t<Node::Async>{},
[p=make_moc(std::move(p)), f=std::forward<F>(f), args...]
(bool cancel) mutable {
if constexpr(std::is_same_v<R, void>) {
if(!cancel) {
f(args...);
}
p.object.set_value();
}
else {
p.object.set_value(cancel ? std::nullopt : std::make_optional(f(args...)));
}
},
std::move(tpg)
);
node->_name = name;
if(auto w = _this_worker(); w) {
_schedule(*w, node);
}
else{
_schedule(node);
}
return fu;
}
// Function: async
template <typename F, typename... ArgsT>
auto Executor::async(F&& f, ArgsT&&... args) {
return named_async("", std::forward<F>(f), std::forward<ArgsT>(args)...);
}
// Function: named_silent_async
template <typename F, typename... ArgsT>
void Executor::named_silent_async(
const std::string& name, F&& f, ArgsT&&... args
) {
_increment_topology();
Node* node = node_pool.animate(
std::in_place_type_t<Node::SilentAsync>{},
[f=std::forward<F>(f), args...] () mutable {
f(args...);
}
);
node->_name = name;
if(auto w = _this_worker(); w) {
_schedule(*w, node);
}
else {
_schedule(node);
}
}
// Function: silent_async
template <typename F, typename... ArgsT>
void Executor::silent_async(F&& f, ArgsT&&... args) {
named_silent_async("", std::forward<F>(f), std::forward<ArgsT>(args)...);
}
// Function: this_worker_id
inline int Executor::this_worker_id() const {
auto i = _wids.find(std::this_thread::get_id());
return i == _wids.end() ? -1 : static_cast<int>(_workers[i->second]._id);
}
// Procedure: _spawn
inline void Executor::_spawn(size_t N) {
std::mutex mutex;
std::condition_variable cond;
size_t n=0;
for(size_t id=0; id<N; ++id) {
_workers[id]._id = id;
_workers[id]._vtm = id;
_workers[id]._executor = this;
_workers[id]._waiter = &_notifier._waiters[id];
_threads.emplace_back([this] (
Worker& w, std::mutex& mutex, std::condition_variable& cond, size_t& n
) -> void {
// enables the mapping
{
std::scoped_lock lock(mutex);
_wids[std::this_thread::get_id()] = w._id;
if(n++; n == num_workers()) {
cond.notify_one();
}
}
//this_worker().worker = &w;
Node* t = nullptr;
// must use 1 as condition instead of !done
while(1) {
// execute the tasks.
_exploit_task(w, t);
// wait for tasks
if(_wait_for_task(w, t) == false) {
break;
}
}
}, std::ref(_workers[id]), std::ref(mutex), std::ref(cond), std::ref(n));
}
std::unique_lock<std::mutex> lock(mutex);
cond.wait(lock, [&](){ return n==N; });
}
// Function: _consume_task
inline void Executor::_consume_task(Worker& w, Node* p) {
std::uniform_int_distribution<size_t> rdvtm(0, _workers.size()-1);
while(p->_join_counter != 0) {
exploit:
if(auto t = w._wsq.pop(); t) {
_invoke(w, t);
}
else {
size_t num_steals = 0;
//size_t num_pauses = 0;
size_t max_steals = ((_workers.size() + 1) << 1);
explore:
t = (w._id == w._vtm) ? _wsq.steal() : _workers[w._vtm]._wsq.steal();
if(t) {
_invoke(w, t);
goto exploit;
}
else if(p->_join_counter != 0){
if(num_steals++ > max_steals) {
//(num_pauses++ < 100) ? relax_cpu() : std::this_thread::yield();
std::this_thread::yield();
}
//std::this_thread::yield();
w._vtm = rdvtm(w._rdgen);
goto explore;
}
else {
break;
}
}
}
}
// Function: _explore_task
inline void Executor::_explore_task(Worker& w, Node*& t) {
//assert(_workers[w].wsq.empty());
//assert(!t);
size_t num_steals = 0;
size_t num_yields = 0;
size_t max_steals = ((_workers.size() + 1) << 1);
std::uniform_int_distribution<size_t> rdvtm(0, _workers.size()-1);
do {
t = (w._id == w._vtm) ? _wsq.steal() : _workers[w._vtm]._wsq.steal();
if(t) {
break;
}
if(num_steals++ > max_steals) {
std::this_thread::yield();
if(num_yields++ > 100) {
break;
}
}
w._vtm = rdvtm(w._rdgen);
} while(!_done);
}
// Procedure: _exploit_task
inline void Executor::_exploit_task(Worker& w, Node*& t) {
if(t) {
if(_num_actives.fetch_add(1) == 0 && _num_thieves == 0) {
_notifier.notify(false);
}
while(t) {
_invoke(w, t);
t = w._wsq.pop();
}
--_num_actives;
}
}
// Function: _wait_for_task
inline bool Executor::_wait_for_task(Worker& worker, Node*& t) {
wait_for_task:
//assert(!t);
++_num_thieves;
explore_task:
_explore_task(worker, t);
if(t) {
if(_num_thieves.fetch_sub(1) == 1) {
_notifier.notify(false);
}
return true;
}
_notifier.prepare_wait(worker._waiter);
//if(auto vtm = _find_vtm(me); vtm != _workers.size()) {
if(!_wsq.empty()) {
_notifier.cancel_wait(worker._waiter);
//t = (vtm == me) ? _wsq.steal() : _workers[vtm].wsq.steal();
t = _wsq.steal(); // must steal here
if(t) {
if(_num_thieves.fetch_sub(1) == 1) {
_notifier.notify(false);
}
return true;
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