Hi,
Recently we faced with some weird behavior in TBB... Sometimes it "forgets" about normal priority tasks when high priority task is created.
Code to reproduce:
std::vector<std::unique_ptr<tbb::task_group>> groups;
groups.resize(10000);
const auto main_thread_id = std::this_thread::get_id();
// Normal priority tasks
std::atomic<size_t> normal_counter = 0;
for (auto& group : groups) {
group = std::make_unique<tbb::task_group>();
group->run([&](){
if (std::this_thread::get_id() != main_thread_id)
std::cout << 1;
++normal_counter;
std::this_thread::sleep_for(std::chrono::milliseconds { 10 });
});
}
std::this_thread::sleep_for(std::chrono::seconds { 1 });
// High priority task, should start during running normal priority task.
tbb::task_group_context context;
context.set_priority(tbb::priority_high);
std::atomic<size_t> high_counter = 0;
tbb::parallel_for(0, 1000, [&](size_t){
if (std::this_thread::get_id() != main_thread_id)
std::cout << 2;
++high_counter;
std::this_thread::sleep_for(std::chrono::milliseconds { 10 });
}, context);
// Big sleep. Normal priority tasks should be already processed by workers when main thread awake.
std::this_thread::sleep_for(std::chrono::seconds { 20 });
// It is expected to see "10000 1000" on the screen
std::cout << std::endl << normal_counter << " " << high_counter << std::endl;
for (auto& group : groups)
group->wait();
// It is expected to see "10000 1000" on the screen
std::cout << std::endl << normal_counter << " " << high_counter << std::endl;
In the code above we have normal priority work (10000 x tbb::task_group) that is being processed by worker threads and high priority work (tbb::parallel_for) that is created simultaneously. Sometimes worker threads complete about ~660 tasks with normal priority and sometimes everyhing. I intentionally used big amount of task groups because this simulates our real usage, so it is not possible to have one big task group :).
One interesting thing: when ::wait() is executed on "forgotten" task it will be executed.
TBB Version: 2019 Update 2.
Is this a bug in TBB?
P.S. Worker threads use 100% cpu time during waiting for "forgotten" tasks.
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