Add first, experimental version of 'sleep on empty flag'.

lib/pls/CMakeLists.txt

@@ -43,7 +43,7 @@ add_library(pls STATIC
 
         include/pls/internal/profiling/dag_node.h src/internal/profiling/dag_node.cpp
         include/pls/internal/profiling/profiler.h
-        include/pls/internal/profiling/thread_stats.h src/internal/profiling/thread_stats.cpp src/internal/profiling/profiler.cpp)
+        include/pls/internal/profiling/thread_stats.h src/internal/profiling/thread_stats.cpp src/internal/profiling/profiler.cpp include/pls/internal/build_flavour.h include/pls/internal/base/futex_wrapper.h)
 
 # Dependencies for pls
 target_link_libraries(pls Threads::Threads)

lib/pls/include/pls/internal/base/futex_wrapper.h

+
+#ifndef PLS_INTERNAL_BASE_FUTEX_WRAPPER_H_
+#define PLS_INTERNAL_BASE_FUTEX_WRAPPER_H_
+
+#include <unistd.h>
+#include <sys/syscall.h>
+#include <linux/futex.h>
+#include <ctime>
+#include <cerrno>
+#include <cstdint>
+
+namespace pls::internal::base {
+
+int futex(int *uaddr, int futex_op, int val, const struct timespec *timeout, int *uaddr2, int val3) {
+  return syscall(SYS_futex, uaddr, futex_op, val, timeout, uaddr2, val3);
+}
+
+int futex_wait(int32_t *uaddr, int32_t val) {
+  return futex(uaddr, FUTEX_WAIT_PRIVATE, val, nullptr, nullptr, 0);
+}
+
+int futex_wakeup(int32_t *uaddr, int num_waked) {
+  return futex(uaddr, FUTEX_WAKE_PRIVATE, num_waked, nullptr, nullptr, 0);
+}
+
+}
+
+#endif //PLS_INTERNAL_BASE_FUTEX_WRAPPER_H_

lib/pls/include/pls/internal/build_flavour.h

+
+#ifndef PLS_INTERNAL_BUILD_FLAVOUR_H_
+#define PLS_INTERNAL_BUILD_FLAVOUR_H_
+
+#define PLS_SLEEP_WORKERS_ON_EMPTY true
+
+#endif //PLS_INTERNAL_BUILD_FLAVOUR_H_

lib/pls/include/pls/internal/scheduling/scheduler.h

@@ -7,6 +7,8 @@
 #include <vector>
 #include <memory>
 
+#include "pls/internal/build_flavour.h"
+
 #include "pls/internal/base/barrier.h"
 #include "pls/internal/base/stack_allocator.h"
 
@@ -130,6 +132,15 @@ class scheduler {
 
   std::shared_ptr<base::stack_allocator> stack_allocator_;
 
+#if PLS_SLEEP_WORKERS_ON_EMPTY
+  PLS_CACHE_ALIGN std::atomic<int32_t> empty_queue_counter_{0};
+  PLS_CACHE_ALIGN std::atomic<int32_t> threads_sleeping_{0};
+
+  void empty_queue_try_sleep_worker();
+  void empty_queue_increase_counter();
+  void empty_queue_decrease_counter_and_wake();
+#endif
+
 #if PLS_PROFILING_ENABLED
   profiling::profiler profiler_;
 #endif

lib/pls/include/pls/internal/scheduling/scheduler_impl.h

@@ -86,6 +86,11 @@ class scheduler::init_function_impl : public init_function {
       thread_state::get().get_scheduler().work_section_done_.store(true);
       PLS_ASSERT(thread_state::get().main_continuation().valid(), "Must return valid continuation from main task.");
 
+#if PLS_SLEEP_WORKERS_ON_EMPTY
+      thread_state::get().get_scheduler().empty_queue_counter_.store(-1000);
+      thread_state::get().get_scheduler().empty_queue_decrease_counter_and_wake();
+#endif
+
 #if PLS_PROFILING_ENABLED
       thread_state::get().get_scheduler().profiler_.task_finish_stack_measure(thread_state::get().get_thread_id(),
                                                                               root_task->stack_memory_,
@@ -113,6 +118,13 @@ void scheduler::perform_work(Function work_section) {
   auto *root_node = profiler_.start_profiler_run();
   root_task->profiling_node_ = root_node;
 #endif
+#if PLS_SLEEP_WORKERS_ON_EMPTY
+  empty_queue_counter_.store(0);
+  for (auto &thread_state : thread_states_) {
+    thread_state->get_queue_empty_flag().store({EMPTY_QUEUE_STATE::QUEUE_NON_EMPTY});
+  }
+#endif
+
   work_section_done_ = false;
   if (reuse_thread_) {
     auto &my_state = thread_state_for(0);
@@ -155,6 +167,34 @@ void scheduler::spawn(Function &&lambda) {
     spawned_task->is_synchronized_ = true;
     spawning_state.set_active_task(spawned_task);
     spawning_state.get_task_manager().push_local_task(last_task);
+#if PLS_SLEEP_WORKERS_ON_EMPTY
+    // TODO: relax atomic operations on empty flag
+    data_structures::stamped_integer queue_empty_flag = spawning_state.get_queue_empty_flag().load();
+    switch (queue_empty_flag.value) {
+      case EMPTY_QUEUE_STATE::QUEUE_NON_EMPTY: {
+        // The queue was not found empty, ignore it.
+        break;
+      }
+      case EMPTY_QUEUE_STATE::QUEUE_MAYBE_EMPTY: {
+        // Someone tries to mark us empty and might be re-stealing right now.
+        data_structures::stamped_integer
+            queue_non_empty_flag{queue_empty_flag.stamp++, EMPTY_QUEUE_STATE::QUEUE_NON_EMPTY};
+        auto actual_empty_flag = spawning_state.get_queue_empty_flag().exchange(queue_non_empty_flag);
+        if (actual_empty_flag.value == EMPTY_QUEUE_STATE::QUEUE_EMPTY) {
+          spawning_state.get_scheduler().empty_queue_decrease_counter_and_wake();
+        }
+        break;
+      }
+      case EMPTY_QUEUE_STATE::QUEUE_EMPTY: {
+        // Someone already marked the queue empty, we must revert its action on the central queue.
+        data_structures::stamped_integer
+            queue_non_empty_flag{queue_empty_flag.stamp++, EMPTY_QUEUE_STATE::QUEUE_NON_EMPTY};
+        spawning_state.get_queue_empty_flag().store(queue_non_empty_flag);
+        spawning_state.get_scheduler().empty_queue_decrease_counter_and_wake();
+        break;
+      }
+    }
+#endif
 
     // execute the lambda itself, which could lead to a different thread returning.
 #if PLS_PROFILING_ENABLED

lib/pls/include/pls/internal/scheduling/thread_state.h

@@ -2,19 +2,29 @@
 #ifndef PLS_THREAD_STATE_H
 #define PLS_THREAD_STATE_H
 
+#include <atomic>
 #include <random>
 #include <chrono>
 #include <utility>
 
 #include "context_switcher/continuation.h"
 
+#include "pls/internal/build_flavour.h"
+
 #include "pls/internal/base/system_details.h"
+#include "pls/internal/data_structures/stamped_integer.h"
 
 #include "pls/internal/scheduling/base_task.h"
 #include "pls/internal/scheduling/task_manager.h"
 
 namespace pls::internal::scheduling {
 
+enum EMPTY_QUEUE_STATE : int32_t {
+  QUEUE_NON_EMPTY = 0,
+  QUEUE_MAYBE_EMPTY = 1,
+  QUEUE_EMPTY = 2,
+};
+
 class scheduler;
 /**
  * Proxy-Object for thread local state needed during scheduling.
@@ -34,6 +44,10 @@ struct PLS_CACHE_ALIGN thread_state {
   std::minstd_rand random_;
   base_task *active_task_;
 
+#if PLS_SLEEP_WORKERS_ON_EMPTY
+  PLS_CACHE_ALIGN std::atomic<data_structures::stamped_integer> queue_empty_{EMPTY_QUEUE_STATE::QUEUE_NON_EMPTY};
+#endif
+
  public:
   explicit thread_state(scheduler &scheduler,
                         unsigned thread_id,
@@ -75,6 +89,13 @@ struct PLS_CACHE_ALIGN thread_state {
 
   void set_active_task(base_task *active_task) { active_task_ = active_task; }
   base_task *get_active_task() const { return active_task_; }
+
+#if PLS_SLEEP_WORKERS_ON_EMPTY
+  [[nodiscard]] std::atomic<data_structures::stamped_integer> &get_queue_empty_flag() {
+    return queue_empty_;
+  };
+#endif
+
 };
 
 }

lib/pls/src/internal/scheduling/scheduler.cpp

 #include "pls/internal/scheduling/scheduler.h"
 
 #include "context_switcher/context_switcher.h"
+
+#include "pls/internal/build_flavour.h"
+#include "pls/internal/base/futex_wrapper.h"
 #include "pls/internal/base/error_handling.h"
 
 #include <thread>
@@ -59,6 +62,11 @@ void scheduler::work_thread_work_section() {
     } while (target == my_state.get_thread_id());
 
     thread_state &target_state = my_state.get_scheduler().thread_state_for(target);
+#if PLS_SLEEP_WORKERS_ON_EMPTY
+    queue_empty_flag_retry_steal:
+    // TODO: relax atomics for empty flag
+    data_structures::stamped_integer target_queue_empty_flag = target_state.get_queue_empty_flag().load();
+#endif
 
     base_task *stolen_task;
     base_task *chain_after_stolen_task;
@@ -69,7 +77,7 @@ void scheduler::work_thread_work_section() {
 #endif
       std::tie(stolen_task, chain_after_stolen_task, cas_success) =
           target_state.get_task_manager().steal_task(my_state);
-    } while (!cas_success);
+    } while (!cas_success); // re-try on cas-conflicts, as in classic ws
 
     if (stolen_task) {
       // Keep task chain consistent. We want to appear as if we are working an a branch upwards of the stolen task.
@@ -93,13 +101,48 @@ void scheduler::work_thread_work_section() {
       // We will continue execution in this line when we finished the stolen work.
       failed_steals = 0;
     } else {
+      // TODO: tune value for when we start yielding
+      const unsigned YIELD_AFTER = 1;
+
       failed_steals++;
-      if (failed_steals >= num_threads) {
+      if (failed_steals >= YIELD_AFTER) {
         std::this_thread::yield();
-      }
 #if PLS_PROFILING_ENABLED
-      my_state.get_scheduler().profiler_.stealing_end(my_state.get_thread_id(), false);
+        my_state.get_scheduler().profiler_.stealing_end(my_state.get_thread_id(), false);
 #endif
+#if PLS_SLEEP_WORKERS_ON_EMPTY
+        switch (target_queue_empty_flag.value) {
+          case EMPTY_QUEUE_STATE::QUEUE_NON_EMPTY: {
+            // We found the queue empty, but the flag says it should still be full.
+            // We want to declare it empty, bet we need to re-check the queue in a sub-step to avoid races.
+            data_structures::stamped_integer
+                maybe_empty_flag{target_queue_empty_flag.stamp + 1, EMPTY_QUEUE_STATE::QUEUE_MAYBE_EMPTY};
+            if (target_state.get_queue_empty_flag().compare_exchange_strong(target_queue_empty_flag,
+                                                                            maybe_empty_flag)) {
+              goto queue_empty_flag_retry_steal;
+            }
+            break;
+          }
+          case EMPTY_QUEUE_STATE::QUEUE_MAYBE_EMPTY: {
+            // We found the queue empty and it was already marked as maybe empty.
+            // We can safely mark it empty and increment the central counter.
+            data_structures::stamped_integer
+                empty_flag{target_queue_empty_flag.stamp + 1, EMPTY_QUEUE_STATE::QUEUE_EMPTY};
+            if (target_state.get_queue_empty_flag().compare_exchange_strong(target_queue_empty_flag, empty_flag)) {
+              // We marked it empty, now its our duty to modify the central counter
+              my_state.get_scheduler().empty_queue_increase_counter();
+            }
+            break;
+          }
+          case EMPTY_QUEUE_STATE::QUEUE_EMPTY: {
+            // The queue was already marked empty, just do nothing
+            break;
+          }
+        }
+        // Disregarding if we found the thread empty, we should check if we can put ourself to sleep
+        my_state.get_scheduler().empty_queue_try_sleep_worker();
+#endif
+      }
     }
   }
 }
@@ -253,4 +296,34 @@ bool scheduler::check_task_chain(base_task &start_task) {
   return check_task_chain_backward(start_task) && check_task_chain_forward(start_task);
 }
 
+#if PLS_SLEEP_WORKERS_ON_EMPTY
+// TODO: relax memory orderings
+
+void scheduler::empty_queue_try_sleep_worker() {
+  int32_t counter_value = empty_queue_counter_.load();
+//  printf("try sleep thread %d...\n", counter_value);
+  if (counter_value == num_threads() - 1) {
+//    printf("sleeping thread...\n");
+    threads_sleeping_++;
+    base::futex_wait((int32_t *) &empty_queue_counter_, num_threads() - 1);
+    base::futex_wakeup((int32_t *) &empty_queue_counter_, 1);
+    threads_sleeping_--;
+//    printf("waked thread...\n");
+  }
+}
+
+void scheduler::empty_queue_increase_counter() {
+  int32_t old_value = empty_queue_counter_.fetch_add(1);
+//  printf("increase counter from %d", old_value);
+}
+
+void scheduler::empty_queue_decrease_counter_and_wake() {
+  empty_queue_counter_.fetch_sub(1);
+  if (threads_sleeping_.load() > 0) {
+    // Threads could be sleeping, we MUST wake them up
+    base::futex_wakeup((int32_t *) &empty_queue_counter_, 1);
+  }
+}
+#endif
+
 }