1 files changed, 132 insertions, 0 deletions
diff --git a/src/crimson/os/alienstore/thread_pool.h b/src/crimson/os/alienstore/thread_pool.h
new file mode 100644
index 000000000..27840da18
--- /dev/null
+++ b/src/crimson/os/alienstore/thread_pool.h
@@ -0,0 +1,132 @@
+// -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*- 
+// vim: ts=8 sw=2 smarttab
+#pragma once
+
+#include <atomic>
+#include <condition_variable>
+#include <tuple>
+#include <type_traits>
+#include <boost/lockfree/queue.hpp>
+#include <boost/optional.hpp>
+#include <seastar/core/future.hh>
+#include <seastar/core/gate.hh>
+#include <seastar/core/reactor.hh>
+#include <seastar/core/semaphore.hh>
+#include <seastar/core/sharded.hh>
+
+namespace crimson::os {
+
+struct WorkItem {
+  virtual ~WorkItem() {}
+  virtual void process() = 0;
+};
+
+template<typename Func>
+struct Task final : WorkItem {
+  using T = std::invoke_result_t<Func>;
+  using future_stored_type_t =
+    std::conditional_t<std::is_void_v<T>,
+		       seastar::internal::future_stored_type_t<>,
+		       seastar::internal::future_stored_type_t<T>>;
+  using futurator_t = seastar::futurize<T>;
+public:
+  explicit Task(Func&& f)
+    : func(std::move(f))
+  {}
+  void process() override {
+    try {
+      if constexpr (std::is_void_v<T>) {
+        func();
+        state.set();
+      } else {
+        state.set(func());
+      }
+    } catch (...) {
+      state.set_exception(std::current_exception());
+    }
+    on_done.write_side().signal(1);
+  }
+  typename futurator_t::type get_future() {
+    return on_done.wait().then([this](size_t) {
+      if (state.failed()) {
+	return futurator_t::make_exception_future(state.get_exception());
+      } else {
+	return futurator_t::from_tuple(state.get_value());
+      }
+    });
+  }
+private:
+  Func func;
+  seastar::future_state<future_stored_type_t> state;
+  seastar::readable_eventfd on_done;
+};
+
+struct SubmitQueue {
+  seastar::semaphore free_slots;
+  seastar::gate pending_tasks;
+  explicit SubmitQueue(size_t num_free_slots)
+    : free_slots(num_free_slots)
+  {}
+  seastar::future<> stop() {
+    return pending_tasks.close();
+  }
+};
+
+/// an engine for scheduling non-seastar tasks from seastar fibers
+class ThreadPool {
+  std::atomic<bool> stopping = false;
+  std::mutex mutex;
+  std::condition_variable cond;
+  std::vector<std::thread> threads;
+  seastar::sharded<SubmitQueue> submit_queue;
+  const size_t queue_size;
+  boost::lockfree::queue<WorkItem*> pending;
+
+  void loop(std::chrono::milliseconds queue_max_wait);
+  bool is_stopping() const {
+    return stopping.load(std::memory_order_relaxed);
+  }
+  static void pin(unsigned cpu_id);
+  seastar::semaphore& local_free_slots() {
+    return submit_queue.local().free_slots;
+  }
+  ThreadPool(const ThreadPool&) = delete;
+  ThreadPool& operator=(const ThreadPool&) = delete;
+public:
+  /**
+   * @param queue_sz the depth of pending queue. before a task is scheduled,
+   *                 it waits in this queue. we will round this number to
+   *                 multiple of the number of cores.
+   * @param n_threads the number of threads in this thread pool.
+   * @param cpu the CPU core to which this thread pool is assigned
+   * @note each @c Task has its own crimson::thread::Condition, which possesses
+   * an fd, so we should keep the size of queue under a reasonable limit.
+   */
+  ThreadPool(size_t n_threads, size_t queue_sz, long cpu);
+  ~ThreadPool();
+  seastar::future<> start();
+  seastar::future<> stop();
+  template<typename Func, typename...Args>
+  auto submit(Func&& func, Args&&... args) {
+    auto packaged = [func=std::move(func),
+                     args=std::forward_as_tuple(args...)] {
+      return std::apply(std::move(func), std::move(args));
+    };
+    return seastar::with_gate(submit_queue.local().pending_tasks,
+      [packaged=std::move(packaged), this] {
+        return local_free_slots().wait()
+          .then([packaged=std::move(packaged), this] {
+            auto task = new Task{std::move(packaged)};
+            auto fut = task->get_future();
+            pending.push(task);
+            cond.notify_one();
+            return fut.finally([task, this] {
+              local_free_slots().signal();
+              delete task;
+            });
+          });
+        });
+  }
+};
+
+} // namespace crimson::os