update source to Ceph Pacific 16.2.2

[ceph.git] / ceph / src / rocksdb / third-party / folly / folly / synchronization / WaitOptions.h

diff --git a/ceph/src/rocksdb/third-party/folly/folly/synchronization/WaitOptions.h b/ceph/src/rocksdb/third-party/folly/folly/synchronization/WaitOptions.h
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+++ b/ceph/src/rocksdb/third-party/folly/folly/synchronization/WaitOptions.h
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+//  Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.
+//  This source code is licensed under both the GPLv2 (found in the
+//  COPYING file in the root directory) and Apache 2.0 License
+//  (found in the LICENSE.Apache file in the root directory).
+
+#pragma once
+
+#include <chrono>
+
+namespace folly {
+
+/// WaitOptions
+///
+/// Various synchronization primitives as well as various concurrent data
+/// structures built using them have operations which might wait. This type
+/// represents a set of options for controlling such waiting.
+class WaitOptions {
+ public:
+  struct Defaults {
+    /// spin_max
+    ///
+    /// If multiple threads are actively using a synchronization primitive,
+    /// whether indirectly via a higher-level concurrent data structure or
+    /// directly, where the synchronization primitive has an operation which
+    /// waits and another operation which wakes the waiter, it is common for
+    /// wait and wake events to happen almost at the same time. In this state,
+    /// we lose big 50% of the time if the wait blocks immediately.
+    ///
+    /// We can improve our chances of being waked immediately, before blocking,
+    /// by spinning for a short duration, although we have to balance this
+    /// against the extra cpu utilization, latency reduction, power consumption,
+    /// and priority inversion effect if we end up blocking anyway.
+    ///
+    /// We use a default maximum of 2 usec of spinning. As partial consolation,
+    /// since spinning as implemented in folly uses the pause instruction where
+    /// available, we give a small speed boost to the colocated hyperthread.
+    ///
+    /// On circa-2013 devbox hardware, it costs about 7 usec to FUTEX_WAIT and
+    /// then be awoken. Spins on this hw take about 7 nsec, where all but 0.5
+    /// nsec is the pause instruction.
+    static constexpr std::chrono::nanoseconds spin_max =
+        std::chrono::microseconds(2);
+  };
+
+  std::chrono::nanoseconds spin_max() const {
+    return spin_max_;
+  }
+  WaitOptions& spin_max(std::chrono::nanoseconds dur) {
+    spin_max_ = dur;
+    return *this;
+  }
+
+ private:
+  std::chrono::nanoseconds spin_max_ = Defaults::spin_max;
+};
+
+} // namespace folly