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