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118 lines
3.1 KiB
C++
118 lines
3.1 KiB
C++
// SPDX-License-Identifier: BSD-2-Clause
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// Copyright CM4all GmbH
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// author: Max Kellermann <mk@cm4all.com>
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#pragma once
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#include "Chrono.hxx"
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#include "util/IntrusiveList.hxx"
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#include <array>
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#include <algorithm>
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class CoarseTimerEvent;
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/**
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* A list of #CoarseTimerEvent instances managed in a circular timer
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* wheel.
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*/
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class TimerWheel final {
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static constexpr Event::Duration RESOLUTION = std::chrono::seconds(1);
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static constexpr Event::Duration SPAN = std::chrono::minutes(2);
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static_assert(SPAN % RESOLUTION == Event::Duration::zero());
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static constexpr std::size_t N_BUCKETS = SPAN / RESOLUTION;
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using List = IntrusiveList<CoarseTimerEvent>;
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/**
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* Each bucket contains a doubly linked list of
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* #CoarseTimerEvent instances scheduled for one #RESOLUTION.
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*
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* Timers scheduled far into the future (more than #SPAN) may
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* also sit in between, so anybody walking those lists should
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* check the due time.
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*/
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std::array<List, N_BUCKETS> buckets;
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/**
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* A list of timers which are already ready. This can happen
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* if they are scheduled with a zero duration or scheduled in
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* the past.
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*/
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List ready;
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/**
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* The last time Run() was invoked. This is needed to
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* determine the range of buckets to be checked, because we
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* can't rely on getting a caller for every bucket; there may
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* be arbitrary delays.
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*/
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Event::TimePoint last_time{};
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/**
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* If this flag is true, then all buckets are guaranteed to be
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* empty. If it is false, the buckets may or may not be
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* empty; if so, the next full scan will set it back to true.
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*
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* This field is "mutable" so the "const" method GetSleep()
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* can update it.
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*/
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mutable bool empty = true;
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public:
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TimerWheel() noexcept;
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~TimerWheel() noexcept;
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bool IsEmpty() const noexcept {
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return ready.empty() &&
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std::all_of(buckets.begin(), buckets.end(),
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[](const auto &list){
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return list.empty();
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});
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}
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void Insert(CoarseTimerEvent &t,
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Event::TimePoint now) noexcept;
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/**
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* Invoke all expired #CoarseTimerEvent instances and return
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* the duration until the next timer expires. Returns a
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* negative duration if there is no timeout.
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*/
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Event::Duration Run(Event::TimePoint now) noexcept;
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private:
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static constexpr std::size_t NextBucketIndex(std::size_t i) noexcept {
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return (i + 1) % N_BUCKETS;
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}
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static constexpr std::size_t BucketIndexAt(Event::TimePoint t) noexcept {
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return std::size_t(t.time_since_epoch() / RESOLUTION)
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% N_BUCKETS;
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}
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static constexpr Event::TimePoint GetBucketStartTime(Event::TimePoint t) noexcept {
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return t - t.time_since_epoch() % RESOLUTION;
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}
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/**
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* What is the end time of the next non-empty bucket?
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*
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* @param bucket_index start searching at this bucket index
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* @return the bucket end time or max() if the wheel is empty
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*/
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[[gnu::pure]]
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Event::TimePoint GetNextDue(std::size_t bucket_index,
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Event::TimePoint bucket_start_time) const noexcept;
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[[gnu::pure]]
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Event::Duration GetSleep(Event::TimePoint now) const noexcept;
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/**
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* Run all due timers in this bucket.
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*/
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static void Run(List &list, Event::TimePoint now) noexcept;
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};
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