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/*
* Copyright 2007-2021 CM4all GmbH
* All rights reserved.
*
* author: Max Kellermann <mk@cm4all.com>
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
*
* - Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
*
* - Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the
* distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
* FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
* FOUNDATION OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
* (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
* SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
* STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
* OF THE POSSIBILITY OF SUCH DAMAGE.
*/
#pragma once
#include "Chrono.hxx"
#include "util/IntrusiveList.hxx"
#include <array>
#include <algorithm>
class CoarseTimerEvent;
/**
* A list of #CoarseTimerEvent instances managed in a circular timer
* wheel.
*/
class TimerWheel final {
static constexpr Event::Duration RESOLUTION = std::chrono::milliseconds(250);
static constexpr Event::Duration SPAN = std::chrono::minutes(1);
static_assert(SPAN % RESOLUTION == Event::Duration::zero());
static constexpr std::size_t N_BUCKETS = SPAN / RESOLUTION;
using List = IntrusiveList<CoarseTimerEvent>;
/**
* Each bucket contains a doubly linked list of
* #CoarseTimerEvent instances scheduled for one #RESOLUTION.
*
* Timers scheduled far into the future (more than #SPAN) may
* also sit in between, so anybody walking those lists should
* check the due time.
*/
std::array<List, N_BUCKETS> buckets;
/**
* A list of timers which are already ready. This can happen
* if they are scheduled with a zero duration or scheduled in
* the past.
*/
List ready;
/**
* The last time Run() was invoked. This is needed to
* determine the range of buckets to be checked, because we
* can't rely on getting a caller for every bucket; there may
* be arbitrary delays.
*/
Event::TimePoint last_time{};
/**
* If this flag is true, then all buckets are guaranteed to be
* empty. If it is false, the buckets may or may not be
* empty; if so, the next full scan will set it back to true.
*
* This field is "mutable" so the "const" method GetSleep()
* can update it.
*/
mutable bool empty = true;
public:
TimerWheel() noexcept;
~TimerWheel() noexcept;
bool IsEmpty() const noexcept {
return ready.empty() &&
std::all_of(buckets.begin(), buckets.end(),
[](const auto &list){
return list.empty();
});
}
void Insert(CoarseTimerEvent &t,
Event::TimePoint now) noexcept;
/**
* Invoke all expired #CoarseTimerEvent instances and return
* the duration until the next timer expires. Returns a
* negative duration if there is no timeout.
*/
Event::Duration Run(Event::TimePoint now) noexcept;
private:
static constexpr std::size_t NextBucketIndex(std::size_t i) noexcept {
return (i + 1) % N_BUCKETS;
}
static constexpr std::size_t BucketIndexAt(Event::TimePoint t) noexcept {
return std::size_t(t.time_since_epoch() / RESOLUTION)
% N_BUCKETS;
}
static constexpr Event::TimePoint GetBucketStartTime(Event::TimePoint t) noexcept {
return t - t.time_since_epoch() % RESOLUTION;
}
/**
* What is the end time of the next non-empty bucket?
*
* @param bucket_index start searching at this bucket index
* @return the bucket end time or max() if the wheel is empty
*/
[[gnu::pure]]
Event::TimePoint GetNextDue(std::size_t bucket_index,
Event::TimePoint bucket_start_time) const noexcept;
[[gnu::pure]]
Event::Duration GetSleep(Event::TimePoint now) const noexcept;
/**
* Run all due timers in this bucket.
*/
static void Run(List &list, Event::TimePoint now) noexcept;
};
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