event/TimerWheel: optimized container for CoarseTimerEvent

src/event/CoarseTimerEvent.cxx

+/*
+ * 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.
+ */
+
+#include "CoarseTimerEvent.hxx"
+#include "Loop.hxx"
+
+void
+CoarseTimerEvent::Schedule(Event::Duration d) noexcept
+{
+	Cancel();
+
+	due = loop.SteadyNow() + d;
+	loop.Insert(*this);
+}
+
+void
+CoarseTimerEvent::ScheduleEarlier(Event::Duration d) noexcept
+{
+	const auto new_due = loop.SteadyNow() + d;
+
+	if (IsPending()) {
+		if (new_due >= due)
+			return;
+
+		Cancel();
+	}
+
+	due = new_due;
+	loop.Insert(*this);
+}

src/event/CoarseTimerEvent.hxx

@@ -32,7 +32,11 @@
 
 #pragma once
 
-#include "TimerEvent.hxx"
+#include "Chrono.hxx"
+#include "util/BindMethod.hxx"
+#include "util/IntrusiveList.hxx"
+
+class EventLoop;
 
 /**
  * This class invokes a callback function after a certain amount of
@@ -47,5 +51,53 @@
  * thread that runs the #EventLoop, except where explicitly documented
  * as thread-safe.
  */
-using CoarseTimerEvent = TimerEvent;
-// TODO: implement
+class CoarseTimerEvent final : AutoUnlinkIntrusiveListHook
+{
+	friend class TimerWheel;
+	friend class IntrusiveList<CoarseTimerEvent>;
+
+	EventLoop &loop;
+
+	using Callback = BoundMethod<void() noexcept>;
+	const Callback callback;
+
+	/**
+	 * When is this timer due?  This is only valid if IsPending()
+	 * returns true.
+	 */
+	Event::TimePoint due;
+
+public:
+	CoarseTimerEvent(EventLoop &_loop, Callback _callback) noexcept
+		:loop(_loop), callback(_callback) {}
+
+	auto &GetEventLoop() const noexcept {
+		return loop;
+	}
+
+	constexpr auto GetDue() const noexcept {
+		return due;
+	}
+
+	bool IsPending() const noexcept {
+		return is_linked();
+	}
+
+	void Schedule(Event::Duration d) noexcept;
+
+	/**
+	 * Like Schedule(), but is a no-op if there is a due time
+	 * earlier than the given one.
+	 */
+	void ScheduleEarlier(Event::Duration d) noexcept;
+
+	void Cancel() noexcept {
+		if (IsPending())
+			unlink();
+	}
+
+private:
+	void Run() noexcept {
+		callback();
+	}
+};

src/event/Loop.cxx

@@ -143,6 +143,13 @@ EventLoop::AbandonFD(SocketEvent &event)  noexcept
 }
 
 void
+EventLoop::Insert(CoarseTimerEvent &t) noexcept
+{
+	coarse_timers.Insert(t);
+	again = true;
+}
+
+void
 EventLoop::Insert(FineTimerEvent &t) noexcept
 {
 	assert(IsInside());
@@ -154,7 +161,15 @@ EventLoop::Insert(FineTimerEvent &t) noexcept
 inline Event::Duration
 EventLoop::HandleTimers() noexcept
 {
-	return timers.Run(SteadyNow());
+	const auto now = SteadyNow();
+
+	auto fine_timeout = timers.Run(now);
+	auto coarse_timeout = coarse_timers.Run(now);
+
+	return fine_timeout.count() < 0 ||
+		(coarse_timeout.count() >= 0 && coarse_timeout < fine_timeout)
+		? coarse_timeout
+		: fine_timeout;
 }
 
 void

src/event/Loop.hxx

@@ -21,6 +21,7 @@
 #define EVENT_LOOP_HXX
 
 #include "Chrono.hxx"
+#include "TimerWheel.hxx"
 #include "TimerList.hxx"
 #include "Backend.hxx"
 #include "SocketEvent.hxx"
@@ -65,6 +66,7 @@ class EventLoop final
 	SocketEvent wake_event{*this, BIND_THIS_METHOD(OnSocketReady), wake_fd.GetSocket()};
 #endif
 
+	TimerWheel coarse_timers;
 	TimerList timers;
 
 	using DeferList = IntrusiveList<DeferEvent>;
@@ -204,6 +206,7 @@ public:
 	 */
 	bool AbandonFD(SocketEvent &event) noexcept;
 
+	void Insert(CoarseTimerEvent &t) noexcept;
 	void Insert(FineTimerEvent &t) noexcept;
 
 	/**

src/event/TimerWheel.cxx

+/*
+ * 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.
+ */
+
+#include "TimerWheel.hxx"
+#include "CoarseTimerEvent.hxx"
+
+#include <cassert>
+
+TimerWheel::TimerWheel() noexcept = default;
+TimerWheel::~TimerWheel() noexcept = default;
+
+void
+TimerWheel::Insert(CoarseTimerEvent &t) noexcept
+{
+	/* if this timer's due time is already in the past, don't
+	   insert it into an older bucket because Run() won't look at
+	   it in this iteration */
+	const auto due = std::max(t.GetDue(), last_time);
+
+	buckets[BucketIndexAt(due)].push_back(t);
+}
+
+void
+TimerWheel::Run(List &list, Event::TimePoint now) noexcept
+{
+	/* move all timers to a temporary list to avoid problems with
+	   canceled timers while we traverse the list */
+	auto tmp = std::move(list);
+
+	tmp.clear_and_dispose([&](auto *t){
+		if (t->GetDue() <= now) {
+			/* this timer is due: run it */
+			t->Run();
+		} else {
+			/* not yet due: move it back to the given
+			   list */
+			list.push_back(*t);
+		}
+	});
+}
+
+inline Event::TimePoint
+TimerWheel::GetNextDue(const std::size_t bucket_index,
+		       const Event::TimePoint bucket_start_time) const noexcept
+{
+	Event::TimePoint t = bucket_start_time;
+
+	for (std::size_t i = bucket_index;;) {
+		t += RESOLUTION;
+
+		if (!buckets[i].empty())
+			/* found a non-empty bucket; return this
+			   bucket's end time */
+			return t;
+
+		i = NextBucketIndex(i);
+		if (i == bucket_index)
+			/* no timer scheduled - no wakeup */
+			return Event::TimePoint::max();
+	}
+}
+
+inline Event::Duration
+TimerWheel::GetSleep(Event::TimePoint now) const noexcept
+{
+	auto t = GetNextDue(BucketIndexAt(now), GetBucketStartTime(now));
+	assert(t > now);
+	if (t == Event::TimePoint::max())
+		return Event::Duration(-1);
+
+	return t - now;
+}
+
+Event::Duration
+TimerWheel::Run(const Event::TimePoint now) noexcept
+{
+	/* check all buckets between the last time we were invoked and
+	   now */
+	const std::size_t start_bucket = BucketIndexAt(last_time);
+	std::size_t end_bucket;
+
+	if (now < last_time || now >= last_time + SPAN - RESOLUTION) {
+		/* too much time has passed (or time warp): check all
+		   buckets */
+		end_bucket = start_bucket;
+	} else {
+		/* check only the relevant range of buckets (between
+		   the last run and now) */
+		/* note, we're not checking the current bucket index,
+		   we stop at the one before that; all timers in the
+		   same bucket shall be combined, so we only execute
+		   it when the bucket end has passed by */
+		end_bucket = BucketIndexAt(now);
+
+		if (start_bucket == end_bucket)
+			/* still on the same bucket - don't run any
+			   timers, instead wait until this bucket end
+			   has passed by */
+			return GetSleep(now);
+	}
+
+	last_time = GetBucketStartTime(now);
+	assert(BucketIndexAt(last_time) == BucketIndexAt(now));
+
+	/* run those buckets */
+
+	for (std::size_t i = start_bucket;;) {
+		Run(buckets[i], now);
+
+		i = NextBucketIndex(i);
+		if (i == end_bucket)
+			break;
+	}
+
+	/* now determine how much time remains until the next
+	   non-empty bucket passes */
+
+	return GetSleep(now);
+}

src/event/TimerWheel.hxx

+/*
+ * 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::seconds(1);
+	static constexpr Event::Duration SPAN = std::chrono::minutes(2);
+
+	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;
+
+	/**
+	 * 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{};
+
+public:
+	TimerWheel() noexcept;
+	~TimerWheel() noexcept;
+
+	bool IsEmpty() const noexcept {
+		return std::all_of(buckets.begin(), buckets.end(),
+				   [](const auto &list){
+					   return list.empty();
+				   });
+	}
+
+	void Insert(CoarseTimerEvent &t) 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;
+};

src/event/meson.build

@@ -22,7 +22,9 @@ endif
 event = static_library(
   'event',
   'SignalMonitor.cxx',
+  'TimerWheel.cxx',
   'TimerList.cxx',
+  'CoarseTimerEvent.cxx',
   'FineTimerEvent.cxx',
   'IdleEvent.cxx',
   'InjectEvent.cxx',