165 lines
7.4 KiB
C++
165 lines
7.4 KiB
C++
/*
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* WriteOnlySet.h
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*
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* This source file is part of the FoundationDB open source project
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*
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* Copyright 2013-2021 Apple Inc. and the FoundationDB project authors
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#pragma once
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#include "flow/Error.h"
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#include "flow/FastRef.h"
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#include "flow/Trace.h"
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#include <boost/lockfree/queue.hpp>
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#ifdef ENABLE_SAMPLING
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/**
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* This is a Write-Only set that supports copying the whole content. This data structure is lock-free and allows a user
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* to insert and remove objects up to a given capacity (passed by a template).
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*
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* Template parameters:
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* \param T The type to store.
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* \param IndexType The type used as an index
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* \param CAPACITY The maximum number of object this structure can store (if a user tries to store more, insert will
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* fail gracefully)
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* \pre T implements `void addref() const` and `void delref() const`
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* \pre IndexType must have a copy constructor
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* \pre IndexType must have a trivial assignment operator
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* \pre IndexType must have a trivial destructor
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* \pre IndexType can be used as an index into a std::vector
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*/
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template <class T, class IndexType, IndexType CAPACITY>
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class WriteOnlySet {
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public:
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// The type we use for lookup into the set. Gets assigned during insert
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using Index = IndexType;
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// For now we use a fixed size capacity
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constexpr static Index npos = std::numeric_limits<Index>::max();
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constexpr static IndexType capacity = CAPACITY;
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explicit WriteOnlySet();
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WriteOnlySet(const WriteOnlySet&) = delete;
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WriteOnlySet(WriteOnlySet&&) = delete;
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WriteOnlySet& operator=(const WriteOnlySet&) = delete;
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WriteOnlySet& operator=(WriteOnlySet&&) = delete;
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/**
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* Attempts to insert \p lineage into the set. This method can fail if the set is full (its size is equal to its
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* capacity). Calling insert on a full set is safe but the method will return \ref npos if the operation fails.
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*
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* \param lineage A reference to the object the user wants to insert.
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* \ret An index that can later be used to erase the value again or \ref npos if the insert failed.
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* \pre lineage.getPtr() % 2 == 0 (the memory for lineage has to be at least 2 byte aligned)
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*/
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[[nodiscard]] Index insert(const Reference<T>& lineage);
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/**
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* Erases the object associated with \p idx from the set.
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*
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* \ret Whether the reference count was decremented. Usually the return value is only interesting for testing and
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* benchmarking purposes and will in most cases be ignored. If \ref delref wasn't called, it will be called
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* later. Note that at the time the return value is checked, \ref delref might already have been called.
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*/
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bool erase(Index idx);
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/**
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* Replaces the object associated with \p idx with \p lineage.
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*
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* \ret Whether the reference count of the replaced object was decremented. Usually the return value is only
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* interesting for testing and benchmarking purposes and will in most cases be ignored. If \ref delref
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* wasn't called, it will be called later. Note that at the time the return value is checked, \ref delref
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* might already have been called.
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*/
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bool replace(Index idx, const Reference<T>& lineage);
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/**
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* Copies all elements that are stored in the set into a vector. This copy operation does NOT provide a snapshot of
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* the data structure. The contract is weak:
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* - All object that were in the set before copy is called and weren't removed until after copy returned are
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* guaranteed to be in the result.
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* - Any object that was inserted while copy is running might be in the result.
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* - Any object that was erased while copy is running might be in the result.
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*/
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std::vector<Reference<T>> copy();
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protected:
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// the implementation of erase -- the wrapper just makes the function a bit more readable.
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bool eraseImpl(Index idx);
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// the last bit of a pointer within the set is used like a boolean and true means that the object is locked. Locking
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// an object is only relevant for memory management. A locked pointer can still be erased from the set, but the
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// erase won't call delref on the object. Instead it will push the pointer into the \ref freeList and copy will call
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// delref later.
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static constexpr uintptr_t LOCK = 0b1;
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// The actual memory
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std::vector<std::atomic<std::uintptr_t>> _set;
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static_assert(std::atomic<Index>::is_always_lock_free, "Index type can't be used as a lock-free type");
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static_assert(std::atomic<uintptr_t>::is_always_lock_free, "uintptr_t can't be used as a lock-free type");
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// The freeQueue. On creation all indexes (0..capacity-1) are pushed into this queue. On insert one element from
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// this queue is consumed and the resulting number is used as an index into the set. On erase the index is given
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// back to the freeQueue.
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boost::lockfree::queue<Index, boost::lockfree::fixed_sized<true>, boost::lockfree::capacity<CAPACITY>> freeQueue;
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// The freeList is used for memory management. Generally copying a shared pointer can't be done in a lock-free way.
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// Instead, when we copy the data structure we first copy the address, then attempt to set the last bit to 1 and
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// only if that succeeds we will increment the reference count. Whenever we attempt to remove an object
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// in \ref erase we remove the object from the set (using an atomic compare and swap) and only decrement the
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// reference count if the last bit is 0. If it's not we'll push the pointer into this free list.
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// \ref copy will consume all elements from this freeList each time it runs and decrements the refcount for each
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// element.
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boost::lockfree::queue<T*, boost::lockfree::fixed_sized<true>, boost::lockfree::capacity<CAPACITY>> freeList;
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};
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/**
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* Provides a thread safe, lock-free write only variable.
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*
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* Template parameters:
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* \param T The type to store.
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* \param IndexType The type used as an index
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* \pre T implements `void addref() const` and `void delref() const`
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* \pre IndexType must have a copy constructor
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* \pre IndexType must have a trivial assignment operator
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* \pre IndexType must have a trivial destructor
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* \pre IndexType can be used as an index into a std::vector
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*/
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template <class T, class IndexType>
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class WriteOnlyVariable : private WriteOnlySet<T, IndexType, 1> {
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public:
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explicit WriteOnlyVariable();
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/**
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* Returns a copied reference to the stored variable.
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*/
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Reference<T> get();
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/**
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* Replaces the variable with \p lineage. \p lineage is permitted to be an invalid pointer.
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*
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* \ret Whether the reference count of the replaced object was decremented. Note that if the reference being
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* replaced is invalid, this function will always return false. If \ref delref wasn't called and the reference was
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* valid, it will be called later. Note that at the time the return value is checked, \ref delref might already have
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* been called.
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*/
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bool replace(const Reference<T>& element);
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};
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class ActorLineage;
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extern template class WriteOnlySet<ActorLineage, unsigned, 1024>;
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using ActorLineageSet = WriteOnlySet<ActorLineage, unsigned, 1024>;
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#endif
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