2 * Copyright 2014 Facebook, Inc.
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
8 * http://www.apache.org/licenses/LICENSE-2.0
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
18 * Improved thread local storage for non-trivial types (similar speed as
19 * pthread_getspecific but only consumes a single pthread_key_t, and 4x faster
20 * than boost::thread_specific_ptr).
22 * Also includes an accessor interface to walk all the thread local child
23 * objects of a parent. accessAllThreads() initializes an accessor which holds
24 * a global lock *that blocks all creation and destruction of ThreadLocal
25 * objects with the same Tag* and can be used as an iterable container.
27 * Intended use is for frequent write, infrequent read data access patterns such
30 * There are two classes here - ThreadLocal and ThreadLocalPtr. ThreadLocalPtr
31 * has semantics similar to boost::thread_specific_ptr. ThreadLocal is a thin
32 * wrapper around ThreadLocalPtr that manages allocation automatically.
34 * @author Spencer Ahrens (sahrens)
37 #ifndef FOLLY_THREADLOCAL_H_
38 #define FOLLY_THREADLOCAL_H_
40 #include "folly/Portability.h"
41 #include <boost/iterator/iterator_facade.hpp>
42 #include "folly/Likely.h"
43 #include <type_traits>
47 enum class TLPDestructionMode {
53 #include "folly/detail/ThreadLocalDetail.h"
57 template<class T, class Tag> class ThreadLocalPtr;
59 template<class T, class Tag=void>
66 if (LIKELY(ptr != nullptr)) {
70 // separated new item creation out to speed up the fast path.
74 T* operator->() const {
78 T& operator*() const {
82 void reset(T* newPtr = nullptr) {
86 typedef typename ThreadLocalPtr<T,Tag>::Accessor Accessor;
87 Accessor accessAllThreads() const {
88 return tlp_.accessAllThreads();
92 ThreadLocal(ThreadLocal&&) = default;
93 ThreadLocal& operator=(ThreadLocal&&) = default;
97 ThreadLocal(const ThreadLocal&) = delete;
98 ThreadLocal& operator=(const ThreadLocal&) = delete;
106 mutable ThreadLocalPtr<T,Tag> tlp_;
110 * The idea here is that __thread is faster than pthread_getspecific, so we
111 * keep a __thread array of pointers to objects (ThreadEntry::elements) where
112 * each array has an index for each unique instance of the ThreadLocalPtr
113 * object. Each ThreadLocalPtr object has a unique id that is an index into
114 * these arrays so we can fetch the correct object from thread local storage
117 * In order to prevent unbounded growth of the id space and thus huge
118 * ThreadEntry::elements, arrays, for example due to continuous creation and
119 * destruction of ThreadLocalPtr objects, we keep a set of all active
120 * instances. When an instance is destroyed we remove it from the active
121 * set and insert the id into freeIds_ for reuse. These operations require a
122 * global mutex, but only happen at construction and destruction time.
124 * We use a single global pthread_key_t per Tag to manage object destruction and
125 * memory cleanup upon thread exit because there is a finite number of
126 * pthread_key_t's available per machine.
128 * NOTE: Apple platforms don't support the same semantics for __thread that
129 * Linux does (and it's only supported at all on i386). For these, use
130 * pthread_setspecific()/pthread_getspecific() for the per-thread
131 * storage. Windows (MSVC and GCC) does support the same semantics
132 * with __declspec(thread)
135 template<class T, class Tag=void>
136 class ThreadLocalPtr {
138 ThreadLocalPtr() : id_(threadlocal_detail::StaticMeta<Tag>::create()) { }
140 ThreadLocalPtr(ThreadLocalPtr&& other) : id_(other.id_) {
144 ThreadLocalPtr& operator=(ThreadLocalPtr&& other) {
145 assert(this != &other);
157 return static_cast<T*>(threadlocal_detail::StaticMeta<Tag>::get(id_).ptr);
160 T* operator->() const {
164 T& operator*() const {
168 void reset(T* newPtr = nullptr) {
169 threadlocal_detail::ElementWrapper& w =
170 threadlocal_detail::StaticMeta<Tag>::get(id_);
171 if (w.ptr != newPtr) {
172 w.dispose(TLPDestructionMode::THIS_THREAD);
177 explicit operator bool() const {
178 return get() != nullptr;
182 * reset() with a custom deleter:
183 * deleter(T* ptr, TLPDestructionMode mode)
184 * "mode" is ALL_THREADS if we're destructing this ThreadLocalPtr (and thus
185 * deleting pointers for all threads), and THIS_THREAD if we're only deleting
186 * the member for one thread (because of thread exit or reset())
188 template <class Deleter>
189 void reset(T* newPtr, Deleter deleter) {
190 threadlocal_detail::ElementWrapper& w =
191 threadlocal_detail::StaticMeta<Tag>::get(id_);
192 if (w.ptr != newPtr) {
193 w.dispose(TLPDestructionMode::THIS_THREAD);
194 w.set(newPtr, deleter);
198 // Holds a global lock for iteration through all thread local child objects.
199 // Can be used as an iterable container.
200 // Use accessAllThreads() to obtain one.
202 friend class ThreadLocalPtr<T,Tag>;
204 threadlocal_detail::StaticMeta<Tag>& meta_;
210 friend class Iterator;
212 // The iterators obtained from Accessor are bidirectional iterators.
213 class Iterator : public boost::iterator_facade<
216 boost::bidirectional_traversal_tag> { // traversal
217 friend class Accessor;
218 friend class boost::iterator_core_access;
219 const Accessor* const accessor_;
220 threadlocal_detail::ThreadEntry* e_;
232 T& dereference() const {
233 return *static_cast<T*>(e_->elements[accessor_->id_].ptr);
236 bool equal(const Iterator& other) const {
237 return (accessor_->id_ == other.accessor_->id_ &&
241 explicit Iterator(const Accessor* accessor)
242 : accessor_(accessor),
243 e_(&accessor_->meta_.head_) {
247 return (e_->elements &&
248 accessor_->id_ < e_->elementsCapacity &&
249 e_->elements[accessor_->id_].ptr);
252 void incrementToValid() {
253 for (; e_ != &accessor_->meta_.head_ && !valid(); e_ = e_->next) { }
256 void decrementToValid() {
257 for (; e_ != &accessor_->meta_.head_ && !valid(); e_ = e_->prev) { }
265 Iterator begin() const {
266 return ++Iterator(this);
269 Iterator end() const {
270 return Iterator(this);
273 Accessor(const Accessor&) = delete;
274 Accessor& operator=(const Accessor&) = delete;
276 Accessor(Accessor&& other) noexcept
277 : meta_(other.meta_),
281 other.lock_ = nullptr;
284 Accessor& operator=(Accessor&& other) noexcept {
285 // Each Tag has its own unique meta, and accessors with different Tags
286 // have different types. So either *this is empty, or this and other
287 // have the same tag. But if they have the same tag, they have the same
288 // meta (and lock), so they'd both hold the lock at the same time,
289 // which is impossible, which leaves only one possible scenario --
290 // *this is empty. Assert it.
291 assert(&meta_ == &other.meta_);
292 assert(lock_ == nullptr);
294 swap(lock_, other.lock_);
295 swap(id_, other.id_);
299 : meta_(threadlocal_detail::StaticMeta<Tag>::instance()),
305 explicit Accessor(int id)
306 : meta_(threadlocal_detail::StaticMeta<Tag>::instance()),
307 lock_(&meta_.lock_) {
321 // accessor allows a client to iterate through all thread local child
322 // elements of this ThreadLocal instance. Holds a global lock for each <Tag>
323 Accessor accessAllThreads() const {
324 static_assert(!std::is_same<Tag, void>::value,
325 "Must use a unique Tag to use the accessAllThreads feature");
326 return Accessor(id_);
332 threadlocal_detail::StaticMeta<Tag>::destroy(id_);
337 ThreadLocalPtr(const ThreadLocalPtr&) = delete;
338 ThreadLocalPtr& operator=(const ThreadLocalPtr&) = delete;
340 int id_; // every instantiation has a unique id
345 #endif /* FOLLY_THREADLOCAL_H_ */