2 * Copyright 2017 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.
19 #include <type_traits>
23 #include <boost/noncopyable.hpp>
24 #include <folly/AtomicStruct.h>
25 #include <folly/detail/CacheLocality.h>
26 #include <folly/portability/SysMman.h>
27 #include <folly/portability/Unistd.h>
29 // Ignore shadowing warnings within this file, so includers can use -Wshadow.
30 #pragma GCC diagnostic push
31 #pragma GCC diagnostic ignored "-Wshadow"
36 template <typename Pool>
37 struct IndexedMemPoolRecycler;
42 bool EagerRecycleWhenTrivial = false,
43 bool EagerRecycleWhenNotTrivial = true>
44 struct IndexedMemPoolTraits {
45 static constexpr bool eagerRecycle() {
46 return std::is_trivial<T>::value ? EagerRecycleWhenTrivial
47 : EagerRecycleWhenNotTrivial;
50 /// Called when the element pointed to by ptr is allocated for the
52 static void initialize(T* ptr) {
53 if (!eagerRecycle()) {
58 /// Called when the element pointed to by ptr is freed at the pool
60 static void cleanup(T* ptr) {
61 if (!eagerRecycle()) {
66 /// Called when the element is allocated with the arguments forwarded from
67 /// IndexedMemPool::allocElem.
68 template <typename... Args>
69 static void onAllocate(T* ptr, Args&&... args) {
71 sizeof...(Args) == 0 || eagerRecycle(),
72 "emplace-style allocation requires eager recycle, "
73 "which is defaulted only for non-trivial types");
75 new (ptr) T(std::forward<Args>(args)...);
79 /// Called when the element is recycled.
80 static void onRecycle(T* ptr) {
87 /// IndexedMemPool traits that implements the lazy lifecycle strategy. In this
88 /// strategy elements are default-constructed the first time they are allocated,
89 /// and destroyed when the pool itself is destroyed.
91 using IndexedMemPoolTraitsLazyRecycle = IndexedMemPoolTraits<T, false, false>;
93 /// IndexedMemPool traits that implements the eager lifecycle strategy. In this
94 /// strategy elements are constructed when they are allocated from the pool and
95 /// destroyed when recycled.
97 using IndexedMemPoolTraitsEagerRecycle = IndexedMemPoolTraits<T, true, true>;
99 /// Instances of IndexedMemPool dynamically allocate and then pool their
100 /// element type (T), returning 4-byte integer indices that can be passed
101 /// to the pool's operator[] method to access or obtain pointers to the
102 /// actual elements. The memory backing items returned from the pool
103 /// will always be readable, even if items have been returned to the pool.
104 /// These two features are useful for lock-free algorithms. The indexing
105 /// behavior makes it easy to build tagged pointer-like-things, since
106 /// a large number of elements can be managed using fewer bits than a
107 /// full pointer. The access-after-free behavior makes it safe to read
108 /// from T-s even after they have been recycled, since it is guaranteed
109 /// that the memory won't have been returned to the OS and unmapped
110 /// (the algorithm must still use a mechanism to validate that the read
111 /// was correct, but it doesn't have to worry about page faults), and if
112 /// the elements use internal sequence numbers it can be guaranteed that
113 /// there won't be an ABA match due to the element being overwritten with
114 /// a different type that has the same bit pattern.
116 /// The object lifecycle strategy is controlled by the Traits parameter.
117 /// One strategy, implemented by IndexedMemPoolTraitsEagerRecycle, is to
118 /// construct objects when they are allocated from the pool and destroy
119 /// them when they are recycled. In this mode allocIndex and allocElem
120 /// have emplace-like semantics. In another strategy, implemented by
121 /// IndexedMemPoolTraitsLazyRecycle, objects are default-constructed the
122 /// first time they are removed from the pool, and deleted when the pool
123 /// itself is deleted. By default the first mode is used for non-trivial
124 /// T, and the second is used for trivial T. Clients can customize the
125 /// object lifecycle by providing their own Traits implementation.
126 /// See IndexedMemPoolTraits for a Traits example.
128 /// IMPORTANT: Space for extra elements is allocated to account for those
129 /// that are inaccessible because they are in other local lists, so the
130 /// actual number of items that can be allocated ranges from capacity to
131 /// capacity + (NumLocalLists_-1)*LocalListLimit_. This is important if
132 /// you are trying to maximize the capacity of the pool while constraining
133 /// the bit size of the resulting pointers, because the pointers will
134 /// actually range up to the boosted capacity. See maxIndexForCapacity
135 /// and capacityForMaxIndex.
137 /// To avoid contention, NumLocalLists_ free lists of limited (less than
138 /// or equal to LocalListLimit_) size are maintained, and each thread
139 /// retrieves and returns entries from its associated local list. If the
140 /// local list becomes too large then elements are placed in bulk in a
141 /// global free list. This allows items to be efficiently recirculated
142 /// from consumers to producers. AccessSpreader is used to access the
143 /// local lists, so there is no performance advantage to having more
144 /// local lists than L1 caches.
146 /// The pool mmap-s the entire necessary address space when the pool is
147 /// constructed, but delays element construction. This means that only
148 /// elements that are actually returned to the caller get paged into the
149 /// process's resident set (RSS).
152 uint32_t NumLocalLists_ = 32,
153 uint32_t LocalListLimit_ = 200,
154 template <typename> class Atom = std::atomic,
155 typename Traits = IndexedMemPoolTraits<T>>
156 struct IndexedMemPool : boost::noncopyable {
157 typedef T value_type;
159 typedef std::unique_ptr<T, detail::IndexedMemPoolRecycler<IndexedMemPool>>
162 static_assert(LocalListLimit_ <= 255, "LocalListLimit must fit in 8 bits");
164 NumLocalLists = NumLocalLists_,
165 LocalListLimit = LocalListLimit_
168 // these are public because clients may need to reason about the number
169 // of bits required to hold indices from a pool, given its capacity
171 static constexpr uint32_t maxIndexForCapacity(uint32_t capacity) {
172 // index of std::numeric_limits<uint32_t>::max() is reserved for isAllocated
174 return uint32_t(std::min(
175 uint64_t(capacity) + (NumLocalLists - 1) * LocalListLimit,
176 uint64_t(std::numeric_limits<uint32_t>::max() - 1)));
179 static constexpr uint32_t capacityForMaxIndex(uint32_t maxIndex) {
180 return maxIndex - (NumLocalLists - 1) * LocalListLimit;
184 /// Constructs a pool that can allocate at least _capacity_ elements,
185 /// even if all the local lists are full
186 explicit IndexedMemPool(uint32_t capacity)
187 : actualCapacity_(maxIndexForCapacity(capacity))
189 , globalHead_(TaggedPtr{})
191 const size_t needed = sizeof(Slot) * (actualCapacity_ + 1);
192 size_t pagesize = size_t(sysconf(_SC_PAGESIZE));
193 mmapLength_ = ((needed - 1) & ~(pagesize - 1)) + pagesize;
194 assert(needed <= mmapLength_ && mmapLength_ < needed + pagesize);
195 assert((mmapLength_ % pagesize) == 0);
197 slots_ = static_cast<Slot*>(mmap(nullptr, mmapLength_,
198 PROT_READ | PROT_WRITE,
199 MAP_PRIVATE | MAP_ANONYMOUS, -1, 0));
200 if (slots_ == MAP_FAILED) {
201 assert(errno == ENOMEM);
202 throw std::bad_alloc();
206 /// Destroys all of the contained elements
208 for (uint32_t i = maxAllocatedIndex(); i > 0; --i) {
209 Traits::cleanup(&slots_[i].elem);
211 munmap(slots_, mmapLength_);
214 /// Returns a lower bound on the number of elements that may be
215 /// simultaneously allocated and not yet recycled. Because of the
216 /// local lists it is possible that more elements than this are returned
218 uint32_t capacity() {
219 return capacityForMaxIndex(actualCapacity_);
222 /// Returns the maximum index of elements ever allocated in this pool
223 /// including elements that have been recycled.
224 uint32_t maxAllocatedIndex() const {
225 // Take the minimum since it is possible that size_ > actualCapacity_.
226 // This can happen if there are multiple concurrent requests
227 // when size_ == actualCapacity_ - 1.
228 return std::min(uint32_t(size_), uint32_t(actualCapacity_));
231 /// Finds a slot with a non-zero index, emplaces a T there if we're
232 /// using the eager recycle lifecycle mode, and returns the index,
233 /// or returns 0 if no elements are available. Passes a pointer to
234 /// the element to Traits::onAllocate before the slot is marked as
236 template <typename ...Args>
237 uint32_t allocIndex(Args&&... args) {
238 auto idx = localPop(localHead());
241 Traits::onAllocate(&s.elem, std::forward<Args>(args)...);
247 /// If an element is available, returns a std::unique_ptr to it that will
248 /// recycle the element to the pool when it is reclaimed, otherwise returns
249 /// a null (falsy) std::unique_ptr. Passes a pointer to the element to
250 /// Traits::onAllocate before the slot is marked as allocated.
251 template <typename ...Args>
252 UniquePtr allocElem(Args&&... args) {
253 auto idx = allocIndex(std::forward<Args>(args)...);
254 T* ptr = idx == 0 ? nullptr : &slot(idx).elem;
255 return UniquePtr(ptr, typename UniquePtr::deleter_type(this));
258 /// Gives up ownership previously granted by alloc()
259 void recycleIndex(uint32_t idx) {
260 assert(isAllocated(idx));
261 Traits::onRecycle(&slot(idx).elem);
262 localPush(localHead(), idx);
265 /// Provides access to the pooled element referenced by idx
266 T& operator[](uint32_t idx) {
267 return slot(idx).elem;
270 /// Provides access to the pooled element referenced by idx
271 const T& operator[](uint32_t idx) const {
272 return slot(idx).elem;
275 /// If elem == &pool[idx], then pool.locateElem(elem) == idx. Also,
276 /// pool.locateElem(nullptr) == 0
277 uint32_t locateElem(const T* elem) const {
282 static_assert(std::is_standard_layout<Slot>::value, "offsetof needs POD");
284 auto slot = reinterpret_cast<const Slot*>(
285 reinterpret_cast<const char*>(elem) - offsetof(Slot, elem));
286 auto rv = uint32_t(slot - slots_);
288 // this assert also tests that rv is in range
289 assert(elem == &(*this)[rv]);
293 /// Returns true iff idx has been alloc()ed and not recycleIndex()ed
294 bool isAllocated(uint32_t idx) const {
295 return slot(idx).localNext.load(std::memory_order_acquire) == uint32_t(-1);
304 Atom<uint32_t> localNext;
305 Atom<uint32_t> globalNext;
307 Slot() : localNext{}, globalNext{} {}
313 // size is bottom 8 bits, tag in top 24. g++'s code generation for
314 // bitfields seems to depend on the phase of the moon, plus we can
315 // do better because we can rely on other checks to avoid masking
320 SizeMask = (1U << SizeBits) - 1,
321 TagIncr = 1U << SizeBits,
324 uint32_t size() const {
325 return tagAndSize & SizeMask;
328 TaggedPtr withSize(uint32_t repl) const {
329 assert(repl <= LocalListLimit);
330 return TaggedPtr{ idx, (tagAndSize & ~SizeMask) | repl };
333 TaggedPtr withSizeIncr() const {
334 assert(size() < LocalListLimit);
335 return TaggedPtr{ idx, tagAndSize + 1 };
338 TaggedPtr withSizeDecr() const {
340 return TaggedPtr{ idx, tagAndSize - 1 };
343 TaggedPtr withIdx(uint32_t repl) const {
344 return TaggedPtr{ repl, tagAndSize + TagIncr };
347 TaggedPtr withEmpty() const {
348 return withIdx(0).withSize(0);
352 struct FOLLY_ALIGN_TO_AVOID_FALSE_SHARING LocalList {
353 AtomicStruct<TaggedPtr,Atom> head;
355 LocalList() : head(TaggedPtr{}) {}
360 /// the number of bytes allocated from mmap, which is a multiple of
361 /// the page size of the machine
364 /// the actual number of slots that we will allocate, to guarantee
365 /// that we will satisfy the capacity requested at construction time.
366 /// They will be numbered 1..actualCapacity_ (note the 1-based counting),
367 /// and occupy slots_[1..actualCapacity_].
368 uint32_t actualCapacity_;
370 /// this records the number of slots that have actually been constructed.
371 /// To allow use of atomic ++ instead of CAS, we let this overflow.
372 /// The actual number of constructed elements is min(actualCapacity_,
374 Atom<uint32_t> size_;
376 /// raw storage, only 1..min(size_,actualCapacity_) (inclusive) are
377 /// actually constructed. Note that slots_[0] is not constructed or used
378 FOLLY_ALIGN_TO_AVOID_FALSE_SHARING Slot* slots_;
380 /// use AccessSpreader to find your list. We use stripes instead of
381 /// thread-local to avoid the need to grow or shrink on thread start
382 /// or join. These are heads of lists chained with localNext
383 LocalList local_[NumLocalLists];
385 /// this is the head of a list of node chained by globalNext, that are
386 /// themselves each the head of a list chained by localNext
387 FOLLY_ALIGN_TO_AVOID_FALSE_SHARING AtomicStruct<TaggedPtr,Atom> globalHead_;
389 ///////////// private methods
391 uint32_t slotIndex(uint32_t idx) const {
393 idx <= actualCapacity_ &&
394 idx <= size_.load(std::memory_order_acquire));
398 Slot& slot(uint32_t idx) {
399 return slots_[slotIndex(idx)];
402 const Slot& slot(uint32_t idx) const {
403 return slots_[slotIndex(idx)];
406 // localHead references a full list chained by localNext. s should
407 // reference slot(localHead), it is passed as a micro-optimization
408 void globalPush(Slot& s, uint32_t localHead) {
410 TaggedPtr gh = globalHead_.load(std::memory_order_acquire);
411 s.globalNext.store(gh.idx, std::memory_order_relaxed);
412 if (globalHead_.compare_exchange_strong(gh, gh.withIdx(localHead))) {
419 // idx references a single node
420 void localPush(AtomicStruct<TaggedPtr,Atom>& head, uint32_t idx) {
422 TaggedPtr h = head.load(std::memory_order_acquire);
424 s.localNext.store(h.idx, std::memory_order_relaxed);
426 if (h.size() == LocalListLimit) {
427 // push will overflow local list, steal it instead
428 if (head.compare_exchange_strong(h, h.withEmpty())) {
429 // steal was successful, put everything in the global list
434 // local list has space
435 if (head.compare_exchange_strong(h, h.withIdx(idx).withSizeIncr())) {
440 // h was updated by failing CAS
444 // returns 0 if empty
445 uint32_t globalPop() {
447 TaggedPtr gh = globalHead_.load(std::memory_order_acquire);
449 globalHead_.compare_exchange_strong(
452 slot(gh.idx).globalNext.load(std::memory_order_relaxed)))) {
453 // global list is empty, or pop was successful
459 // returns 0 if allocation failed
460 uint32_t localPop(AtomicStruct<TaggedPtr,Atom>& head) {
462 TaggedPtr h = head.load(std::memory_order_acquire);
464 // local list is non-empty, try to pop
465 Slot& s = slot(h.idx);
466 auto next = s.localNext.load(std::memory_order_relaxed);
467 if (head.compare_exchange_strong(h, h.withIdx(next).withSizeDecr())) {
474 uint32_t idx = globalPop();
476 // global list is empty, allocate and construct new slot
477 if (size_.load(std::memory_order_relaxed) >= actualCapacity_ ||
478 (idx = ++size_) > actualCapacity_) {
482 Traits::initialize(&slot(idx).elem);
487 auto next = s.localNext.load(std::memory_order_relaxed);
488 if (head.compare_exchange_strong(
489 h, h.withIdx(next).withSize(LocalListLimit))) {
490 // global list moved to local list, keep head for us
493 // local bulk push failed, return idx to the global list and try again
498 AtomicStruct<TaggedPtr,Atom>& localHead() {
499 auto stripe = detail::AccessSpreader<Atom>::current(NumLocalLists);
500 return local_[stripe].head;
503 void markAllocated(Slot& slot) {
504 slot.localNext.store(uint32_t(-1), std::memory_order_release);
510 /// This is a stateful Deleter functor, which allows std::unique_ptr
511 /// to track elements allocated from an IndexedMemPool by tracking the
512 /// associated pool. See IndexedMemPool::allocElem.
513 template <typename Pool>
514 struct IndexedMemPoolRecycler {
517 explicit IndexedMemPoolRecycler(Pool* pool) : pool(pool) {}
519 IndexedMemPoolRecycler(const IndexedMemPoolRecycler<Pool>& rhs)
521 IndexedMemPoolRecycler& operator= (const IndexedMemPoolRecycler<Pool>& rhs)
524 void operator()(typename Pool::value_type* elem) const {
525 pool->recycleIndex(pool->locateElem(elem));
533 # pragma GCC diagnostic pop