verdnatura-chat/ios/Pods/Folly/folly/IndexedMemPool.h

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C++

/*
* Copyright 2014-present Facebook, Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#pragma once
#include <assert.h>
#include <errno.h>
#include <stdint.h>
#include <type_traits>
#include <boost/noncopyable.hpp>
#include <folly/Portability.h>
#include <folly/concurrency/CacheLocality.h>
#include <folly/portability/SysMman.h>
#include <folly/portability/Unistd.h>
#include <folly/synchronization/AtomicStruct.h>
// Ignore shadowing warnings within this file, so includers can use -Wshadow.
FOLLY_PUSH_WARNING
FOLLY_GNU_DISABLE_WARNING("-Wshadow")
namespace folly {
namespace detail {
template <typename Pool>
struct IndexedMemPoolRecycler;
} // namespace detail
template <
typename T,
bool EagerRecycleWhenTrivial = false,
bool EagerRecycleWhenNotTrivial = true>
struct IndexedMemPoolTraits {
static constexpr bool eagerRecycle() {
return std::is_trivial<T>::value ? EagerRecycleWhenTrivial
: EagerRecycleWhenNotTrivial;
}
/// Called when the element pointed to by ptr is allocated for the
/// first time.
static void initialize(T* ptr) {
if (!eagerRecycle()) {
new (ptr) T();
}
}
/// Called when the element pointed to by ptr is freed at the pool
/// destruction time.
static void cleanup(T* ptr) {
if (!eagerRecycle()) {
ptr->~T();
}
}
/// Called when the element is allocated with the arguments forwarded from
/// IndexedMemPool::allocElem.
template <typename... Args>
static void onAllocate(T* ptr, Args&&... args) {
static_assert(
sizeof...(Args) == 0 || eagerRecycle(),
"emplace-style allocation requires eager recycle, "
"which is defaulted only for non-trivial types");
if (eagerRecycle()) {
new (ptr) T(std::forward<Args>(args)...);
}
}
/// Called when the element is recycled.
static void onRecycle(T* ptr) {
if (eagerRecycle()) {
ptr->~T();
}
}
};
/// IndexedMemPool traits that implements the lazy lifecycle strategy. In this
/// strategy elements are default-constructed the first time they are allocated,
/// and destroyed when the pool itself is destroyed.
template <typename T>
using IndexedMemPoolTraitsLazyRecycle = IndexedMemPoolTraits<T, false, false>;
/// IndexedMemPool traits that implements the eager lifecycle strategy. In this
/// strategy elements are constructed when they are allocated from the pool and
/// destroyed when recycled.
template <typename T>
using IndexedMemPoolTraitsEagerRecycle = IndexedMemPoolTraits<T, true, true>;
/// Instances of IndexedMemPool dynamically allocate and then pool their
/// element type (T), returning 4-byte integer indices that can be passed
/// to the pool's operator[] method to access or obtain pointers to the
/// actual elements. The memory backing items returned from the pool
/// will always be readable, even if items have been returned to the pool.
/// These two features are useful for lock-free algorithms. The indexing
/// behavior makes it easy to build tagged pointer-like-things, since
/// a large number of elements can be managed using fewer bits than a
/// full pointer. The access-after-free behavior makes it safe to read
/// from T-s even after they have been recycled, since it is guaranteed
/// that the memory won't have been returned to the OS and unmapped
/// (the algorithm must still use a mechanism to validate that the read
/// was correct, but it doesn't have to worry about page faults), and if
/// the elements use internal sequence numbers it can be guaranteed that
/// there won't be an ABA match due to the element being overwritten with
/// a different type that has the same bit pattern.
///
/// The object lifecycle strategy is controlled by the Traits parameter.
/// One strategy, implemented by IndexedMemPoolTraitsEagerRecycle, is to
/// construct objects when they are allocated from the pool and destroy
/// them when they are recycled. In this mode allocIndex and allocElem
/// have emplace-like semantics. In another strategy, implemented by
/// IndexedMemPoolTraitsLazyRecycle, objects are default-constructed the
/// first time they are removed from the pool, and deleted when the pool
/// itself is deleted. By default the first mode is used for non-trivial
/// T, and the second is used for trivial T. Clients can customize the
/// object lifecycle by providing their own Traits implementation.
/// See IndexedMemPoolTraits for a Traits example.
///
/// IMPORTANT: Space for extra elements is allocated to account for those
/// that are inaccessible because they are in other local lists, so the
/// actual number of items that can be allocated ranges from capacity to
/// capacity + (NumLocalLists_-1)*LocalListLimit_. This is important if
/// you are trying to maximize the capacity of the pool while constraining
/// the bit size of the resulting pointers, because the pointers will
/// actually range up to the boosted capacity. See maxIndexForCapacity
/// and capacityForMaxIndex.
///
/// To avoid contention, NumLocalLists_ free lists of limited (less than
/// or equal to LocalListLimit_) size are maintained, and each thread
/// retrieves and returns entries from its associated local list. If the
/// local list becomes too large then elements are placed in bulk in a
/// global free list. This allows items to be efficiently recirculated
/// from consumers to producers. AccessSpreader is used to access the
/// local lists, so there is no performance advantage to having more
/// local lists than L1 caches.
///
/// The pool mmap-s the entire necessary address space when the pool is
/// constructed, but delays element construction. This means that only
/// elements that are actually returned to the caller get paged into the
/// process's resident set (RSS).
template <
typename T,
uint32_t NumLocalLists_ = 32,
uint32_t LocalListLimit_ = 200,
template <typename> class Atom = std::atomic,
typename Traits = IndexedMemPoolTraits<T>>
struct IndexedMemPool : boost::noncopyable {
typedef T value_type;
typedef std::unique_ptr<T, detail::IndexedMemPoolRecycler<IndexedMemPool>>
UniquePtr;
static_assert(LocalListLimit_ <= 255, "LocalListLimit must fit in 8 bits");
enum {
NumLocalLists = NumLocalLists_,
LocalListLimit = LocalListLimit_,
};
// these are public because clients may need to reason about the number
// of bits required to hold indices from a pool, given its capacity
static constexpr uint32_t maxIndexForCapacity(uint32_t capacity) {
// index of std::numeric_limits<uint32_t>::max() is reserved for isAllocated
// tracking
return uint32_t(std::min(
uint64_t(capacity) + (NumLocalLists - 1) * LocalListLimit,
uint64_t(std::numeric_limits<uint32_t>::max() - 1)));
}
static constexpr uint32_t capacityForMaxIndex(uint32_t maxIndex) {
return maxIndex - (NumLocalLists - 1) * LocalListLimit;
}
/// Constructs a pool that can allocate at least _capacity_ elements,
/// even if all the local lists are full
explicit IndexedMemPool(uint32_t capacity)
: actualCapacity_(maxIndexForCapacity(capacity)),
size_(0),
globalHead_(TaggedPtr{}) {
const size_t needed = sizeof(Slot) * (actualCapacity_ + 1);
size_t pagesize = size_t(sysconf(_SC_PAGESIZE));
mmapLength_ = ((needed - 1) & ~(pagesize - 1)) + pagesize;
assert(needed <= mmapLength_ && mmapLength_ < needed + pagesize);
assert((mmapLength_ % pagesize) == 0);
slots_ = static_cast<Slot*>(mmap(
nullptr,
mmapLength_,
PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS,
-1,
0));
if (slots_ == MAP_FAILED) {
assert(errno == ENOMEM);
throw std::bad_alloc();
}
}
/// Destroys all of the contained elements
~IndexedMemPool() {
for (uint32_t i = maxAllocatedIndex(); i > 0; --i) {
Traits::cleanup(&slots_[i].elem);
}
munmap(slots_, mmapLength_);
}
/// Returns a lower bound on the number of elements that may be
/// simultaneously allocated and not yet recycled. Because of the
/// local lists it is possible that more elements than this are returned
/// successfully
uint32_t capacity() {
return capacityForMaxIndex(actualCapacity_);
}
/// Returns the maximum index of elements ever allocated in this pool
/// including elements that have been recycled.
uint32_t maxAllocatedIndex() const {
// Take the minimum since it is possible that size_ > actualCapacity_.
// This can happen if there are multiple concurrent requests
// when size_ == actualCapacity_ - 1.
return std::min(uint32_t(size_), uint32_t(actualCapacity_));
}
/// Finds a slot with a non-zero index, emplaces a T there if we're
/// using the eager recycle lifecycle mode, and returns the index,
/// or returns 0 if no elements are available. Passes a pointer to
/// the element to Traits::onAllocate before the slot is marked as
/// allocated.
template <typename... Args>
uint32_t allocIndex(Args&&... args) {
auto idx = localPop(localHead());
if (idx != 0) {
Slot& s = slot(idx);
Traits::onAllocate(&s.elem, std::forward<Args>(args)...);
markAllocated(s);
}
return idx;
}
/// If an element is available, returns a std::unique_ptr to it that will
/// recycle the element to the pool when it is reclaimed, otherwise returns
/// a null (falsy) std::unique_ptr. Passes a pointer to the element to
/// Traits::onAllocate before the slot is marked as allocated.
template <typename... Args>
UniquePtr allocElem(Args&&... args) {
auto idx = allocIndex(std::forward<Args>(args)...);
T* ptr = idx == 0 ? nullptr : &slot(idx).elem;
return UniquePtr(ptr, typename UniquePtr::deleter_type(this));
}
/// Gives up ownership previously granted by alloc()
void recycleIndex(uint32_t idx) {
assert(isAllocated(idx));
localPush(localHead(), idx);
}
/// Provides access to the pooled element referenced by idx
T& operator[](uint32_t idx) {
return slot(idx).elem;
}
/// Provides access to the pooled element referenced by idx
const T& operator[](uint32_t idx) const {
return slot(idx).elem;
}
/// If elem == &pool[idx], then pool.locateElem(elem) == idx. Also,
/// pool.locateElem(nullptr) == 0
uint32_t locateElem(const T* elem) const {
if (!elem) {
return 0;
}
static_assert(std::is_standard_layout<Slot>::value, "offsetof needs POD");
auto slot = reinterpret_cast<const Slot*>(
reinterpret_cast<const char*>(elem) - offsetof(Slot, elem));
auto rv = uint32_t(slot - slots_);
// this assert also tests that rv is in range
assert(elem == &(*this)[rv]);
return rv;
}
/// Returns true iff idx has been alloc()ed and not recycleIndex()ed
bool isAllocated(uint32_t idx) const {
return slot(idx).localNext.load(std::memory_order_acquire) == uint32_t(-1);
}
private:
///////////// types
struct Slot {
T elem;
Atom<uint32_t> localNext;
Atom<uint32_t> globalNext;
Slot() : localNext{}, globalNext{} {}
};
struct TaggedPtr {
uint32_t idx;
// size is bottom 8 bits, tag in top 24. g++'s code generation for
// bitfields seems to depend on the phase of the moon, plus we can
// do better because we can rely on other checks to avoid masking
uint32_t tagAndSize;
enum : uint32_t {
SizeBits = 8,
SizeMask = (1U << SizeBits) - 1,
TagIncr = 1U << SizeBits,
};
uint32_t size() const {
return tagAndSize & SizeMask;
}
TaggedPtr withSize(uint32_t repl) const {
assert(repl <= LocalListLimit);
return TaggedPtr{idx, (tagAndSize & ~SizeMask) | repl};
}
TaggedPtr withSizeIncr() const {
assert(size() < LocalListLimit);
return TaggedPtr{idx, tagAndSize + 1};
}
TaggedPtr withSizeDecr() const {
assert(size() > 0);
return TaggedPtr{idx, tagAndSize - 1};
}
TaggedPtr withIdx(uint32_t repl) const {
return TaggedPtr{repl, tagAndSize + TagIncr};
}
TaggedPtr withEmpty() const {
return withIdx(0).withSize(0);
}
};
struct alignas(hardware_destructive_interference_size) LocalList {
AtomicStruct<TaggedPtr, Atom> head;
LocalList() : head(TaggedPtr{}) {}
};
////////// fields
/// the number of bytes allocated from mmap, which is a multiple of
/// the page size of the machine
size_t mmapLength_;
/// the actual number of slots that we will allocate, to guarantee
/// that we will satisfy the capacity requested at construction time.
/// They will be numbered 1..actualCapacity_ (note the 1-based counting),
/// and occupy slots_[1..actualCapacity_].
uint32_t actualCapacity_;
/// this records the number of slots that have actually been constructed.
/// To allow use of atomic ++ instead of CAS, we let this overflow.
/// The actual number of constructed elements is min(actualCapacity_,
/// size_)
Atom<uint32_t> size_;
/// raw storage, only 1..min(size_,actualCapacity_) (inclusive) are
/// actually constructed. Note that slots_[0] is not constructed or used
alignas(hardware_destructive_interference_size) Slot* slots_;
/// use AccessSpreader to find your list. We use stripes instead of
/// thread-local to avoid the need to grow or shrink on thread start
/// or join. These are heads of lists chained with localNext
LocalList local_[NumLocalLists];
/// this is the head of a list of node chained by globalNext, that are
/// themselves each the head of a list chained by localNext
alignas(hardware_destructive_interference_size)
AtomicStruct<TaggedPtr, Atom> globalHead_;
///////////// private methods
uint32_t slotIndex(uint32_t idx) const {
assert(
0 < idx && idx <= actualCapacity_ &&
idx <= size_.load(std::memory_order_acquire));
return idx;
}
Slot& slot(uint32_t idx) {
return slots_[slotIndex(idx)];
}
const Slot& slot(uint32_t idx) const {
return slots_[slotIndex(idx)];
}
// localHead references a full list chained by localNext. s should
// reference slot(localHead), it is passed as a micro-optimization
void globalPush(Slot& s, uint32_t localHead) {
while (true) {
TaggedPtr gh = globalHead_.load(std::memory_order_acquire);
s.globalNext.store(gh.idx, std::memory_order_relaxed);
if (globalHead_.compare_exchange_strong(gh, gh.withIdx(localHead))) {
// success
return;
}
}
}
// idx references a single node
void localPush(AtomicStruct<TaggedPtr, Atom>& head, uint32_t idx) {
Slot& s = slot(idx);
TaggedPtr h = head.load(std::memory_order_acquire);
while (true) {
s.localNext.store(h.idx, std::memory_order_release);
Traits::onRecycle(&slot(idx).elem);
if (h.size() == LocalListLimit) {
// push will overflow local list, steal it instead
if (head.compare_exchange_strong(h, h.withEmpty())) {
// steal was successful, put everything in the global list
globalPush(s, idx);
return;
}
} else {
// local list has space
if (head.compare_exchange_strong(h, h.withIdx(idx).withSizeIncr())) {
// success
return;
}
}
// h was updated by failing CAS
}
}
// returns 0 if empty
uint32_t globalPop() {
while (true) {
TaggedPtr gh = globalHead_.load(std::memory_order_acquire);
if (gh.idx == 0 ||
globalHead_.compare_exchange_strong(
gh,
gh.withIdx(
slot(gh.idx).globalNext.load(std::memory_order_relaxed)))) {
// global list is empty, or pop was successful
return gh.idx;
}
}
}
// returns 0 if allocation failed
uint32_t localPop(AtomicStruct<TaggedPtr, Atom>& head) {
while (true) {
TaggedPtr h = head.load(std::memory_order_acquire);
if (h.idx != 0) {
// local list is non-empty, try to pop
Slot& s = slot(h.idx);
auto next = s.localNext.load(std::memory_order_relaxed);
if (head.compare_exchange_strong(h, h.withIdx(next).withSizeDecr())) {
// success
return h.idx;
}
continue;
}
uint32_t idx = globalPop();
if (idx == 0) {
// global list is empty, allocate and construct new slot
if (size_.load(std::memory_order_relaxed) >= actualCapacity_ ||
(idx = ++size_) > actualCapacity_) {
// allocation failed
return 0;
}
Traits::initialize(&slot(idx).elem);
return idx;
}
Slot& s = slot(idx);
auto next = s.localNext.load(std::memory_order_relaxed);
if (head.compare_exchange_strong(
h, h.withIdx(next).withSize(LocalListLimit))) {
// global list moved to local list, keep head for us
return idx;
}
// local bulk push failed, return idx to the global list and try again
globalPush(s, idx);
}
}
AtomicStruct<TaggedPtr, Atom>& localHead() {
auto stripe = AccessSpreader<Atom>::current(NumLocalLists);
return local_[stripe].head;
}
void markAllocated(Slot& slot) {
slot.localNext.store(uint32_t(-1), std::memory_order_release);
}
public:
static constexpr std::size_t kSlotSize = sizeof(Slot);
};
namespace detail {
/// This is a stateful Deleter functor, which allows std::unique_ptr
/// to track elements allocated from an IndexedMemPool by tracking the
/// associated pool. See IndexedMemPool::allocElem.
template <typename Pool>
struct IndexedMemPoolRecycler {
Pool* pool;
explicit IndexedMemPoolRecycler(Pool* pool) : pool(pool) {}
IndexedMemPoolRecycler(const IndexedMemPoolRecycler<Pool>& rhs) = default;
IndexedMemPoolRecycler& operator=(const IndexedMemPoolRecycler<Pool>& rhs) =
default;
void operator()(typename Pool::value_type* elem) const {
pool->recycleIndex(pool->locateElem(elem));
}
};
} // namespace detail
} // namespace folly
FOLLY_POP_WARNING