/*
- * Copyright 2014 Facebook, Inc.
+ * Copyright 2017 Facebook, Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* limitations under the License.
*/
-#ifndef FOLLY_INDEXEDMEMPOOL_H
-#define FOLLY_INDEXEDMEMPOOL_H
+#pragma once
-#include <stdint.h>
+#include <type_traits>
#include <assert.h>
-#include <unistd.h>
-#include <sys/mman.h>
+#include <errno.h>
+#include <stdint.h>
#include <boost/noncopyable.hpp>
#include <folly/AtomicStruct.h>
#include <folly/detail/CacheLocality.h>
+#include <folly/portability/SysMman.h>
+#include <folly/portability/Unistd.h>
// Ignore shadowing warnings within this file, so includers can use -Wshadow.
#pragma GCC diagnostic push
struct IndexedMemPoolRecycler;
}
-/// 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. Once they are constructed, elements are
-/// never destroyed. 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 pooling 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.
+/// 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.
+///
+/// IndexedMemPool has two object lifecycle strategies. The first
+/// 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 the second mode, 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.
///
/// IMPORTANT: Space for extra elements is allocated to account for those
/// that are inaccessible because they are in other local lists, so the
/// 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,
- int NumLocalLists_ = 32,
- int LocalListLimit_ = 200,
- template<typename> class Atom = std::atomic>
+template <
+ typename T,
+ uint32_t NumLocalLists_ = 32,
+ uint32_t LocalListLimit_ = 200,
+ template <typename> class Atom = std::atomic,
+ bool EagerRecycleWhenTrivial = false,
+ bool EagerRecycleWhenNotTrivial = true>
struct IndexedMemPool : boost::noncopyable {
typedef T value_type;
};
+ static constexpr bool eagerRecycle() {
+ return std::is_trivial<T>::value
+ ? EagerRecycleWhenTrivial : EagerRecycleWhenNotTrivial;
+ }
+
// 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 uint32_t(-1) == UINT32_MAX is reserved for isAllocated tracking
- return std::min(uint64_t(capacity) + (NumLocalLists - 1) * LocalListLimit,
- uint64_t(uint32_t(-1) - 1));
+ // 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) {
, globalHead_(TaggedPtr{})
{
const size_t needed = sizeof(Slot) * (actualCapacity_ + 1);
- long pagesize = sysconf(_SC_PAGESIZE);
+ 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_ == nullptr) {
+ if (slots_ == MAP_FAILED) {
assert(errno == ENOMEM);
throw std::bad_alloc();
}
/// Destroys all of the contained elements
~IndexedMemPool() {
- for (size_t i = size_; i > 0; --i) {
- slots_[i].~Slot();
+ if (!eagerRecycle()) {
+ // Take the minimum since it is possible that size_ > actualCapacity_.
+ // This can happen if there are multiple concurrent requests
+ // when size_ == actualCapacity_ - 1.
+ uint32_t last = std::min(uint32_t(size_), uint32_t(actualCapacity_));
+ for (uint32_t i = last; i > 0; --i) {
+ slots_[i].~Slot();
+ }
}
munmap(slots_, mmapLength_);
}
/// simultaneously allocated and not yet recycled. Because of the
/// local lists it is possible that more elements than this are returned
/// successfully
- size_t capacity() {
+ uint32_t capacity() {
return capacityForMaxIndex(actualCapacity_);
}
- /// Grants ownership of (*this)[retval], or returns 0 if no elements
- /// are available
- uint32_t allocIndex() {
- return localPop(localHead());
+ /// 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.
+ template <typename ...Args>
+ uint32_t allocIndex(Args&&... args) {
+ static_assert(sizeof...(Args) == 0 || eagerRecycle(),
+ "emplace-style allocation requires eager recycle, "
+ "which is defaulted only for non-trivial types");
+ auto idx = localPop(localHead());
+ if (idx != 0 && eagerRecycle()) {
+ T* ptr = &slot(idx).elem;
+ new (ptr) T(std::forward<Args>(args)...);
+ }
+ 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
- UniquePtr allocElem() {
- auto idx = allocIndex();
- auto ptr = idx == 0 ? nullptr : &slot(idx).elem;
+ 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));
+ if (eagerRecycle()) {
+ slot(idx).elem.~T();
+ }
localPush(localHead(), idx);
}
auto slot = reinterpret_cast<const Slot*>(
reinterpret_cast<const char*>(elem) - offsetof(Slot, elem));
- auto rv = slot - slots_;
+ auto rv = uint32_t(slot - slots_);
// this assert also tests that rv is in range
assert(elem == &(*this)[rv]);
/// Returns true iff idx has been alloc()ed and not recycleIndex()ed
bool isAllocated(uint32_t idx) const {
- return slot(idx).localNext == uint32_t(-1);
+ return slot(idx).localNext.load(std::memory_order_relaxed) == uint32_t(-1);
}
struct Slot {
T elem;
- uint32_t localNext;
- uint32_t globalNext;
+ Atom<uint32_t> localNext;
+ Atom<uint32_t> globalNext;
Slot() : localNext{}, globalNext{} {}
};
////////// 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_].
- size_t actualCapacity_;
-
- /// the number of bytes allocated from mmap, which is a multiple of
- /// the page size of the machine
- size_t mmapLength_;
+ 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_)
- std::atomic<uint32_t> 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
- Slot* FOLLY_ALIGN_TO_AVOID_FALSE_SHARING slots_;
+ FOLLY_ALIGN_TO_AVOID_FALSE_SHARING 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
/// this is the head of a list of node chained by globalNext, that are
/// themselves each the head of a list chained by localNext
- AtomicStruct<TaggedPtr,Atom> FOLLY_ALIGN_TO_AVOID_FALSE_SHARING globalHead_;
+ FOLLY_ALIGN_TO_AVOID_FALSE_SHARING AtomicStruct<TaggedPtr,Atom> globalHead_;
///////////// private methods
- size_t slotIndex(uint32_t idx) const {
+ uint32_t slotIndex(uint32_t idx) const {
assert(0 < idx &&
idx <= actualCapacity_ &&
idx <= size_.load(std::memory_order_acquire));
void globalPush(Slot& s, uint32_t localHead) {
while (true) {
TaggedPtr gh = globalHead_.load(std::memory_order_acquire);
- s.globalNext = gh.idx;
+ s.globalNext.store(gh.idx, std::memory_order_relaxed);
if (globalHead_.compare_exchange_strong(gh, gh.withIdx(localHead))) {
// success
return;
Slot& s = slot(idx);
TaggedPtr h = head.load(std::memory_order_acquire);
while (true) {
- s.localNext = h.idx;
+ s.localNext.store(h.idx, std::memory_order_relaxed);
if (h.size() == LocalListLimit) {
// push will overflow local list, steal it instead
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))) {
+ 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;
}
if (h.idx != 0) {
// local list is non-empty, try to pop
Slot& s = slot(h.idx);
- if (head.compare_exchange_strong(
- h, h.withIdx(s.localNext).withSizeDecr())) {
+ auto next = s.localNext.load(std::memory_order_relaxed);
+ if (head.compare_exchange_strong(h, h.withIdx(next).withSizeDecr())) {
// success
- s.localNext = uint32_t(-1);
+ s.localNext.store(uint32_t(-1), std::memory_order_relaxed);
return h.idx;
}
continue;
// allocation failed
return 0;
}
- // construct it
- new (&slot(idx)) T();
- slot(idx).localNext = uint32_t(-1);
+ // default-construct it now if we aren't going to construct and
+ // destroy on each allocation
+ if (!eagerRecycle()) {
+ T* ptr = &slot(idx).elem;
+ new (ptr) T();
+ }
+ slot(idx).localNext.store(uint32_t(-1), std::memory_order_relaxed);
return idx;
}
Slot& s = slot(idx);
+ auto next = s.localNext.load(std::memory_order_relaxed);
if (head.compare_exchange_strong(
- h, h.withIdx(s.localNext).withSize(LocalListLimit))) {
+ h, h.withIdx(next).withSize(LocalListLimit))) {
// global list moved to local list, keep head for us
- s.localNext = uint32_t(-1);
+ s.localNext.store(uint32_t(-1), std::memory_order_relaxed);
return idx;
}
// local bulk push failed, return idx to the global list and try again
} // namespace folly
# pragma GCC diagnostic pop
-#endif
projects / folly.git / blobdiff