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rocksdb/cache/fast_lru_cache.h

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// Copyright (c) 2011-present, Facebook, Inc. All rights reserved
// This source code is licensed under both the GPLv2 (found in the
// COPYING file in the root directory) and Apache 2.0 License
// (found in the LICENSE.Apache file in the root directory).
//
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.
#pragma once
#include <array>
#include <memory>
#include <string>
#include "cache/cache_key.h"
#include "cache/sharded_cache.h"
#include "port/lang.h"
#include "port/malloc.h"
#include "port/port.h"
#include "rocksdb/secondary_cache.h"
#include "util/autovector.h"
#include "util/distributed_mutex.h"
namespace ROCKSDB_NAMESPACE {
namespace fast_lru_cache {
// Forward declaration of friend class.
class FastLRUCacheTest;
// LRU cache implementation using an open-address hash table.
//
// Every slot in the hash table is an LRUHandle. Because handles can be
// referenced externally, we can't discard them immediately once they are
// deleted (via a delete or an LRU eviction) or replaced by a new version
// (via an insert of the same key). The state of an element is defined by
// the following two properties:
// (R) Referenced: An element can be referenced externally (refs > 0), or not.
// Importantly, an element can be evicted if and only if it's not
// referenced. In particular, when an element becomes referenced, it's
// temporarily taken out of the LRU list until all references to it
// are dropped.
// (V) Visible: An element can visible for lookups (IS_VISIBLE set), or not.
// Initially, every element is visible. An element that is not visible is
// called a ghost.
// These properties induce 4 different states, with transitions defined as
// follows:
// - V --> not V: When a visible element is deleted or replaced by a new
// version.
// - Not V --> V: This cannot happen. A ghost remains in that state until it's
// not referenced any more, at which point it's ready to be removed from the
// hash table. (A ghost simply waits to transition to the afterlife---it will
// never be visible again.)
// - R --> not R: When all references to an element are dropped.
// - Not R --> R: When an unreferenced element becomes referenced. This can only
// happen if the element is V, since references to an element can only be
// created when it's visible.
//
// Internally, the cache uses an open-addressed hash table to index the handles.
// We use tombstone counters to keep track of displacements.
// Because of the tombstones and the two possible visibility states of an
// element, the table slots can be in 4 different states:
// 1. Visible element (IS_ELEMENT set and IS_VISIBLE set): The slot contains a
// key-value element.
// 2. Ghost element (IS_ELEMENT set and IS_VISIBLE unset): The slot contains an
// element that has been removed, but it's still referenced. It's invisible
// to lookups.
// 3. Tombstone (IS_ELEMENT unset and displacements > 0): The slot contains a
// tombstone.
// 4. Empty (IS_ELEMENT unset and displacements == 0): The slot is unused.
// A slot that is an element can further have IS_VISIBLE set or not.
// When a ghost is removed from the table, it can either transition to being a
// tombstone or an empty slot, depending on the number of displacements of the
// slot. In any case, the slot becomes available. When a handle is inserted
// into that slot, it becomes a visible element again.
// The load factor p is a real number in (0, 1) such that at all
// times at most a fraction p of all slots, without counting tombstones,
// are occupied by elements. This means that the probability that a
// random probe hits an empty slot is at most p, and thus at most 1/p probes
// are required on average. We use p = 70%, so between 1 and 2 probes are
// needed on average.
// Because the size of the hash table is always rounded up to the next
// power of 2, p is really an upper bound on the actual load factor---the
// actual load factor is anywhere between p/2 and p. This is a bit wasteful,
// but bear in mind that slots only hold metadata, not actual values.
// Since space cost is dominated by the values (the LSM blocks),
// overprovisioning the table with metadata only increases the total cache space
// usage by a tiny fraction.
constexpr double kLoadFactor = 0.7;
// Arbitrary seeds.
constexpr uint32_t kProbingSeed1 = 0xbc9f1d34;
constexpr uint32_t kProbingSeed2 = 0x7a2bb9d5;
// An experimental (under development!) alternative to LRUCache
struct LRUHandle {
void* value;
Cache::DeleterFn deleter;
LRUHandle* next;
LRUHandle* prev;
size_t total_charge; // TODO(opt): Only allow uint32_t?
// The hash of key(). Used for fast sharding and comparisons.
uint32_t hash;
// The number of external refs to this entry. The cache itself is not counted.
uint32_t refs;
enum Flags : uint8_t {
// Whether the handle is visible to Lookups.
IS_VISIBLE = (1 << 0),
// Whether the slot is in use by an element.
IS_ELEMENT = (1 << 1),
};
uint8_t flags;
// The number of elements that hash to this slot or a lower one,
// but wind up in a higher slot.
uint32_t displacements;
std::array<char, kCacheKeySize> key_data;
LRUHandle() {
value = nullptr;
deleter = nullptr;
next = nullptr;
prev = nullptr;
total_charge = 0;
hash = 0;
refs = 0;
flags = 0;
displacements = 0;
key_data.fill(0);
}
Slice key() const { return Slice(key_data.data(), kCacheKeySize); }
// Increase the reference count by 1.
void Ref() { refs++; }
// Just reduce the reference count by 1. Return true if it was last reference.
bool Unref() {
assert(refs > 0);
refs--;
return refs == 0;
}
// Return true if there are external refs, false otherwise.
bool HasRefs() const { return refs > 0; }
bool IsVisible() const { return flags & IS_VISIBLE; }
void SetIsVisible(bool is_visible) {
if (is_visible) {
flags |= IS_VISIBLE;
} else {
flags &= ~IS_VISIBLE;
}
}
bool IsElement() const { return flags & IS_ELEMENT; }
void SetIsElement(bool is_element) {
if (is_element) {
flags |= IS_ELEMENT;
} else {
flags &= ~IS_ELEMENT;
}
}
void FreeData() {
assert(refs == 0);
if (deleter) {
(*deleter)(key(), value);
}
}
// Calculate the memory usage by metadata.
inline size_t CalcMetaCharge(
CacheMetadataChargePolicy metadata_charge_policy) const {
if (metadata_charge_policy != kFullChargeCacheMetadata) {
return 0;
} else {
// #ifdef ROCKSDB_MALLOC_USABLE_SIZE
// return malloc_usable_size(
// const_cast<void*>(static_cast<const void*>(this)));
// #else
// TODO(Guido) malloc_usable_size only works when we call it on
// a pointer allocated with malloc. Because our handles are all
// allocated in a single shot as an array, the user can't call
// CalcMetaCharge (or CalcTotalCharge or GetCharge) on a handle
// pointer returned by the cache. Moreover, malloc_usable_size
// expects a heap-allocated handle, but sometimes in our code we
// wish to pass a stack-allocated handle (this is only a performance
// concern).
// What is the right way to compute metadata charges with pre-allocated
// handles?
return sizeof(LRUHandle);
// #endif
}
}
inline void CalcTotalCharge(
size_t charge, CacheMetadataChargePolicy metadata_charge_policy) {
total_charge = charge + CalcMetaCharge(metadata_charge_policy);
}
inline size_t GetCharge(
CacheMetadataChargePolicy metadata_charge_policy) const {
size_t meta_charge = CalcMetaCharge(metadata_charge_policy);
assert(total_charge >= meta_charge);
return total_charge - meta_charge;
}
inline bool IsEmpty() {
return !this->IsElement() && this->displacements == 0;
}
inline bool IsTombstone() {
return !this->IsElement() && this->displacements > 0;
}
inline bool Matches(const Slice& some_key, uint32_t some_hash) {
return this->IsElement() && this->hash == some_hash &&
this->key() == some_key;
}
};
// TODO(Guido) Update the following comment.
// We provide our own simple hash table since it removes a whole bunch
// of porting hacks and is also faster than some of the built-in hash
// table implementations in some of the compiler/runtime combinations
// we have tested. E.g., readrandom speeds up by ~5% over the g++
// 4.4.3's builtin hashtable.
class LRUHandleTable {
public:
explicit LRUHandleTable(uint8_t hash_bits);
~LRUHandleTable();
// Returns a pointer to a visible element matching the key/hash, or
// nullptr if not present.
LRUHandle* Lookup(const Slice& key, uint32_t hash);
// Inserts a copy of h into the hash table.
// Returns a pointer to the inserted handle, or nullptr if no slot
// available was found. If an existing visible element matching the
// key/hash is already present in the hash table, the argument old
// is set to pointe to it; otherwise, it's set to nullptr.
LRUHandle* Insert(LRUHandle* h, LRUHandle** old);
// Removes h from the hash table. The handle must already be off
// the LRU list.
void Remove(LRUHandle* h);
// Turns a visible element h into a ghost (i.e., not visible).
void Exclude(LRUHandle* h);
// Assigns a copy of h to the given slot.
void Assign(int slot, LRUHandle* h);
template <typename T>
void ApplyToEntriesRange(T func, uint32_t index_begin, uint32_t index_end) {
for (uint32_t i = index_begin; i < index_end; i++) {
LRUHandle* h = &array_[i];
if (h->IsVisible()) {
func(h);
}
}
}
uint8_t GetLengthBits() const { return length_bits_; }
uint32_t GetOccupancy() const { return occupancy_; }
private:
int FindVisibleElement(const Slice& key, uint32_t hash, int& probe,
int displacement);
int FindAvailableSlot(const Slice& key, int& probe, int displacement);
int FindVisibleElementOrAvailableSlot(const Slice& key, uint32_t hash,
int& probe, int displacement);
// Returns the index of the first slot probed (hashing with
// the given key) with a handle e such that cond(e) is true.
// Otherwise, if no match is found, returns -1.
// For every handle e probed except the final slot, updates
// e->displacements += displacement.
// The argument probe is modified such that consecutive calls
// to FindSlot continue probing right after where the previous
// call left.
int FindSlot(const Slice& key, std::function<bool(LRUHandle*)> cond,
int& probe, int displacement);
// Number of hash bits used for table index.
// The size of the table is 1 << length_bits_.
uint8_t length_bits_;
// Number of elements in the table.
uint32_t occupancy_;
std::unique_ptr<LRUHandle[]> array_;
};
// A single shard of sharded cache.
class ALIGN_AS(CACHE_LINE_SIZE) LRUCacheShard final : public CacheShard {
public:
LRUCacheShard(size_t capacity, size_t estimated_value_size,
bool strict_capacity_limit,
CacheMetadataChargePolicy metadata_charge_policy);
~LRUCacheShard() override = default;
// Separate from constructor so caller can easily make an array of LRUCache
// if current usage is more than new capacity, the function will attempt to
// free the needed space.
void SetCapacity(size_t capacity) override;
// Set the flag to reject insertion if cache if full.
void SetStrictCapacityLimit(bool strict_capacity_limit) override;
// Like Cache methods, but with an extra "hash" parameter.
// Insert an item into the hash table and, if handle is null, insert into
// the LRU list. Older items are evicted as necessary. If the cache is full
// and free_handle_on_fail is true, the item is deleted and handle is set to
// nullptr.
Status Insert(const Slice& key, uint32_t hash, void* value, size_t charge,
Cache::DeleterFn deleter, Cache::Handle** handle,
Cache::Priority priority) override;
Status Insert(const Slice& key, uint32_t hash, void* value,
const Cache::CacheItemHelper* helper, size_t charge,
Cache::Handle** handle, Cache::Priority priority) override {
return Insert(key, hash, value, charge, helper->del_cb, handle, priority);
}
Cache::Handle* Lookup(const Slice& key, uint32_t hash,
const Cache::CacheItemHelper* /*helper*/,
const Cache::CreateCallback& /*create_cb*/,
Cache::Priority /*priority*/, bool /*wait*/,
Statistics* /*stats*/) override {
return Lookup(key, hash);
}
Cache::Handle* Lookup(const Slice& key, uint32_t hash) override;
bool Release(Cache::Handle* handle, bool /*useful*/,
bool erase_if_last_ref) override {
return Release(handle, erase_if_last_ref);
}
bool IsReady(Cache::Handle* /*handle*/) override { return true; }
void Wait(Cache::Handle* /*handle*/) override {}
bool Ref(Cache::Handle* handle) override;
bool Release(Cache::Handle* handle, bool erase_if_last_ref = false) override;
void Erase(const Slice& key, uint32_t hash) override;
size_t GetUsage() const override;
size_t GetPinnedUsage() const override;
void ApplyToSomeEntries(
const std::function<void(const Slice& key, void* value, size_t charge,
DeleterFn deleter)>& callback,
uint32_t average_entries_per_lock, uint32_t* state) override;
void EraseUnRefEntries() override;
std::string GetPrintableOptions() const override;
private:
friend class LRUCache;
friend class FastLRUCacheTest;
void LRU_Remove(LRUHandle* e);
void LRU_Insert(LRUHandle* e);
// Free some space following strict LRU policy until enough space
// to hold (usage_ + charge) is freed or the lru list is empty
// This function is not thread safe - it needs to be executed while
// holding the mutex_.
void EvictFromLRU(size_t charge, autovector<LRUHandle>* deleted);
// Returns the charge of a single handle.
static size_t CalcEstimatedHandleCharge(
size_t estimated_value_size,
CacheMetadataChargePolicy metadata_charge_policy);
// Returns the number of bits used to hash an element in the hash
// table.
static uint8_t CalcHashBits(size_t capacity, size_t estimated_value_size,
CacheMetadataChargePolicy metadata_charge_policy);
// Initialized before use.
size_t capacity_;
// Whether to reject insertion if cache reaches its full capacity.
bool strict_capacity_limit_;
// Dummy head of LRU list.
// lru.prev is newest entry, lru.next is oldest entry.
// LRU contains items which can be evicted, ie reference only by cache
LRUHandle lru_;
// Pointer to head of low-pri pool in LRU list.
LRUHandle* lru_low_pri_;
// ------------^^^^^^^^^^^^^-----------
// Not frequently modified data members
// ------------------------------------
//
// We separate data members that are updated frequently from the ones that
// are not frequently updated so that they don't share the same cache line
// which will lead into false cache sharing
//
// ------------------------------------
// Frequently modified data members
// ------------vvvvvvvvvvvvv-----------
LRUHandleTable table_;
// Memory size for entries residing in the cache.
size_t usage_;
// Memory size for entries residing only in the LRU list.
size_t lru_usage_;
// mutex_ protects the following state.
// We don't count mutex_ as the cache's internal state so semantically we
// don't mind mutex_ invoking the non-const actions.
mutable DMutex mutex_;
};
class LRUCache
#ifdef NDEBUG
final
#endif
: public ShardedCache {
public:
LRUCache(size_t capacity, size_t estimated_value_size, int num_shard_bits,
bool strict_capacity_limit,
CacheMetadataChargePolicy metadata_charge_policy =
kDontChargeCacheMetadata);
~LRUCache() override;
const char* Name() const override { return "LRUCache"; }
CacheShard* GetShard(uint32_t shard) override;
const CacheShard* GetShard(uint32_t shard) const override;
void* Value(Handle* handle) override;
size_t GetCharge(Handle* handle) const override;
uint32_t GetHash(Handle* handle) const override;
DeleterFn GetDeleter(Handle* handle) const override;
void DisownData() override;
private:
LRUCacheShard* shards_ = nullptr;
int num_shards_ = 0;
};
} // namespace fast_lru_cache
std::shared_ptr<Cache> NewFastLRUCache(
size_t capacity, size_t estimated_value_size, int num_shard_bits,
bool strict_capacity_limit,
CacheMetadataChargePolicy metadata_charge_policy);
} // namespace ROCKSDB_NAMESPACE