rocksdb/cache/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 <string>
#include "cache/sharded_cache.h"
#include "port/malloc.h"
#include "port/port.h"
#include "util/autovector.h"
namespace rocksdb {
// LRU cache implementation. This class is not thread-safe.
// An entry is a variable length heap-allocated structure.
// Entries are referenced by cache and/or by any external entity.
// The cache keeps all its entries in a hash table. Some elements
// are also stored on LRU list.
//
// LRUHandle can be in these states:
// 1. Referenced externally AND in hash table.
// In that case the entry is *not* in the LRU list
// (refs >= 1 && in_cache == true)
// 2. Not referenced externally AND in hash table.
// In that case the entry is in the LRU list and can be freed.
// (refs == 0 && in_cache == true)
// 3. Referenced externally AND not in hash table.
// In that case the entry is not in the LRU list and not in hash table.
// The entry can be freed when refs becomes 0.
// (refs >= 1 && in_cache == false)
//
// All newly created LRUHandles are in state 1. If you call
// LRUCacheShard::Release on entry in state 1, it will go into state 2.
// To move from state 1 to state 3, either call LRUCacheShard::Erase or
// LRUCacheShard::Insert with the same key (but possibly different value).
// To move from state 2 to state 1, use LRUCacheShard::Lookup.
// Before destruction, make sure that no handles are in state 1. This means
// that any successful LRUCacheShard::Lookup/LRUCacheShard::Insert have a
// matching LRUCache::Release (to move into state 2) or LRUCacheShard::Erase
// (to move into state 3).
struct LRUHandle {
void* value;
void (*deleter)(const Slice&, void* value);
LRUHandle* next_hash;
LRUHandle* next;
LRUHandle* prev;
size_t charge; // TODO(opt): Only allow uint32_t?
size_t key_length;
// 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 this entry is referenced by the hash table.
IN_CACHE = (1 << 0),
// Whether this entry is high priority entry.
IS_HIGH_PRI = (1 << 1),
// Whether this entry is in high-pri pool.
IN_HIGH_PRI_POOL = (1 << 2),
// Wwhether this entry has had any lookups (hits).
HAS_HIT = (1 << 3),
};
uint8_t flags;
// Beginning of the key (MUST BE THE LAST FIELD IN THIS STRUCT!)
char key_data[1];
Slice key() const { return Slice(key_data, key_length); }
// 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 InCache() const { return flags & IN_CACHE; }
bool IsHighPri() const { return flags & IS_HIGH_PRI; }
bool InHighPriPool() const { return flags & IN_HIGH_PRI_POOL; }
bool HasHit() const { return flags & HAS_HIT; }
void SetInCache(bool in_cache) {
if (in_cache) {
flags |= IN_CACHE;
} else {
flags &= ~IN_CACHE;
}
}
void SetPriority(Cache::Priority priority) {
if (priority == Cache::Priority::HIGH) {
flags |= IS_HIGH_PRI;
} else {
flags &= ~IS_HIGH_PRI;
}
}
void SetInHighPriPool(bool in_high_pri_pool) {
if (in_high_pri_pool) {
flags |= IN_HIGH_PRI_POOL;
} else {
flags &= ~IN_HIGH_PRI_POOL;
}
}
void SetHit() { flags |= HAS_HIT; }
void Free() {
assert(refs == 0);
if (deleter) {
(*deleter)(key(), value);
}
delete[] reinterpret_cast<char*>(this);
}
// Caclculate the memory usage by metadata
inline size_t CalcTotalCharge(
CacheMetadataChargePolicy metadata_charge_policy) {
size_t meta_charge = 0;
if (metadata_charge_policy == kFullChargeCacheMetadata) {
#ifdef ROCKSDB_MALLOC_USABLE_SIZE
meta_charge += malloc_usable_size(static_cast<void*>(this));
#else
// This is the size that is used when a new handle is created
meta_charge += sizeof(LRUHandle) - 1 + key_length;
#endif
}
return charge + meta_charge;
}
};
// 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:
LRUHandleTable();
~LRUHandleTable();
LRUHandle* Lookup(const Slice& key, uint32_t hash);
LRUHandle* Insert(LRUHandle* h);
LRUHandle* Remove(const Slice& key, uint32_t hash);
template <typename T>
void ApplyToAllCacheEntries(T func) {
for (uint32_t i = 0; i < length_; i++) {
LRUHandle* h = list_[i];
while (h != nullptr) {
auto n = h->next_hash;
assert(h->InCache());
func(h);
h = n;
}
}
}
private:
// Return a pointer to slot that points to a cache entry that
// matches key/hash. If there is no such cache entry, return a
// pointer to the trailing slot in the corresponding linked list.
LRUHandle** FindPointer(const Slice& key, uint32_t hash);
void Resize();
// The table consists of an array of buckets where each bucket is
// a linked list of cache entries that hash into the bucket.
LRUHandle** list_;
uint32_t length_;
uint32_t elems_;
};
// A single shard of sharded cache.
class ALIGN_AS(CACHE_LINE_SIZE) LRUCacheShard final : public CacheShard {
public:
LRUCacheShard(size_t capacity, bool strict_capacity_limit,
double high_pri_pool_ratio, bool use_adaptive_mutex,
CacheMetadataChargePolicy metadata_charge_policy);
virtual ~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
virtual void SetCapacity(size_t capacity) override;
// Set the flag to reject insertion if cache if full.
virtual void SetStrictCapacityLimit(bool strict_capacity_limit) override;
// Set percentage of capacity reserved for high-pri cache entries.
void SetHighPriorityPoolRatio(double high_pri_pool_ratio);
// Like Cache methods, but with an extra "hash" parameter.
virtual Status Insert(const Slice& key, uint32_t hash, void* value,
size_t charge,
void (*deleter)(const Slice& key, void* value),
Cache::Handle** handle,
Cache::Priority priority) override;
virtual Cache::Handle* Lookup(const Slice& key, uint32_t hash) override;
virtual bool Ref(Cache::Handle* handle) override;
virtual bool Release(Cache::Handle* handle,
bool force_erase = false) override;
virtual void Erase(const Slice& key, uint32_t hash) override;
// Although in some platforms the update of size_t is atomic, to make sure
// GetUsage() and GetPinnedUsage() work correctly under any platform, we'll
// protect them with mutex_.
virtual size_t GetUsage() const override;
virtual size_t GetPinnedUsage() const override;
virtual void ApplyToAllCacheEntries(void (*callback)(void*, size_t),
bool thread_safe) override;
virtual void EraseUnRefEntries() override;
virtual std::string GetPrintableOptions() const override;
void TEST_GetLRUList(LRUHandle** lru, LRUHandle** lru_low_pri);
// Retrieves number of elements in LRU, for unit test purpose only
// not threadsafe
size_t TEST_GetLRUSize();
// Retrives high pri pool ratio
double GetHighPriPoolRatio();
private:
void LRU_Remove(LRUHandle* e);
void LRU_Insert(LRUHandle* e);
// Overflow the last entry in high-pri pool to low-pri pool until size of
// high-pri pool is no larger than the size specify by high_pri_pool_pct.
void MaintainPoolSize();
// 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);
// Initialized before use.
size_t capacity_;
// Memory size for entries in high-pri pool.
size_t high_pri_pool_usage_;
// Whether to reject insertion if cache reaches its full capacity.
bool strict_capacity_limit_;
// Ratio of capacity reserved for high priority cache entries.
double high_pri_pool_ratio_;
// High-pri pool size, equals to capacity * high_pri_pool_ratio.
// Remember the value to avoid recomputing each time.
double high_pri_pool_capacity_;
// 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 port::Mutex mutex_;
};
class LRUCache
#ifdef NDEBUG
final
#endif
: public ShardedCache {
public:
LRUCache(size_t capacity, int num_shard_bits, bool strict_capacity_limit,
double high_pri_pool_ratio,
std::shared_ptr<MemoryAllocator> memory_allocator = nullptr,
bool use_adaptive_mutex = kDefaultToAdaptiveMutex,
CacheMetadataChargePolicy metadata_charge_policy =
kDontChargeCacheMetadata);
virtual ~LRUCache();
virtual const char* Name() const override { return "LRUCache"; }
virtual CacheShard* GetShard(int shard) override;
virtual const CacheShard* GetShard(int shard) const override;
virtual void* Value(Handle* handle) override;
virtual size_t GetCharge(Handle* handle) const override;
virtual uint32_t GetHash(Handle* handle) const override;
virtual void DisownData() override;
// Retrieves number of elements in LRU, for unit test purpose only
size_t TEST_GetLRUSize();
// Retrives high pri pool ratio
double GetHighPriPoolRatio();
private:
LRUCacheShard* shards_ = nullptr;
int num_shards_ = 0;
};
} // namespace rocksdb