cache.cc

// Copyright (c) 2015, Baidu.com, Inc. All Rights Reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// 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.

#include "cache.h"

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>

#include "mutex.h"
#include "hash.h"

namespace baidu {
namespace common {

Cache::~Cache() {
}

namespace {

// LRU cache implementation

// An entry is a variable length heap-allocated structure.  Entries
// are kept in a circular doubly linked list ordered by access time.
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;
    uint32_t refs;
    uint32_t hash;      // Hash of key(); used for fast sharding and comparisons
    char key_data[1];   // Beginning of key

    Slice key() const {
        // For cheaper lookups, we allow a temporary Handle object
        // to store a pointer to a key in "value".
        if (next == this) {
            return *(reinterpret_cast<Slice*>(value));
        } else {
            return Slice(key_data, key_length);
        }
    }
};

// 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 HandleTable {
public:
    HandleTable() : length_(0), elems_(0), list_(NULL) { Resize(); }
    ~HandleTable() { delete[] list_; }

    LRUHandle* Lookup(const Slice& key, uint32_t hash) {
        return *FindPointer(key, hash);
    }

    LRUHandle* Insert(LRUHandle* h) {
        LRUHandle** ptr = FindPointer(h->key(), h->hash);
        LRUHandle* old = *ptr;
        h->next_hash = (old == NULL ? NULL : old->next_hash);
        *ptr = h;
        if (old == NULL) {
            ++elems_;
            if (elems_ > length_) {
                // Since each cache entry is fairly large, we aim for a small
                // average linked list length (<= 1).
                Resize();
            }
        }
        return old;
    }

    LRUHandle* Remove(const Slice& key, uint32_t hash) {
        LRUHandle** ptr = FindPointer(key, hash);
        LRUHandle* result = *ptr;
        if (result != NULL) {
            *ptr = result->next_hash;
            --elems_;
        }
        return result;
    }

private:
    // The table consists of an array of buckets where each bucket is
    // a linked list of cache entries that hash into the bucket.
    uint32_t length_;
    uint32_t elems_;
    LRUHandle** list_;

    // 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) {
        LRUHandle** ptr = &list_[hash & (length_ - 1)];
        while (*ptr != NULL &&
                ((*ptr)->hash != hash || key != (*ptr)->key())) {
            ptr = &(*ptr)->next_hash;
        }
        return ptr;
    }

    void Resize() {
        uint32_t new_length = 4;
        while (new_length < elems_) {
            new_length *= 2;
        }
        LRUHandle** new_list = new LRUHandle*[new_length];
        memset(new_list, 0, sizeof(new_list[0]) * new_length);
        uint32_t count = 0;
        for (uint32_t i = 0; i < length_; i++) {
            LRUHandle* h = list_[i];
            while (h != NULL) {
                LRUHandle* next = h->next_hash;
                uint32_t hash = h->hash;
                LRUHandle** ptr = &new_list[hash & (new_length - 1)];
                h->next_hash = *ptr;
                *ptr = h;
                h = next;
                count++;
            }
        }
        assert(elems_ == count);
        delete[] list_;
        list_ = new_list;
        length_ = new_length;
    }
};

// A single shard of sharded cache.
class LRUCache {
public:
    LRUCache();
    ~LRUCache();

    // Separate from constructor so caller can easily make an array of LRUCache
    void SetCapacity(size_t capacity) { capacity_ = capacity; }

    // Like Cache methods, but with an extra "hash" parameter.
    Cache::Handle* Insert(const Slice& key, uint32_t hash,
            void* value, size_t charge,
            void (*deleter)(const Slice& key, void* value));
    Cache::Handle* Lookup(const Slice& key, uint32_t hash);
    void Release(Cache::Handle* handle);
    void Erase(const Slice& key, uint32_t hash);

private:
    void LRU_Remove(LRUHandle* e);
    void LRU_Append(LRUHandle* e);
    void Unref(LRUHandle* e);

    // Initialized before use.
    size_t capacity_;

    // mutex_ protects the following state.
    Mutex mutex_;
    size_t usage_;

    // Dummy head of LRU list.
    // lru.prev is newest entry, lru.next is oldest entry.
    LRUHandle lru_;

    HandleTable table_;
};

LRUCache::LRUCache()
    : usage_(0) {
        // Make empty circular linked list
        lru_.next = &lru_;
        lru_.prev = &lru_;
    }

LRUCache::~LRUCache() {
    for (LRUHandle* e = lru_.next; e != &lru_; ) {
        LRUHandle* next = e->next;
        assert(e->refs == 1);  // Error if caller has an unreleased handle
        Unref(e);
        e = next;
    }
}

void LRUCache::Unref(LRUHandle* e) {
    assert(e->refs > 0);
    e->refs--;
    if (e->refs <= 0) {
        usage_ -= e->charge;
        (*e->deleter)(e->key(), e->value);
        free(e);
    }
}

void LRUCache::LRU_Remove(LRUHandle* e) {
    e->next->prev = e->prev;
    e->prev->next = e->next;
}

void LRUCache::LRU_Append(LRUHandle* e) {
    // Make "e" newest entry by inserting just before lru_
    e->next = &lru_;
    e->prev = lru_.prev;
    e->prev->next = e;
    e->next->prev = e;
}

Cache::Handle* LRUCache::Lookup(const Slice& key, uint32_t hash) {
    MutexLock l(&mutex_);
    LRUHandle* e = table_.Lookup(key, hash);
    if (e != NULL) {
        e->refs++;
        LRU_Remove(e);
        LRU_Append(e);
    }
    return reinterpret_cast<Cache::Handle*>(e);
}

void LRUCache::Release(Cache::Handle* handle) {
    MutexLock l(&mutex_);
    Unref(reinterpret_cast<LRUHandle*>(handle));
}

Cache::Handle* LRUCache::Insert(
        const Slice& key, uint32_t hash, void* value, size_t charge,
        void (*deleter)(const Slice& key, void* value)) {
    MutexLock l(&mutex_);

    LRUHandle* e = reinterpret_cast<LRUHandle*>(
            malloc(sizeof(LRUHandle)-1 + key.size()));
    e->value = value;
    e->deleter = deleter;
    e->charge = charge;
    e->key_length = key.size();
    e->hash = hash;
    e->refs = 2;  // One from LRUCache, one for the returned handle
    memcpy(e->key_data, key.data(), key.size());
    LRU_Append(e);
    usage_ += charge;

    LRUHandle* old = table_.Insert(e);
    if (old != NULL) {
        LRU_Remove(old);
        Unref(old);
    }

    while (usage_ > capacity_ && lru_.next != &lru_) {
        LRUHandle* old = lru_.next;
        LRU_Remove(old);
        table_.Remove(old->key(), old->hash);
        Unref(old);
    }

    return reinterpret_cast<Cache::Handle*>(e);
}

void LRUCache::Erase(const Slice& key, uint32_t hash) {
    MutexLock l(&mutex_);
    LRUHandle* e = table_.Remove(key, hash);
    if (e != NULL) {
        LRU_Remove(e);
        Unref(e);
    }
}

static const int kNumShardBits = 4;
static const int kNumShards = 1 << kNumShardBits;

class ShardedLRUCache : public Cache {
private:
    LRUCache shard_[kNumShards];
    Mutex id_mutex_;
    uint64_t last_id_;

    static inline uint32_t HashSlice(const Slice& s) {
        return Hash(s.data(), s.size(), 0);
    }

    static uint32_t Shard(uint32_t hash) {
        return hash >> (32 - kNumShardBits);
    }

public:
    explicit ShardedLRUCache(size_t capacity)
        : last_id_(0) {
            const size_t per_shard = (capacity + (kNumShards - 1)) / kNumShards;
            for (int s = 0; s < kNumShards; s++) {
                shard_[s].SetCapacity(per_shard);
            }
        }
    virtual ~ShardedLRUCache() { }
    virtual Handle* Insert(const Slice& key, void* value, size_t charge,
                           void (*deleter)(const Slice& key, void* value)) {
        const uint32_t hash = HashSlice(key);
        return shard_[Shard(hash)].Insert(key, hash, value, charge, deleter);
    }
    virtual Handle* Lookup(const Slice& key) {
        const uint32_t hash = HashSlice(key);
        return shard_[Shard(hash)].Lookup(key, hash);
    }
    virtual void Release(Handle* handle) {
        LRUHandle* h = reinterpret_cast<LRUHandle*>(handle);
        shard_[Shard(h->hash)].Release(handle);
    }
    virtual void Erase(const Slice& key) {
        const uint32_t hash = HashSlice(key);
        shard_[Shard(hash)].Erase(key, hash);
    }
    virtual void* Value(Handle* handle) {
        return reinterpret_cast<LRUHandle*>(handle)->value;
    }
    virtual uint64_t NewId() {
        MutexLock l(&id_mutex_);
        return ++(last_id_);
    }
};

}  // end anonymous namespace

Cache* NewLRUCache(size_t capacity) {
    return new ShardedLRUCache(capacity);
}

}  // namespace common
}