diff --git a/util/cache.cc b/util/cache.cc index 0881bce..ce46886 100644 --- a/util/cache.cc +++ b/util/cache.cc @@ -19,6 +19,23 @@ Cache::~Cache() { namespace { // LRU cache implementation +// +// Cache entries have an "in_cache" boolean indicating whether the cache has a +// reference on the entry. The only ways that this can become false without the +// entry being passed to its "deleter" are via Erase(), via Insert() when +// an element with a duplicate key is inserted, or on destruction of the cache. +// +// The cache keeps two linked lists of items in the cache. All items in the +// cache are in one list or the other, and never both. Items still referenced +// by clients but erased from the cache are in neither list. The lists are: +// - in-use: contains the items currently referenced by clients, in no +// particular order. (This list is used for invariant checking. If we +// removed the check, elements that would otherwise be on this list could be +// left as disconnected singleton lists.) +// - LRU: contains the items not currently referenced by clients, in LRU order +// Elements are moved between these lists by the Ref() and Unref() methods, +// when they detect an element in the cache acquiring or losing its only +// external reference. // An entry is a variable length heap-allocated structure. Entries // are kept in a circular doubly linked list ordered by access time. @@ -30,7 +47,8 @@ struct LRUHandle { LRUHandle* prev; size_t charge; // TODO(opt): Only allow uint32_t? size_t key_length; - uint32_t refs; + bool in_cache; // Whether entry is in the cache. + uint32_t refs; // References, including cache reference, if present. uint32_t hash; // Hash of key(); used for fast sharding and comparisons char key_data[1]; // Beginning of key @@ -155,8 +173,10 @@ class LRUCache { private: void LRU_Remove(LRUHandle* e); - void LRU_Append(LRUHandle* e); + void LRU_Append(LRUHandle*list, LRUHandle* e); + void Ref(LRUHandle* e); void Unref(LRUHandle* e); + bool FinishErase(LRUHandle* e); // Initialized before use. size_t capacity_; @@ -167,34 +187,55 @@ class LRUCache { // Dummy head of LRU list. // lru.prev is newest entry, lru.next is oldest entry. + // Entries have refs==1 and in_cache==true. LRUHandle lru_; + // Dummy head of in-use list. + // Entries are in use by clients, and have refs >= 2 and in_cache==true. + LRUHandle in_use_; + HandleTable table_; }; LRUCache::LRUCache() : usage_(0) { - // Make empty circular linked list + // Make empty circular linked lists. lru_.next = &lru_; lru_.prev = &lru_; + in_use_.next = &in_use_; + in_use_.prev = &in_use_; } LRUCache::~LRUCache() { + assert(in_use_.next == &in_use_); // Error if caller has an unreleased handle for (LRUHandle* e = lru_.next; e != &lru_; ) { LRUHandle* next = e->next; - assert(e->refs == 1); // Error if caller has an unreleased handle + assert(e->in_cache); + e->in_cache = false; + assert(e->refs == 1); // Invariant of lru_ list. Unref(e); e = next; } } +void LRUCache::Ref(LRUHandle* e) { + if (e->refs == 1 && e->in_cache) { // If on lru_ list, move to in_use_ list. + LRU_Remove(e); + LRU_Append(&in_use_, e); + } + e->refs++; +} + void LRUCache::Unref(LRUHandle* e) { assert(e->refs > 0); e->refs--; - if (e->refs <= 0) { - usage_ -= e->charge; + if (e->refs == 0) { // Deallocate. + assert(!e->in_cache); (*e->deleter)(e->key(), e->value); free(e); + } else if (e->in_cache && e->refs == 1) { // No longer in use; move to lru_ list. + LRU_Remove(e); + LRU_Append(&lru_, e); } } @@ -203,10 +244,10 @@ void LRUCache::LRU_Remove(LRUHandle* e) { 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; +void LRUCache::LRU_Append(LRUHandle* list, LRUHandle* e) { + // Make "e" newest entry by inserting just before *list + e->next = list; + e->prev = list->prev; e->prev->next = e; e->next->prev = e; } @@ -215,9 +256,7 @@ 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); + Ref(e); } return reinterpret_cast(e); } @@ -239,46 +278,57 @@ Cache::Handle* LRUCache::Insert( e->charge = charge; e->key_length = key.size(); e->hash = hash; - e->refs = 2; // One from LRUCache, one for the returned handle + e->in_cache = false; + e->refs = 1; // 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); - } + if (capacity_ > 0) { + e->refs++; // for the cache's reference. + e->in_cache = true; + LRU_Append(&in_use_, e); + usage_ += charge; + FinishErase(table_.Insert(e)); + } // else don't cache. (Tests use capacity_==0 to turn off caching.) while (usage_ > capacity_ && lru_.next != &lru_) { LRUHandle* old = lru_.next; - LRU_Remove(old); - table_.Remove(old->key(), old->hash); - Unref(old); + assert(old->refs == 1); + bool erased = FinishErase(table_.Remove(old->key(), old->hash)); + if (!erased) { // to avoid unused variable when compiled NDEBUG + assert(erased); + } } return reinterpret_cast(e); } -void LRUCache::Erase(const Slice& key, uint32_t hash) { - MutexLock l(&mutex_); - LRUHandle* e = table_.Remove(key, hash); +// If e != NULL, finish removing *e from the cache; it has already been removed +// from the hash table. Return whether e != NULL. Requires mutex_ held. +bool LRUCache::FinishErase(LRUHandle* e) { if (e != NULL) { + assert(e->in_cache); LRU_Remove(e); + e->in_cache = false; + usage_ -= e->charge; Unref(e); } + return e != NULL; +} + +void LRUCache::Erase(const Slice& key, uint32_t hash) { + MutexLock l(&mutex_); + FinishErase(table_.Remove(key, hash)); } void LRUCache::Prune() { MutexLock l(&mutex_); - for (LRUHandle* e = lru_.next; e != &lru_; ) { - LRUHandle* next = e->next; - if (e->refs == 1) { - table_.Remove(e->key(), e->hash); - LRU_Remove(e); - Unref(e); + while (lru_.next != &lru_) { + LRUHandle* e = lru_.next; + assert(e->refs == 1); + bool erased = FinishErase(table_.Remove(e->key(), e->hash)); + if (!erased) { // to avoid unused variable when compiled NDEBUG + assert(erased); } - e = next; } } diff --git a/util/cache_test.cc b/util/cache_test.cc index 90af5fe..468f7a6 100644 --- a/util/cache_test.cc +++ b/util/cache_test.cc @@ -59,6 +59,11 @@ class CacheTest { &CacheTest::Deleter)); } + Cache::Handle* InsertAndReturnHandle(int key, int value, int charge = 1) { + return cache_->Insert(EncodeKey(key), EncodeValue(value), charge, + &CacheTest::Deleter); + } + void Erase(int key) { cache_->Erase(EncodeKey(key)); } @@ -135,8 +140,11 @@ TEST(CacheTest, EntriesArePinned) { TEST(CacheTest, EvictionPolicy) { Insert(100, 101); Insert(200, 201); + Insert(300, 301); + Cache::Handle* h = cache_->Lookup(EncodeKey(300)); - // Frequently used entry must be kept around + // Frequently used entry must be kept around, + // as must things that are still in use. for (int i = 0; i < kCacheSize + 100; i++) { Insert(1000+i, 2000+i); ASSERT_EQ(2000+i, Lookup(1000+i)); @@ -144,6 +152,25 @@ TEST(CacheTest, EvictionPolicy) { } ASSERT_EQ(101, Lookup(100)); ASSERT_EQ(-1, Lookup(200)); + ASSERT_EQ(301, Lookup(300)); + cache_->Release(h); +} + +TEST(CacheTest, UseExceedsCacheSize) { + // Overfill the cache, keeping handles on all inserted entries. + std::vector h; + for (int i = 0; i < kCacheSize + 100; i++) { + h.push_back(InsertAndReturnHandle(1000+i, 2000+i)); + } + + // Check that all the entries can be found in the cache. + for (int i = 0; i < h.size(); i++) { + ASSERT_EQ(2000+i, Lookup(1000+i)); + } + + for (int i = 0; i < h.size(); i++) { + cache_->Release(h[i]); + } } TEST(CacheTest, HeavyEntries) {