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c987eb4712
Summary: The blob cache enables an optimization on the read path: when a blob is found in the cache, we can avoid copying it into the buffer provided by the application. Instead, we can simply transfer ownership of the cache handle to the target `PinnableSlice`. (Note: this relies on the `Cleanable` interface, which is implemented by `PinnableSlice`.) This has the potential to save a lot of CPU, especially with large blob values. This task is a part of https://github.com/facebook/rocksdb/issues/10156 Pull Request resolved: https://github.com/facebook/rocksdb/pull/10297 Reviewed By: riversand963 Differential Revision: D37640311 Pulled By: gangliao fbshipit-source-id: 92de0e35cc703d06c87c5c1861cc2899ec52234a
146 lines
3.8 KiB
C++
146 lines
3.8 KiB
C++
// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
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// This source code is licensed under both the GPLv2 (found in the
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// COPYING file in the root directory) and Apache 2.0 License
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// (found in the LICENSE.Apache file in the root directory).
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#pragma once
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#include <cassert>
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#include "rocksdb/cache.h"
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#include "rocksdb/rocksdb_namespace.h"
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namespace ROCKSDB_NAMESPACE {
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// Returns the cached value given a cache handle.
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template <typename T>
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T* GetFromCacheHandle(Cache* cache, Cache::Handle* handle) {
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assert(cache);
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assert(handle);
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return static_cast<T*>(cache->Value(handle));
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}
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// Simple generic deleter for Cache (to be used with Cache::Insert).
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template <typename T>
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void DeleteCacheEntry(const Slice& /* key */, void* value) {
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delete static_cast<T*>(value);
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}
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// Turns a T* into a Slice so it can be used as a key with Cache.
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template <typename T>
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Slice GetSlice(const T* t) {
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return Slice(reinterpret_cast<const char*>(t), sizeof(T));
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}
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// Generic resource management object for cache handles that releases the handle
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// when destroyed. Has unique ownership of the handle, so copying it is not
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// allowed, while moving it transfers ownership.
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template <typename T>
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class CacheHandleGuard {
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public:
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CacheHandleGuard() = default;
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CacheHandleGuard(Cache* cache, Cache::Handle* handle)
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: cache_(cache),
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handle_(handle),
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value_(GetFromCacheHandle<T>(cache, handle)) {
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assert(cache_ && handle_ && value_);
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}
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CacheHandleGuard(const CacheHandleGuard&) = delete;
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CacheHandleGuard& operator=(const CacheHandleGuard&) = delete;
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CacheHandleGuard(CacheHandleGuard&& rhs) noexcept
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: cache_(rhs.cache_), handle_(rhs.handle_), value_(rhs.value_) {
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assert((!cache_ && !handle_ && !value_) || (cache_ && handle_ && value_));
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rhs.ResetFields();
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}
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CacheHandleGuard& operator=(CacheHandleGuard&& rhs) noexcept {
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if (this == &rhs) {
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return *this;
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}
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ReleaseHandle();
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cache_ = rhs.cache_;
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handle_ = rhs.handle_;
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value_ = rhs.value_;
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assert((!cache_ && !handle_ && !value_) || (cache_ && handle_ && value_));
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rhs.ResetFields();
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return *this;
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}
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~CacheHandleGuard() { ReleaseHandle(); }
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bool IsEmpty() const { return !handle_; }
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Cache* GetCache() const { return cache_; }
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Cache::Handle* GetCacheHandle() const { return handle_; }
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T* GetValue() const { return value_; }
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void TransferTo(Cleanable* cleanable) {
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if (cleanable) {
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if (handle_ != nullptr) {
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assert(cache_);
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cleanable->RegisterCleanup(&ReleaseCacheHandle, cache_, handle_);
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}
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}
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ResetFields();
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}
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void Reset() {
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ReleaseHandle();
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ResetFields();
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}
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private:
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void ReleaseHandle() {
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if (IsEmpty()) {
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return;
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}
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assert(cache_);
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cache_->Release(handle_);
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}
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void ResetFields() {
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cache_ = nullptr;
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handle_ = nullptr;
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value_ = nullptr;
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}
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static void ReleaseCacheHandle(void* arg1, void* arg2) {
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Cache* const cache = static_cast<Cache*>(arg1);
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assert(cache);
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Cache::Handle* const cache_handle = static_cast<Cache::Handle*>(arg2);
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assert(cache_handle);
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cache->Release(cache_handle);
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}
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private:
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Cache* cache_ = nullptr;
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Cache::Handle* handle_ = nullptr;
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T* value_ = nullptr;
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};
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// Build an aliasing shared_ptr that keeps `handle` in cache while there
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// are references, but the pointer is to the value for that cache entry,
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// which must be of type T. This is copyable, unlike CacheHandleGuard, but
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// does not provide access to caching details.
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template <typename T>
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std::shared_ptr<T> MakeSharedCacheHandleGuard(Cache* cache,
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Cache::Handle* handle) {
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auto wrapper = std::make_shared<CacheHandleGuard<T>>(cache, handle);
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return std::shared_ptr<T>(wrapper, static_cast<T*>(cache->Value(handle)));
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}
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} // namespace ROCKSDB_NAMESPACE
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