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rocksdb/cache/cache_reservation_manager.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 <atomic>
#include <cstddef>
#include <cstdint>
#include <memory>
#include <vector>
#include "cache/cache_entry_roles.h"
#include "rocksdb/cache.h"
#include "rocksdb/slice.h"
#include "rocksdb/status.h"
#include "table/block_based/block_based_table_reader.h"
#include "util/coding.h"
namespace ROCKSDB_NAMESPACE {
template <CacheEntryRole R>
class CacheReservationHandle;
// CacheReservationManager is for reserving cache space for the memory used
// through inserting/releasing dummy entries in the cache.
//
// This class is NOT thread-safe, except that GetTotalReservedCacheSize()
// can be called without external synchronization.
class CacheReservationManager
: public std::enable_shared_from_this<CacheReservationManager> {
public:
// Construct a CacheReservationManager
// @param cache The cache where dummy entries are inserted and released for
// reserving cache space
// @param delayed_decrease If set true, then dummy entries won't be released
// immediately when memory usage decreases.
// Instead, it will be released when the memory usage
// decreases to 3/4 of what we have reserved so far.
// This is for saving some future dummy entry
// insertion when memory usage increases are likely to
// happen in the near future.
explicit CacheReservationManager(std::shared_ptr<Cache> cache,
bool delayed_decrease = false);
// no copy constructor, copy assignment, move constructor, move assignment
CacheReservationManager(const CacheReservationManager &) = delete;
CacheReservationManager &operator=(const CacheReservationManager &) = delete;
CacheReservationManager(CacheReservationManager &&) = delete;
CacheReservationManager &operator=(CacheReservationManager &&) = delete;
~CacheReservationManager();
template <CacheEntryRole R>
// One of the two ways of reserving/releasing cache,
// see CacheReservationManager::MakeCacheReservation() for the other.
// Use ONLY one of them to prevent unexpected behavior.
//
// Insert and release dummy entries in the cache to
// match the size of total dummy entries with the least multiple of
// kSizeDummyEntry greater than or equal to new_mem_used
//
// Insert dummy entries if new_memory_used > cache_allocated_size_;
//
// Release dummy entries if new_memory_used < cache_allocated_size_
// (and new_memory_used < cache_allocated_size_ * 3/4
// when delayed_decrease is set true);
//
// Keey dummy entries the same if (1) new_memory_used == cache_allocated_size_
// or (2) new_memory_used is in the interval of
// [cache_allocated_size_ * 3/4, cache_allocated_size) when delayed_decrease
// is set true.
//
// @param new_memory_used The number of bytes used by new memory
// The most recent new_memoy_used passed in will be returned
// in GetTotalMemoryUsed() even when the call return non-ok status.
//
// Since the class is NOT thread-safe, external synchronization on the
// order of calling UpdateCacheReservation() is needed if you want
// GetTotalMemoryUsed() indeed returns the latest memory used.
//
// @return On inserting dummy entries, it returns Status::OK() if all dummy
// entry insertions succeed.
// Otherwise, it returns the first non-ok status;
// On releasing dummy entries, it always returns Status::OK().
// On keeping dummy entries the same, it always returns Status::OK().
Status UpdateCacheReservation(std::size_t new_memory_used);
// One of the two ways of reserving/releasing cache,
// see CacheReservationManager::UpdateCacheReservation() for the other.
// Use ONLY one of them to prevent unexpected behavior.
//
// Insert dummy entries in the cache for the incremental memory usage
// to match the size of total dummy entries with the least multiple of
// kSizeDummyEntry greater than or equal to the total memory used.
//
// A CacheReservationHandle is returned as an output parameter.
// The reserved dummy entries are automatically released on the destruction of
// this handle, which achieves better RAII per cache reservation.
//
// WARNING: Deallocate all the handles of the CacheReservationManager object
// before deallocating the object to prevent unexpected behavior.
//
// @param incremental_memory_used The number of bytes increased in memory
// usage.
//
// Calling GetTotalMemoryUsed() afterward will return the total memory
// increased by this number, even when calling MakeCacheReservation()
// returns non-ok status.
//
// Since the class is NOT thread-safe, external synchronization in
// calling MakeCacheReservation() is needed if you want
// GetTotalMemoryUsed() indeed returns the latest memory used.
//
// @param handle An pointer to std::unique_ptr<CacheReservationHandle<R>> that
// manages the lifetime of the handle and its cache reservation.
//
// @return It returns Status::OK() if all dummy
// entry insertions succeed.
// Otherwise, it returns the first non-ok status;
//
// REQUIRES: handle != nullptr
// REQUIRES: The CacheReservationManager object is NOT managed by
// std::unique_ptr as CacheReservationHandle needs to
// shares ownership to the CacheReservationManager object.
template <CacheEntryRole R>
Status MakeCacheReservation(
std::size_t incremental_memory_used,
std::unique_ptr<CacheReservationHandle<R>> *handle);
// Return the size of the cache (which is a multiple of kSizeDummyEntry)
// successfully reserved by calling UpdateCacheReservation().
//
// When UpdateCacheReservation() returns non-ok status,
// calling GetTotalReservedCacheSize() after that might return a slightly
// smaller number than the actual reserved cache size due to
// the returned number will always be a multiple of kSizeDummyEntry
// and cache full might happen in the middle of inserting a dummy entry.
std::size_t GetTotalReservedCacheSize();
// Return the latest total memory used indicated by the most recent call of
// UpdateCacheReservation(std::size_t new_memory_used);
std::size_t GetTotalMemoryUsed();
static constexpr std::size_t GetDummyEntrySize() { return kSizeDummyEntry; }
private:
static constexpr std::size_t kSizeDummyEntry = 256 * 1024;
// The key will be longer than keys for blocks in SST files so they won't
// conflict.
static const std::size_t kCacheKeyPrefixSize =
BlockBasedTable::kMaxCacheKeyPrefixSize + kMaxVarint64Length;
Slice GetNextCacheKey();
template <CacheEntryRole R>
Status IncreaseCacheReservation(std::size_t new_mem_used);
Status DecreaseCacheReservation(std::size_t new_mem_used);
std::shared_ptr<Cache> cache_;
bool delayed_decrease_;
std::atomic<std::size_t> cache_allocated_size_;
std::size_t memory_used_;
std::vector<Cache::Handle *> dummy_handles_;
std::uint64_t next_cache_key_id_ = 0;
// The non-prefix part will be updated according to the ID to use.
char cache_key_[kCacheKeyPrefixSize + kMaxVarint64Length];
};
// CacheReservationHandle is for managing the lifetime of a cache reservation
// This class is NOT thread-safe
template <CacheEntryRole R>
class CacheReservationHandle {
public:
// REQUIRES: cache_res_mgr != nullptr
explicit CacheReservationHandle(
std::size_t incremental_memory_used,
std::shared_ptr<CacheReservationManager> cache_res_mgr);
~CacheReservationHandle();
private:
std::size_t incremental_memory_used_;
std::shared_ptr<CacheReservationManager> cache_res_mgr_;
};
} // namespace ROCKSDB_NAMESPACE