rocksdb/cache/cache_reservation_manager.h
Hui Xiao 0aad4ca0ff Add comment for new_memory_used parameter in CacheReservationManager::UpdateCacheReservation (#8895)
Summary:
Context/Summary: this PR is to clarify what the parameter new_memory_used is in CacheReservationManager::UpdateCacheReservation

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8895

Test Plan:
- Passing existing test
- Make format

Reviewed By: jay-zhuang

Differential Revision: D30844814

Pulled By: hx235

fbshipit-source-id: 3177f7abf5668ea9e73818ceaa355566f03acabc
2021-09-09 15:25:24 -07:00

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4.1 KiB
C++

// 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 {
// CacheReservationManager is for reserving cache space for the memory used
// through inserting/releasing dummy entries in the cache.
// This class is not thread-safe.
class 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>
// Insert and release dummy entries in the cache to
// match the size of total dummy entries with the smallest multiple of
// kSizeDummyEntry that is 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
// @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);
std::size_t GetTotalReservedCacheSize();
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::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];
};
} // namespace ROCKSDB_NAMESPACE