rocksdb/cache/cache_entry_roles.h
Peter Dillinger 416dc7bed7 Clarify some DB::Open,OpenForReadOnly semantics (#8379)
Summary:
Longstanding tech debt

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8379

Test Plan: Better than not having an API contract

Reviewed By: jay-zhuang

Differential Revision: D29011131

Pulled By: pdillinger

fbshipit-source-id: 2c4796177733651954024fc17875f8642ca08d09
2021-06-14 16:09:21 -07:00

123 lines
4.6 KiB
C++

// Copyright (c) Facebook, Inc. and its affiliates. 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).
#pragma once
#include <array>
#include <cstdint>
#include <unordered_map>
#include "rocksdb/cache.h"
namespace ROCKSDB_NAMESPACE {
// Classifications of block cache entries, for reporting statistics
enum class CacheEntryRole {
// Block-based table data block
kDataBlock,
// Block-based table filter block (full or partitioned)
kFilterBlock,
// Block-based table metadata block for partitioned filter
kFilterMetaBlock,
// Block-based table deprecated filter block (old "block-based" filter)
kDeprecatedFilterBlock,
// Block-based table index block
kIndexBlock,
// Other kinds of block-based table block
kOtherBlock,
// WriteBufferManager reservations to account for memtable usage
kWriteBuffer,
// Default bucket, for miscellaneous cache entries. Do not use for
// entries that could potentially add up to large usage.
kMisc,
};
constexpr uint32_t kNumCacheEntryRoles =
static_cast<uint32_t>(CacheEntryRole::kMisc) + 1;
extern std::array<const char*, kNumCacheEntryRoles>
kCacheEntryRoleToCamelString;
extern std::array<const char*, kNumCacheEntryRoles>
kCacheEntryRoleToHyphenString;
// To associate cache entries with their role, we use a hack on the
// existing Cache interface. Because the deleter of an entry can authenticate
// the code origin of an entry, we can elaborate the choice of deleter to
// also encode role information, without inferring false role information
// from entries not choosing to encode a role.
//
// The rest of this file is for handling mappings between deleters and
// roles.
// To infer a role from a deleter, the deleter must be registered. This
// can be done "manually" with this function. This function is thread-safe,
// and the registration mappings go into private but static storage. (Note
// that DeleterFn is a function pointer, not std::function. Registrations
// should not be too many.)
void RegisterCacheDeleterRole(Cache::DeleterFn fn, CacheEntryRole role);
// Gets a copy of the registered deleter -> role mappings. This is the only
// function for reading the mappings made with RegisterCacheDeleterRole.
// Why only this interface for reading?
// * This function has to be thread safe, which could incur substantial
// overhead. We should not pay this overhead for every deleter look-up.
// * This is suitable for preparing for batch operations, like with
// CacheEntryStatsCollector.
// * The number of mappings should be sufficiently small (dozens).
std::unordered_map<Cache::DeleterFn, CacheEntryRole> CopyCacheDeleterRoleMap();
// ************************************************************** //
// An automatic registration infrastructure. This enables code
// to simply ask for a deleter associated with a particular type
// and role, and registration is automatic. In a sense, this is
// a small dependency injection infrastructure, because linking
// in new deleter instantiations is essentially sufficient for
// making stats collection (using CopyCacheDeleterRoleMap) aware
// of them.
namespace cache_entry_roles_detail {
template <typename T, CacheEntryRole R>
struct RegisteredDeleter {
RegisteredDeleter() { RegisterCacheDeleterRole(Delete, R); }
// These have global linkage to help ensure compiler optimizations do not
// break uniqueness for each <T,R>
static void Delete(const Slice& /* key */, void* value) {
delete static_cast<T*>(value);
}
};
template <CacheEntryRole R>
struct RegisteredNoopDeleter {
RegisteredNoopDeleter() { RegisterCacheDeleterRole(Delete, R); }
static void Delete(const Slice& /* key */, void* value) {
(void)value;
assert(value == nullptr);
}
};
} // namespace cache_entry_roles_detail
// Get an automatically registered deleter for value type T and role R.
// Based on C++ semantics, registration is invoked exactly once in a
// thread-safe way on first call to this function, for each <T, R>.
template <typename T, CacheEntryRole R>
Cache::DeleterFn GetCacheEntryDeleterForRole() {
static cache_entry_roles_detail::RegisteredDeleter<T, R> reg;
return reg.Delete;
}
// Get an automatically registered no-op deleter (value should be nullptr)
// and associated with role R. This is used for Cache "reservation" entries
// such as for WriteBufferManager.
template <CacheEntryRole R>
Cache::DeleterFn GetNoopDeleterForRole() {
static cache_entry_roles_detail::RegisteredNoopDeleter<R> reg;
return reg.Delete;
}
} // namespace ROCKSDB_NAMESPACE