MySQL 8.3.0
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1/* Copyright (c) 2020, 2023, Oracle and/or its affiliates.
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23/** @file storage/temptable/include/temptable/sharded_kv_store.h
24TempTable sharded key-value store implementation. */
29#include <array>
35namespace temptable {
37/** Sharded key-value store, a convenience wrapper class around Key_value_store
38 * that creates N_SHARDS instances of Key_value_store and exposes a simple
39 * interface to further manipulate with them. User code, if it wishes so, can
40 * opt-in for different key-value implementation or different locking
41 * mechanisms. Defaults are provided as convenience.
42 *
43 * Mapping between a shard and an actual Key_value_store is done at the level of
44 * implementation detail of this class and it is done via modulo-arithmethic on
45 * THD (connection) identifier. It is guaranteed that the same shard (same
46 * Key_value_store instance) will always be returned given the same input: THD
47 * (connection) identifier. However, there is no guarantee that there will be
48 * only one such mapping (this function is not a bijection).
49 *
50 * In other words, due to the wrap-around property of modulo-arithmetic
51 * and depending on the actual value of N_SHARDS, it is very much possible to
52 * get a reference to the same Key_value_store instance for two different THD
53 * (connection) identifiers. Thread-safety guarantees are exercised at the
54 * level of Key_value_store instance but are controlled through the `Lock`, this
55 * class template parameter.
56 *
57 * Due to aforementioned modulo-arithmethic, and the questionable performance in
58 * its more general form, N_SHARDS must be a number which is a power of two.
59 * This enables us to implement a modulo operation in single bitwise
60 * instruction. Check whether N_SHARDS is a number which is power of two is run
61 * during the compile-time.
62 * */
63template <size_t N_SHARDS, typename Lock = std::shared_timed_mutex,
64 template <typename...> class KeyValueImpl = std::unordered_map>
66 : public Sharded_key_value_store_logger<Sharded_key_value_store<N_SHARDS>,
68 /** Do not break encapsulation when using CRTP. */
72 /** Check whether all compile-time pre-conditions are set. */
73 static_assert(N_SHARDS && !(N_SHARDS & (N_SHARDS - 1)),
74 "N_SHARDS template parameter must be a power of two.");
76 /** Bitmask which enables us to implement modulo-arithmetic operation in
77 * single bitwise instruction. */
78 static constexpr size_t MODULO_MASK = N_SHARDS - 1;
80 /** In the event of inability to express ourselves with something like
81 * std::array<alignas<N> Key_value_store<...>> we have to fallback to this
82 * method.
83 * */
86 };
88 /** N_SHARDS instances of Key_value_store's. */
89 std::array<L1_dcache_aligned_kv_store, N_SHARDS> m_kv_store_shard;
91 public:
92 /** Returns a reference to one of the underlying Key_value_store instances.
93 *
94 * [in] THD (connection) identifier.
95 * @return A reference to Key_value_store instance. */
97 return m_kv_store_shard[thd_id & MODULO_MASK].shard;
98 }
101} // namespace temptable
Key-value store, a convenience wrapper class which models a thread-safe dictionary type.
Definition: kv_store.h:60
Sharded key-value store, a convenience wrapper class around Key_value_store that creates N_SHARDS ins...
Definition: sharded_kv_store.h:67
std::array< L1_dcache_aligned_kv_store, N_SHARDS > m_kv_store_shard
N_SHARDS instances of Key_value_store's.
Definition: sharded_kv_store.h:89
Key_value_store< Lock, KeyValueImpl > & operator[](size_t thd_id)
Returns a reference to one of the underlying Key_value_store instances.
Definition: sharded_kv_store.h:96
static constexpr size_t MODULO_MASK
Check whether all compile-time pre-conditions are set.
Definition: sharded_kv_store.h:78
TempTable key-value store implementation.
Definition: allocator.h:44
constexpr bool DEBUG_BUILD
Store build-type information into the constexpr expression.
Definition: constants.h:75
constexpr size_t L1_DCACHE_SIZE
Store L1-dcache size information into the constexpr expression.
Definition: constants.h:81
std::unordered_map< Key, Value, Hash, Key_equal, ut::allocator< std::pair< const Key, Value > > > unordered_map
Definition: ut0new.h:2897
TempTable sharded key-value store logger implementation.
TempTable constants.
In the event of inability to express ourselves with something like std::array<alignas<N> Key_value_st...
Definition: sharded_kv_store.h:84
Key_value_store< Lock, KeyValueImpl > shard
Definition: sharded_kv_store.h:85
Default Sharded_key_value_store logging facility which turns to no-op in non-debug builds.
Definition: sharded_kv_store_logger.h:41