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    • Vladislav Vaintroub's avatar
    • Vladislav Vaintroub's avatar
    • Daniel Black's avatar
    • Marko Mäkelä's avatar
      MDEV-22871: Reduce InnoDB buf_pool.page_hash contention · 5155a300
      Marko Mäkelä authored
      The rw_lock_s_lock() calls for the buf_pool.page_hash became a
      clear bottleneck after MDEV-15053 reduced the contention on
      buf_pool.mutex. We will replace that use of rw_lock_t with a
      special implementation that is optimized for memory bus traffic.
      
      The hash_table_locks instrumentation will be removed.
      
      buf_pool_t::page_hash: Use a special implementation whose API is
      compatible with hash_table_t, and store the custom rw-locks
      directly in buf_pool.page_hash.array, intentionally sharing
      cache lines with the hash table pointers.
      
      rw_lock: A low-level rw-lock implementation based on std::atomic<uint32_t>
      where read_trylock() becomes a simple fetch_add(1).
      
      buf_pool_t::page_hash_latch: The special of rw_lock for the page_hash.
      
      buf_pool_t::page_hash_latch::read_lock(): Assert that buf_pool.mutex
      is not being held by the caller.
      
      buf_pool_t::page_hash_latch::write_lock() may be called while not holding
      buf_pool.mutex. buf_pool_t::watch_set() is such a caller.
      
      buf_pool_t::page_hash_latch::read_lock_wait(),
      page_hash_latch::write_lock_wait(): The spin loops.
      These will obey the global parameters innodb_sync_spin_loops and
      innodb_sync_spin_wait_delay.
      
      buf_pool_t::freed_page_hash: A singly linked list of copies of
      buf_pool.page_hash that ever existed. The fact that we never
      free any buf_pool.page_hash.array guarantees that all
      page_hash_latch that ever existed will remain valid until shutdown.
      
      buf_pool_t::resize_hash(): Replaces buf_pool_resize_hash().
      Prepend a shallow copy of the old page_hash to freed_page_hash.
      
      buf_pool_t::page_hash_table::n_cells: Declare as Atomic_relaxed.
      
      buf_pool_t::page_hash_table::lock(): Explain what prevents a
      race condition with buf_pool_t::resize_hash().
      5155a300