MDEV-19749 MDL scalability regression after backup locks

MDL_lock::Ticket_list::remove_ticket(): reduce algoritmic complexity from O(N) to O(1) MDL_lock::Ticket_list::clear_bit_if_not_in_list(): removed MDL_lock::Ticket_list::m_type_counters: a map of ticket type to count. Initialization is memset(0) which takes time.

Author: kevgs

sql/mdl.cc

@@ -29,6 +29,7 @@
 #include <pfs_metadata_provider.h>
 #include <mysql/psi/mysql_mdl.h>
 #include <algorithm>
+#include <array>
 
 static PSI_memory_key key_memory_MDL_context_acquire_locks;
 
@@ -344,21 +345,20 @@ class MDL_lock
  {
  using List= ilist<MDL_ticket>;
  public:
- Ticket_list() :m_bitmap(0) {}
+ Ticket_list() :m_bitmap(0) { m_type_counters.fill(0); }
 
  void add_ticket(MDL_ticket *ticket);
  void remove_ticket(MDL_ticket *ticket);
  bool is_empty() const { return m_list.empty(); }
  bitmap_t bitmap() const { return m_bitmap; }
  List::const_iterator begin() const { return m_list.begin(); }
  List::const_iterator end() const { return m_list.end(); }
- private:
- void clear_bit_if_not_in_list(enum_mdl_type type);
  private:
  /** List of tickets. */
  List m_list;
  /** Bitmap of types of tickets in this list. */
  bitmap_t m_bitmap;
+ std::array<uint32_t, MDL_BACKUP_END> m_type_counters; // hash table
  };
 
 
@@ -1196,24 +1196,6 @@ MDL_wait::timed_wait(MDL_context_owner *owner, struct timespec *abs_timeout,
 }
 
 
-/**
- Clear bit corresponding to the type of metadata lock in bitmap representing
- set of such types if list of tickets does not contain ticket with such type.
-
- @param[in,out] bitmap Bitmap representing set of types of locks.
- @param[in] list List to inspect.
- @param[in] type Type of metadata lock to look up in the list.
-*/
-
-void MDL_lock::Ticket_list::clear_bit_if_not_in_list(enum_mdl_type type)
-{
- for (const auto &ticket : m_list)
- if (ticket.get_type() == type)
- return;
- m_bitmap&= ~ MDL_BIT(type);
-}
-
-
 /**
  Add ticket to MDL_lock's list of waiting requests and
  update corresponding bitmap of lock types.
@@ -1251,6 +1233,7 @@ void MDL_lock::Ticket_list::add_ticket(MDL_ticket *ticket)
  m_list.push_back(*ticket);
  }
  m_bitmap|= MDL_BIT(ticket->get_type());
+ m_type_counters[ticket->get_type()]++;
 }
 
 
@@ -1267,12 +1250,9 @@ void MDL_lock::Ticket_list::remove_ticket(MDL_ticket *ticket)
  one which was removed. If there is no such ticket, i.e. we have
  removed last ticket of particular type, then we need to update
  bitmap of waiting ticket's types.
- Note that in most common case, i.e. when shared lock is removed
- from waiting queue, we are likely to find ticket of the same
- type early without performing full iteration through the list.
- So this method should not be too expensive.
  */
- clear_bit_if_not_in_list(ticket->get_type());
+ if (--m_type_counters[ticket->get_type()] == 0)
+ m_bitmap&= ~MDL_BIT(ticket->get_type());
 }