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.

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;
 
@@ -344,7 +345,7 @@ 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);
@@ -352,13 +353,12 @@ class MDL_lock
     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());
 }