Fix for bug#12695572 - "IMPROVE MDL PERFORMANCE IN PRE-VISTA

BY CACHING OR REDUCING CREATEEVENT CALLS".
 
5.5 versions of MySQL server performed worse than 5.1 versions 
under single-connection workload in autocommit mode on Windows XP.
 
Part of this slowdown can be attributed to overhead associated
with constant creation/destruction of MDL_lock objects in the MDL
subsystem. The problem is that creation/destruction of these
objects causes creation and destruction of associated
synchronization primitives, which are expensive on Windows XP.
 
This patch tries to alleviate this problem by introducing a cache
of unused MDL_object_lock objects. Instead of destroying such
objects we put them into the cache and then reuse with a new
key when creation of a new object is requested.

To limit the size of this cache, a new --metadata-locks-cache-size
start-up parameter was introduced.

mysql-test/r/mysqld--help-notwin.result

@@ -336,6 +336,8 @@ The following options may be given as the first argument:
  After this many write locks, allow some read locks to run
  in between
  --memlock           Lock mysqld in memory.
+ --metadata-locks-cache-size=# 
+ Size of unused metadata locks cache
  --min-examined-row-limit=# 
  Don't write queries to slow log that examine fewer rows
  than that
@@ -844,6 +846,7 @@ max-tmp-tables 32
 max-user-connections 0
 max-write-lock-count 18446744073709551615
 memlock FALSE
+metadata-locks-cache-size 1024
 min-examined-row-limit 0
 multi-range-count 256
 myisam-block-size 1024

mysql-test/r/mysqld--help-win.result

@@ -335,6 +335,8 @@ The following options may be given as the first argument:
  After this many write locks, allow some read locks to run
  in between
  --memlock           Lock mysqld in memory.
+ --metadata-locks-cache-size=# 
+ Size of unused metadata locks cache
  --min-examined-row-limit=# 
  Don't write queries to slow log that examine fewer rows
  than that
@@ -847,6 +849,7 @@ max-tmp-tables 32
 max-user-connections 0
 max-write-lock-count 18446744073709551615
 memlock FALSE
+metadata-locks-cache-size 1024
 min-examined-row-limit 0
 multi-range-count 256
 myisam-block-size 1024

mysql-test/suite/sys_vars/r/metadata_locks_cache_size_basic.result

+#
+# Check that the paremeter is correctly set by start-up
+# option (.opt file sets it to 256 while default is 1024).
+select @@global.metadata_locks_cache_size = 256;
+@@global.metadata_locks_cache_size = 256
+1
+#
+# Check that variable is read only
+#
+set @@global.metadata_locks_cache_size= 1024;
+ERROR HY000: Variable 'metadata_locks_cache_size' is a read only variable
+select @@global.metadata_locks_cache_size = 256;
+@@global.metadata_locks_cache_size = 256
+1
+#
+# And only GLOBAL
+#
+select @@session.metadata_locks_cache_size;
+ERROR HY000: Variable 'metadata_locks_cache_size' is a GLOBAL variable
+set @@session.metadata_locks_cache_size= 1024;
+ERROR HY000: Variable 'metadata_locks_cache_size' is a read only variable

mysql-test/suite/sys_vars/t/metadata_locks_cache_size_basic-master.opt

+--metadata-locks-cache-size=256

mysql-test/suite/sys_vars/t/metadata_locks_cache_size_basic.test

+#
+# Basic test coverage for --metadata-locks-cache-size startup
+# parameter and corresponding read-only global @@metadata_locks_cache_size
+# variable.
+#
+
+--echo #
+--echo # Check that the paremeter is correctly set by start-up
+--echo # option (.opt file sets it to 256 while default is 1024).
+select @@global.metadata_locks_cache_size = 256;
+
+--echo #
+--echo # Check that variable is read only
+--echo #
+--error ER_INCORRECT_GLOBAL_LOCAL_VAR
+set @@global.metadata_locks_cache_size= 1024;
+select @@global.metadata_locks_cache_size = 256;
+
+--echo #
+--echo # And only GLOBAL
+--echo #
+--error ER_INCORRECT_GLOBAL_LOCAL_VAR
+select @@session.metadata_locks_cache_size;
+--error ER_INCORRECT_GLOBAL_LOCAL_VAR
+set @@session.metadata_locks_cache_size= 1024;

sql/mdl.h

@@ -851,4 +851,12 @@ extern "C" void thd_exit_cond(MYSQL_THD thd, const char *old_msg);
 extern mysql_mutex_t LOCK_open;
 #endif
 
+
+/*
+  Start-up parameter for the maximum size of the unused MDL_lock objects cache
+  and a constant for its default value.
+*/
+extern ulong mdl_locks_cache_size;
+static const ulong MDL_LOCKS_CACHE_SIZE_DEFAULT = 1024;
+
 #endif

sql/sql_plist.h

@@ -128,6 +128,15 @@ public:
   }
   inline T* front() { return m_first; }
   inline const T *front() const { return m_first; }
+  inline T* pop_front()
+  {
+    T *result= front();
+
+    if (result)
+      remove(result);
+
+    return result;
+  }
   void swap(I_P_List<T, B, C> &rhs)
   {
     swap_variables(T *, m_first, rhs.m_first);

sql/sys_vars.cc

@@ -1156,6 +1156,12 @@ static Sys_var_ulonglong Sys_max_heap_table_size(
        VALID_RANGE(16384, (ulonglong)~(intptr)0), DEFAULT(16*1024*1024),
        BLOCK_SIZE(1024));
 
+static Sys_var_ulong Sys_metadata_locks_cache_size(
+       "metadata_locks_cache_size", "Size of unused metadata locks cache",
+       READ_ONLY GLOBAL_VAR(mdl_locks_cache_size), CMD_LINE(REQUIRED_ARG),
+       VALID_RANGE(1, 1024*1024), DEFAULT(MDL_LOCKS_CACHE_SIZE_DEFAULT),
+       BLOCK_SIZE(1));
+
 static Sys_var_ulong Sys_pseudo_thread_id(
        "pseudo_thread_id",
        "This variable is for internal server use",