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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: Updated test after adding --metadata-locks-cache-size parameter. mysql-test/r/mysqld--help-win.result: Updated test after adding --metadata-locks-cache-size parameter. mysql-test/suite/sys_vars/r/metadata_locks_cache_size_basic.result: Added test coverage for newly introduced --metadata_locks_cache_size start-up parameter and corresponding global read-only variable. mysql-test/suite/sys_vars/t/metadata_locks_cache_size_basic-master.opt: Added test coverage for newly introduced --metadata_locks_cache_size start-up parameter and corresponding global read-only variable. mysql-test/suite/sys_vars/t/metadata_locks_cache_size_basic.test: Added test coverage for newly introduced --metadata_locks_cache_size start-up parameter and corresponding global read-only variable. sql/mdl.cc: Introduced caching of unused MDL_object_lock objects, in order to avoid costs associated with constant creation and destruction of such objects in single-connection workloads run in autocommit mode. Such costs can be pretty high on systems where creation and destruction of synchronization primitives require a system call (e.g. Windows XP). To implement this cache,a list of unused MDL_object_lock instances was added to MDL_map object. Instead of being destroyed MDL_object_lock instances are put into this list and re-used later when creation of a new instance is required. Also added MDL_lock::m_version counter to allow threads having outstanding references to an MDL_object_lock instance to notice that it has been moved to the unused objects list. Added a global variable for a start-up parameter that limits the size of the unused objects list. Note that we don't cache MDL_scoped_lock objects since they are supposed to be created only during execution of DDL statements and therefore should not affect performance much. sql/mdl.h: Added a global variable for start-up parameter that limits the size of the unused MDL_object_lock objects list and constant for its default value. sql/sql_plist.h: Added I_P_List<>::pop_front() function. sql/sys_vars.cc: Introduced --metadata-locks-cache-size start-up parameter for specifying size of the cache of unused MDL_object_lock objects. |
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