Bug#38551 query cache can still consume [very little] cpu time even when it is off.

      
When the query cache is disabled, the server shouldn't attempt to take the 
query cache mutex.
                             
By using the command line option --query_cache_type=0, the user can disable
   
(backport from mysql-pe)

Author: Kristofer Pettersson

mysql-test/t/query_cache_disabled-master.opt

@@ -0,0 +1 @@
+--query_cache_type=0

mysql-test/t/query_cache_disabled.test

@@ -0,0 +1,15 @@
+-- source include/have_query_cache.inc
+#
+# Bug#38551 query cache can still consume [very little] cpu time even when it is off.
+#
+SHOW GLOBAL VARIABLES LIKE 'query_cache_type';
+--error ER_QUERY_CACHE_DISABLED
+SET GLOBAL query_cache_type=ON;
+--error ER_QUERY_CACHE_DISABLED
+SET GLOBAL query_cache_type=DEMAND;
+--error ER_QUERY_CACHE_DISABLED
+SET GLOBAL query_cache_type=OFF;
+SET GLOBAL query_cache_size=1024*1024;
+SHOW GLOBAL VARIABLES LIKE 'query_cache_size';
+SET GLOBAL query_cache_size=0;
+

sql/set_var.cc

@@ -125,8 +125,10 @@ static void fix_net_read_timeout(THD *thd, enum_var_type type);
 static void fix_net_write_timeout(THD *thd, enum_var_type type);
 static void fix_net_retry_count(THD *thd, enum_var_type type);
 static void fix_max_join_size(THD *thd, enum_var_type type);
+#ifdef HAVE_QUERY_CACHE
 static void fix_query_cache_size(THD *thd, enum_var_type type);
 static void fix_query_cache_min_res_unit(THD *thd, enum_var_type type);
+#endif
 static void fix_myisam_max_sort_file_size(THD *thd, enum_var_type type);
 static void fix_max_binlog_size(THD *thd, enum_var_type type);
 static void fix_max_relay_log_size(THD *thd, enum_var_type type);
@@ -493,9 +495,6 @@ static sys_var_thd_ulong	sys_div_precincrement(&vars, "div_precision_increment",
                                               &SV::div_precincrement);
 static sys_var_long_ptr	sys_rpl_recovery_rank(&vars, "rpl_recovery_rank",
 					      &rpl_recovery_rank);
-static sys_var_long_ptr	sys_query_cache_size(&vars, "query_cache_size",
-					     &query_cache_size,
-					     fix_query_cache_size);
 
 static sys_var_thd_ulong	sys_range_alloc_block_size(&vars, "range_alloc_block_size",
 						   &SV::range_alloc_block_size);
@@ -557,14 +556,20 @@ sys_var_enum_const      sys_thread_handling(&vars, "thread_handling",
                                             NULL);
 
 #ifdef HAVE_QUERY_CACHE
+static sys_var_long_ptr	sys_query_cache_size(&vars, "query_cache_size",
+                                             &query_cache_size,
+                                             fix_query_cache_size);
 static sys_var_long_ptr	sys_query_cache_limit(&vars, "query_cache_limit",
-					      &query_cache.query_cache_limit);
-static sys_var_long_ptr        sys_query_cache_min_res_unit(&vars, "query_cache_min_res_unit",
-						     &query_cache_min_res_unit,
-						     fix_query_cache_min_res_unit);
+                                              &query_cache.query_cache_limit);
+static sys_var_long_ptr
+  sys_query_cache_min_res_unit(&vars, "query_cache_min_res_unit",
+                               &query_cache_min_res_unit,
+                               fix_query_cache_min_res_unit);
+static int check_query_cache_type(THD *thd, set_var *var);
 static sys_var_thd_enum	sys_query_cache_type(&vars, "query_cache_type",
 					     &SV::query_cache_type,
-					     &query_cache_type_typelib);
+					     &query_cache_type_typelib, NULL,
+                                             check_query_cache_type);
 static sys_var_thd_bool
 sys_query_cache_wlock_invalidate(&vars, "query_cache_wlock_invalidate",
 				 &SV::query_cache_wlock_invalidate);
@@ -1131,10 +1136,9 @@ static void fix_net_retry_count(THD *thd __attribute__((unused)),
 {}
 #endif /* HAVE_REPLICATION */
 
-
+#ifdef HAVE_QUERY_CACHE
 static void fix_query_cache_size(THD *thd, enum_var_type type)
 {
-#ifdef HAVE_QUERY_CACHE
   ulong new_cache_size= query_cache.resize(query_cache_size);
 
   /*
@@ -1148,11 +1152,35 @@ static void fix_query_cache_size(THD *thd, enum_var_type type)
 			query_cache_size, new_cache_size);
 
   query_cache_size= new_cache_size;
-#endif
 }
 
 
-#ifdef HAVE_QUERY_CACHE
+/**
+  Trigger before query_cache_type variable is updated.
+  @param thd Thread handler
+  @param var Pointer to the new variable status
+
+  @return Status code
+   @retval 1 Failure
+   @retval 0 Success
+*/
+
+static int check_query_cache_type(THD *thd, set_var *var)
+{
+  /*
+    Don't allow changes of the query_cache_type if the query cache
+    is disabled.
+  */
+  if (query_cache.is_disabled())
+  {
+    my_error(ER_QUERY_CACHE_DISABLED,MYF(0));
+    return 1;
+  }
+
+  return 0;
+}
+
+
 static void fix_query_cache_min_res_unit(THD *thd, enum_var_type type)
 {
   query_cache_min_res_unit= 
@@ -3601,6 +3629,16 @@ bool not_all_support_one_shot(List<set_var_base> *var_list)
   Functions to handle SET mysql_internal_variable=const_expr
 *****************************************************************************/
 
+/**
+  Verify that the supplied value is correct.
+
+  @param thd Thread handler
+
+  @return status code
+    @retval -1 Failure
+    @retval 0 Success
+*/
+
 int set_var::check(THD *thd)
 {
   if (var->is_readonly())

sql/set_var.h

@@ -543,10 +543,16 @@ class sys_var_thd_enum :public sys_var_thd
   { chain_sys_var(chain); }
   bool check(THD *thd, set_var *var)
   {
-    int ret= 0;
-    if (check_func)
-      ret= (*check_func)(thd, var);
-    return ret ? ret : check_enum(thd, var, enum_names);
+    /*
+      check_enum fails if the character representation supplied was wrong
+      or that the integer value was wrong or missing.
+    */
+    if (check_enum(thd, var, enum_names))
+      return TRUE;
+    else if ((check_func && (*check_func)(thd, var)))
+      return TRUE;
+    else
+      return FALSE;
   }
   bool update(THD *thd, set_var *var);
   void set_default(THD *thd, enum_var_type type);

sql/share/errmsg-utf8.txt

@@ -6241,3 +6241,6 @@ ER_UNKNOWN_LOCALE
 ER_SLAVE_IGNORE_SERVER_IDS
   eng "The requested server id %d clashes with the slave startup option --replicate-same-server-id"
 
+ER_QUERY_CACHE_DISABLED
+  eng "Query cache is disabled; restart the server with query_cache_type=1 to enable it"
+

sql/sql_cache.cc

@@ -286,6 +286,7 @@ The query cache interfaces with the rest of the server code through 7
          if (and only if) this query has a registered result set writer
          (thd->net.query_cache_query).
  4. Query_cache::invalidate
+    Query_cache::invalidate_locked_for_write
        - Called from various places to invalidate query cache based on data-
          base, table and myisam file name. During an on going invalidation
          the query cache is temporarily disabled.
@@ -851,7 +852,7 @@ Query_cache::insert(Query_cache_tls *query_cache_tls,
   DBUG_ENTER("Query_cache::insert");
 
   /* See the comment on double-check locking usage above. */
-  if (query_cache_tls->first_query_block == NULL)
+  if (is_disabled() || query_cache_tls->first_query_block == NULL)
     DBUG_VOID_RETURN;
 
   DBUG_EXECUTE_IF("wait_in_query_cache_insert",
@@ -913,7 +914,7 @@ Query_cache::abort(Query_cache_tls *query_cache_tls)
   THD *thd= current_thd;
 
   /* See the comment on double-check locking usage above. */
-  if (query_cache_tls->first_query_block == NULL)
+  if (is_disabled() || query_cache_tls->first_query_block == NULL)
     DBUG_VOID_RETURN;
 
   if (try_lock())
@@ -1055,7 +1056,7 @@ Query_cache::Query_cache(ulong query_cache_limit_arg,
    min_result_data_size(ALIGN_SIZE(min_result_data_size_arg)),
    def_query_hash_size(ALIGN_SIZE(def_query_hash_size_arg)),
    def_table_hash_size(ALIGN_SIZE(def_table_hash_size_arg)),
-   initialized(0)
+   initialized(0), m_query_cache_is_disabled(FALSE)
 {
   ulong min_needed= (ALIGN_SIZE(sizeof(Query_cache_block)) +
 		     ALIGN_SIZE(sizeof(Query_cache_block_table)) +
@@ -1362,8 +1363,8 @@ Query_cache::send_result_to_client(THD *thd, char *sql, uint query_length)
 
     See also a note on double-check locking usage above.
   */
-  if (thd->locked_tables || thd->variables.query_cache_type == 0 ||
-      query_cache_size == 0)
+  if (is_disabled() || thd->locked_tables ||
+      thd->variables.query_cache_type == 0 || query_cache_size == 0)
     goto err;
 
   if (!thd->lex->safe_to_cache_query)
@@ -1669,6 +1670,8 @@ void Query_cache::invalidate(THD *thd, TABLE_LIST *tables_used,
 			     my_bool using_transactions)
 {
   DBUG_ENTER("Query_cache::invalidate (table list)");
+  if (is_disabled())
+    DBUG_VOID_RETURN;
 
   using_transactions= using_transactions &&
     (thd->options & (OPTION_NOT_AUTOCOMMIT | OPTION_BEGIN));
@@ -1699,6 +1702,9 @@ void Query_cache::invalidate(THD *thd, TABLE_LIST *tables_used,
 void Query_cache::invalidate(CHANGED_TABLE_LIST *tables_used)
 {
   DBUG_ENTER("Query_cache::invalidate (changed table list)");
+  if (is_disabled())
+    DBUG_VOID_RETURN;
+
   THD *thd= current_thd;
   for (; tables_used; tables_used= tables_used->next)
   {
@@ -1724,8 +1730,11 @@ void Query_cache::invalidate(CHANGED_TABLE_LIST *tables_used)
 */
 void Query_cache::invalidate_locked_for_write(TABLE_LIST *tables_used)
 {
-  THD *thd= current_thd;
   DBUG_ENTER("Query_cache::invalidate_locked_for_write");
+  if (is_disabled())
+    DBUG_VOID_RETURN;
+
+  THD *thd= current_thd;
   for (; tables_used; tables_used= tables_used->next_local)
   {
     thd_proc_info(thd, "invalidating query cache entries (table)");
@@ -1746,7 +1755,9 @@ void Query_cache::invalidate(THD *thd, TABLE *table,
 			     my_bool using_transactions)
 {
   DBUG_ENTER("Query_cache::invalidate (table)");
-  
+  if (is_disabled())
+    DBUG_VOID_RETURN;
+
   using_transactions= using_transactions &&
     (thd->options & (OPTION_NOT_AUTOCOMMIT | OPTION_BEGIN));
   if (using_transactions && 
@@ -1763,6 +1774,8 @@ void Query_cache::invalidate(THD *thd, const char *key, uint32  key_length,
 			     my_bool using_transactions)
 {
   DBUG_ENTER("Query_cache::invalidate (key)");
+  if (is_disabled())
+   DBUG_VOID_RETURN;
 
   using_transactions= using_transactions &&
     (thd->options & (OPTION_NOT_AUTOCOMMIT | OPTION_BEGIN));
@@ -1781,9 +1794,12 @@ void Query_cache::invalidate(THD *thd, const char *key, uint32  key_length,
 
 void Query_cache::invalidate(char *db)
 {
-  bool restart= FALSE;
+  
   DBUG_ENTER("Query_cache::invalidate (db)");
+  if (is_disabled())
+    DBUG_VOID_RETURN;
 
+  bool restart= FALSE;
   /*
     Lock the query cache and queue all invalidation attempts to avoid
     the risk of a race between invalidation, cache inserts and flushes.
@@ -1868,6 +1884,9 @@ void Query_cache::invalidate_by_MyISAM_filename(const char *filename)
 void Query_cache::flush()
 {
   DBUG_ENTER("Query_cache::flush");
+  if (is_disabled())
+    DBUG_VOID_RETURN;
+
   DBUG_EXECUTE_IF("wait_in_query_cache_flush1",
                   debug_wait_for_kill("wait_in_query_cache_flush1"););
 
@@ -1899,6 +1918,9 @@ void Query_cache::pack(ulong join_limit, uint iteration_limit)
 {
   DBUG_ENTER("Query_cache::pack");
 
+  if (is_disabled())
+    DBUG_VOID_RETURN;
+
   /*
     If the entire qc is being invalidated we can bail out early
     instead of waiting for the lock.
@@ -1956,6 +1978,15 @@ void Query_cache::init()
   pthread_cond_init(&COND_cache_status_changed, NULL);
   m_cache_lock_status= Query_cache::UNLOCKED;
   initialized = 1;
+  /*
+    If we explicitly turn off query cache from the command line query cache will
+    be disabled for the reminder of the server life time. This is because we
+    want to avoid locking the QC specific mutex if query cache isn't going to
+    be used.
+  */
+  if (global_system_variables.query_cache_type == 0)
+    query_cache.disable_query_cache();
+
   DBUG_VOID_RETURN;
 }
 
@@ -4660,3 +4691,4 @@ my_bool Query_cache::in_table_list(Query_cache_block_table * root,
 #endif /* DBUG_OFF */
 
 #endif /*HAVE_QUERY_CACHE*/
+

sql/sql_cache.h

@@ -281,8 +281,11 @@ class Query_cache
   enum Cache_lock_status { UNLOCKED, LOCKED_NO_WAIT, LOCKED };
   Cache_lock_status m_cache_lock_status;
 
+  bool m_query_cache_is_disabled;
+
   void free_query_internal(Query_cache_block *point);
   void invalidate_table_internal(THD *thd, uchar *key, uint32 key_length);
+  void disable_query_cache(void) { m_query_cache_is_disabled= TRUE; }
 
 protected:
   /*
@@ -426,6 +429,8 @@ class Query_cache
 	      uint def_query_hash_size = QUERY_CACHE_DEF_QUERY_HASH_SIZE,
 	      uint def_table_hash_size = QUERY_CACHE_DEF_TABLE_HASH_SIZE);
 
+  bool is_disabled(void) { return m_query_cache_is_disabled; }
+
   /* initialize cache (mutex) */
   void init();
   /* resize query cache (return real query size, 0 if disabled) */