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)

1 files changed, 40 insertions, 8 deletions

diff --git a/sql/sql_cache.cc b/sql/sql_cache.cc
index 1141f7009a7..6f550f92987 100644
--- a/sql/sql_cache.cc
+++ b/sql/sql_cache.cc
@@ -286,6 +286,7 @@ functions:
          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 @@ err2:
 #endif /* DBUG_OFF */
 
 #endif /*HAVE_QUERY_CACHE*/
+