#At file:///home/thek/Development/cpp/mysqlbzr/mysql-5.1-bug38551/
2666 Kristofer Pettersson 2008-10-16
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
the query cache permanently and avoid taking the query cache mutex.
modified:
sql/mysqld.cc
sql/sql_cache.cc
sql/sql_cache.h
per-file messages:
sql/sql_cache.cc
* Added checks to see if query cache is disabled to avoid taking the
structure_guard_mutex.
* Shortened execution path while invalidating if the query cache size is 0.
sql/sql_cache.h
* Introduced new member and method for checking if the cache is disabled
=== modified file 'sql/mysqld.cc'
--- a/sql/mysqld.cc 2008-10-13 10:22:36 +0000
+++ b/sql/mysqld.cc 2008-10-16 20:11:04 +0000
@@ -3458,6 +3458,19 @@ You should consider changing lower_case_
files_charset_info :
&my_charset_bin);
+
+ /*
+ 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)
+ {
+ have_query_cache= SHOW_OPTION_NO;
+ query_cache.disable_query_cache();
+ }
+
return 0;
}
@@ -6691,12 +6704,10 @@ The minimum value for this variable is 4
(uchar**) &query_cache_min_res_unit, (uchar**) &query_cache_min_res_unit,
0, GET_ULONG, REQUIRED_ARG, QUERY_CACHE_MIN_RESULT_DATA_SIZE,
0, ULONG_MAX, 0, 1, 0},
-#endif /*HAVE_QUERY_CACHE*/
{"query_cache_size", OPT_QUERY_CACHE_SIZE,
"The memory allocated to store results from old queries.",
(uchar**) &query_cache_size, (uchar**) &query_cache_size, 0, GET_ULONG,
REQUIRED_ARG, 0, 0, (longlong) ULONG_MAX, 0, 1024, 0},
-#ifdef HAVE_QUERY_CACHE
{"query_cache_type", OPT_QUERY_CACHE_TYPE,
"0 = OFF = Don't cache or retrieve results. 1 = ON = Cache all results except SELECT SQL_NO_CACHE ... queries. 2 = DEMAND = Cache only SELECT SQL_CACHE ... queries.",
(uchar**) &global_system_variables.query_cache_type,
=== modified file 'sql/sql_cache.cc'
--- a/sql/sql_cache.cc 2008-09-30 13:47:01 +0000
+++ b/sql/sql_cache.cc 2008-10-16 20:11:04 +0000
@@ -670,7 +670,7 @@ void query_cache_insert(NET *net, const
DBUG_ENTER("query_cache_insert");
/* See the comment on double-check locking usage above. */
- if (net->query_cache_query == 0)
+ if (query_cache.is_disabled() || net->query_cache_query == 0)
DBUG_VOID_RETURN;
DBUG_EXECUTE_IF("wait_in_query_cache_insert",
@@ -778,7 +778,7 @@ void query_cache_end_of_result(THD *thd)
DBUG_ENTER("query_cache_end_of_result");
/* See the comment on double-check locking usage above. */
- if (thd->net.query_cache_query == 0)
+ if (query_cache.is_disabled() || thd->net.query_cache_query == 0)
DBUG_VOID_RETURN;
if (thd->killed)
@@ -891,7 +891,7 @@ Query_cache::Query_cache(ulong query_cac
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)) +
@@ -978,7 +978,7 @@ void Query_cache::store_query(THD *thd,
See also a note on double-check locking usage above.
*/
- if (thd->locked_tables || query_cache_size == 0)
+ if (m_query_cache_is_disabled || thd->locked_tables || query_cache_size == 0)
DBUG_VOID_RETURN;
uint8 tables_type= 0;
@@ -1161,14 +1161,18 @@ end:
Check if the query is in the cache. If it was cached, send it
to the user.
- RESULTS
- 1 Query was not cached.
- 0 The query was cached and user was sent the result.
- -1 The query was cached but we didn't have rights to use it.
- No error is sent to the client yet.
+ @param thd Pointer to the thread handler
+ @param sql A pointer to the sql statement *
+ @param query_length Length of the statement in characters
+
+ @return status code
+ @retval 1 Query was not cached.
+ @retval 0 The query was cached and user was sent the result.
+ @retval -1 The query was cached but we didn't have rights to use it.
+
+ In case of -1, no error is sent to the client.
- NOTE
- This method requires that sql points to allocated memory of size:
+ *) The buffer must be allocated memory of size:
tot_length= query_length + thd->db_length + 1 + QUERY_CACHE_FLAGS_SIZE;
*/
@@ -1183,6 +1187,9 @@ Query_cache::send_result_to_client(THD *
Query_cache_query_flags flags;
DBUG_ENTER("Query_cache::send_result_to_client");
+ if (m_query_cache_is_disabled)
+ DBUG_RETURN(0);
+
/*
Testing 'query_cache_size' without a lock here is safe: the thing
we may loose is that the query won't be served from cache, but we
@@ -2529,7 +2536,17 @@ void Query_cache::invalidate_table(THD *
void Query_cache::invalidate_table(THD *thd, uchar * key, uint32 key_length)
{
bool interrupt;
+
+ if (m_query_cache_is_disabled)
+ return;
+
STRUCT_LOCK(&structure_guard_mutex);
+ if (query_cache_size == 0)
+ {
+ STRUCT_UNLOCK(&structure_guard_mutex);
+ return;
+ }
+
wait_while_table_flush_is_in_progress(&interrupt);
if (interrupt)
{
=== modified file 'sql/sql_cache.h'
--- a/sql/sql_cache.h 2008-07-24 13:41:55 +0000
+++ b/sql/sql_cache.h 2008-10-16 20:11:04 +0000
@@ -279,6 +279,8 @@ private:
Cache_status m_cache_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);
@@ -437,6 +439,14 @@ protected:
/* register query in cache */
void store_query(THD *thd, TABLE_LIST *used_tables);
+ /**
+ At startup the user has an option to disable the query cache
+ to avoid locking the structure_guard_mutex.
+ This option is enabled by explicitly setting query_cache_type=OFF
+ in the command line.
+ */
+ void disable_query_cache(void) { m_query_cache_is_disabled= TRUE; }
+
/*
Check if the query is in the cache and if this is true send the
data to client.
@@ -469,6 +479,8 @@ protected:
friend void query_cache_end_of_result(THD *thd);
friend void query_cache_abort(NET *net);
+ bool is_disabled(void) { return m_query_cache_is_disabled; }
+
bool is_flushing(void)
{
return (m_cache_status != Query_cache::NO_FLUSH_IN_PROGRESS);
