BUG#21051: RESET QUERY CACHE very slow when query_cache_type=0

There were two problems: RESET QUERY CACHE took a long time to complete
and other threads were blocked during this time.

The patch does three things:
  1 fixes a bug with improper use of test-lock-test_again technique.
      AKA Double-Checked Locking is applicable here only in few places.
  2 Somewhat improves performance of RESET QUERY CACHE.
      Do my_hash_reset() instead of deleting elements one by one.  Note
      however that the slowdown also happens when inserting into sorted
      list of free blocks, should be rewritten using balanced tree.
  3 Makes RESET QUERY CACHE non-blocking.
      The patch adjusts the locking protocol of the query cache in the
      following way: it introduces a flag flush_in_progress, which is
      set when Query_cache::flush_cache() is in progress.  This call
      sets the flag on enter, and then releases the lock.  Every other
      call is able to acquire the lock, but does nothing if
      flush_in_progress is set (as if the query cache is disabled).
      The only exception is the concurrent calls to
      Query_cache::flush_cache(), that are blocked until the flush is
      over.  When leaving Query_cache::flush_cache(), the lock is
      acquired and the flag is reset, and one thread waiting on
      Query_cache::flush_cache() (if any) is notified that it may
      proceed.
parent bf10578f
......@@ -210,7 +210,13 @@ typedef struct st_net {
char last_error[MYSQL_ERRMSG_SIZE], sqlstate[SQLSTATE_LENGTH+1];
unsigned int last_errno;
unsigned char error;
/*
'query_cache_query' should be accessed only via query cache
functions and methods to maintain proper locking.
*/
gptr query_cache_query;
my_bool report_error; /* We should report error (we have unreported error) */
my_bool return_errno;
} NET;
......
......@@ -96,8 +96,11 @@ extern uint test_flags;
extern ulong bytes_sent, bytes_received, net_big_packet_count;
extern pthread_mutex_t LOCK_bytes_sent , LOCK_bytes_received;
#ifndef MYSQL_INSTANCE_MANAGER
extern void query_cache_insert(NET *net, const char *packet, ulong length);
#ifdef HAVE_QUERY_CACHE
#define USE_QUERY_CACHE
extern void query_cache_init_query(NET *net);
extern void query_cache_insert(NET *net, const char *packet, ulong length);
#endif // HAVE_QUERY_CACHE
#define update_statistics(A) A
#endif /* MYSQL_INSTANCE_MANGER */
#endif /* defined(MYSQL_SERVER) && !defined(MYSQL_INSTANCE_MANAGER) */
......@@ -133,7 +136,11 @@ my_bool my_net_init(NET *net, Vio* vio)
net->compress=0; net->reading_or_writing=0;
net->where_b = net->remain_in_buf=0;
net->last_errno=0;
net->query_cache_query=0;
#ifdef USE_QUERY_CACHE
query_cache_init_query(net);
#else
net->query_cache_query= 0;
#endif
net->report_error= 0;
if (vio != 0) /* If real connection */
......@@ -552,10 +559,8 @@ net_real_write(NET *net,const char *packet,ulong len)
my_bool net_blocking = vio_is_blocking(net->vio);
DBUG_ENTER("net_real_write");
#if defined(MYSQL_SERVER) && defined(HAVE_QUERY_CACHE) \
&& !defined(MYSQL_INSTANCE_MANAGER)
if (net->query_cache_query != 0)
query_cache_insert(net, packet, len);
#if defined(MYSQL_SERVER) && defined(USE_QUERY_CACHE)
query_cache_insert(net, packet, len);
#endif
if (net->error == 2)
......
This diff is collapsed.
......@@ -195,7 +195,6 @@ extern "C"
byte *query_cache_table_get_key(const byte *record, uint *length,
my_bool not_used);
}
void query_cache_insert(NET *thd, const char *packet, ulong length);
extern "C" void query_cache_invalidate_by_MyISAM_filename(const char* filename);
......@@ -241,6 +240,12 @@ public:
ulong free_memory, queries_in_cache, hits, inserts, refused,
free_memory_blocks, total_blocks, lowmem_prunes;
private:
pthread_cond_t COND_flush_finished;
bool flush_in_progress;
void free_query_internal(Query_cache_block *point);
protected:
/*
The following mutex is locked when searching or changing global
......@@ -249,6 +254,12 @@ protected:
LOCK SEQUENCE (to prevent deadlocks):
1. structure_guard_mutex
2. query block (for operation inside query (query block/results))
Thread doing cache flush releases the mutex once it sets
flush_in_progress flag, so other threads may bypass the cache as
if it is disabled, not waiting for reset to finish. The exception
is other threads that were going to do cache flush---they'll wait
till the end of a flush operation.
*/
pthread_mutex_t structure_guard_mutex;
byte *cache; // cache memory
......@@ -358,6 +369,7 @@ protected:
If query is cacheable return number tables in query
(query without tables not cached)
*/
static
TABLE_COUNTER_TYPE is_cacheable(THD *thd, uint32 query_len, char *query,
LEX *lex, TABLE_LIST *tables_used,
uint8 *tables_type);
......@@ -410,6 +422,7 @@ protected:
void destroy();
friend void query_cache_init_query(NET *net);
friend void query_cache_insert(NET *net, const char *packet, ulong length);
friend void query_cache_end_of_result(THD *thd);
friend void query_cache_abort(NET *net);
......@@ -435,6 +448,8 @@ protected:
extern Query_cache query_cache;
extern TYPELIB query_cache_type_typelib;
void query_cache_init_query(NET *net);
void query_cache_insert(NET *net, const char *packet, ulong length);
void query_cache_end_of_result(THD *thd);
void query_cache_abort(NET *net);
......
......@@ -223,7 +223,7 @@ THD::THD()
#endif
client_capabilities= 0; // minimalistic client
net.last_error[0]=0; // If error on boot
net.query_cache_query=0; // If error on boot
query_cache_init_query(&net); // If error on boot
ull=0;
system_thread= cleanup_done= abort_on_warning= no_warnings_for_error= 0;
peer_port= 0; // For SHOW PROCESSLIST
......
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