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.


include/mysql_com.h:
  Add comment for NET::query_cache_query.
sql/net_serv.cc:
  Use query_cache_init_query() for initialization of
  NET::query_cache_query if query cache is used.
  Do not access net->query_cache_query without a lock.
sql/sql_cache.cc:
  Fix bug with accessing query_cache_size, Query_cache_query::wri and
  thd->net.query_cache_query before acquiring the lock---leave
  double-check locking only in safe places.
  Wherever we check that cache is usable (query_cache_size > 0) we now
  also check that flush_in_progress is false, i.e. we are not in the
  middle of cache flush.
  Add Query_cache::not_in_flush_or_wait() method and use it in
  Query_cache::flush_cache(), so that threads doing cache flush will
  wait it to finish, while other threads will bypass the cache as if
  it is disabled.
  Extract Query_cache::free_query_internal() from Query_cache::free_query(),
  which does not removes elements from the hash, and use it together with
  my_hash_reset() in Query_cache::flush_cache().
sql/sql_cache.h:
  Add declarations for new members and methods.
  Make is_cacheable() a static method.
  Add query_cache_init_query() function.
sql/sql_class.cc:
  Use query_cache_init_query() for initialization of
  NET::query_cache_query.

1 files changed, 11 insertions, 6 deletions

diff --git a/sql/net_serv.cc b/sql/net_serv.cc
index cf9dc6e3f60..e54d3abd29d 100644
--- a/sql/net_serv.cc
+++ b/sql/net_serv.cc
@@ -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)