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From:kpettersson Date:September 3 2007 8:47am
Subject:bk commit into 5.0 tree (thek:1.2480) BUG#21074
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Below is the list of changes that have just been committed into a local
5.0 repository of thek. When thek does a push these changes will
be propagated to the main repository and, within 24 hours after the
push, to the public repository.
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see http://dev.mysql.com/doc/mysql/en/installing-source-tree.html

ChangeSet@stripped, 2007-09-03 10:47:24+02:00, thek@adventure.(none) +1 -0
  Bug #21074 Large query_cache freezes mysql server sporadically under heavy load
  
  Invaldating a subset of a sufficiently large query cache can take a long time.
  During this time the server is efficiently frozen and no other operation can
  be executed. This patch addresses this problem by setting a time limit on
  how long time a dictionary access request can take before giving up on the 
  attempt. This patch does not work for query cache invalidations issued by
  DROP, ALTER or RENAME TABLE operations.

  sql/sql_cache.cc@stripped, 2007-09-03 10:47:23+02:00, thek@adventure.(none) +30 -1
    Changed mutex locking to a timed spinn lock. If access to query cache dictionary
    takes "a long time" (currently more than 0.1 seconds) the system fall backs on
    ordinary statement execution.

diff -Nrup a/sql/sql_cache.cc b/sql/sql_cache.cc
--- a/sql/sql_cache.cc	2007-05-10 14:54:53 +02:00
+++ b/sql/sql_cache.cc	2007-09-03 10:47:23 +02:00
@@ -1023,6 +1023,13 @@ Query_cache::send_result_to_client(THD *
   Query_cache_block_table *block_table, *block_table_end;
   ulong tot_length;
   Query_cache_query_flags flags;
+  const uint spin_treshold= 50000;
+  const double lock_time_treshold= 0.1; /* Time in seconds */
+  uint spin_count= 0;
+  int lock_status= 0;
+  ulong new_time= 0;
+  ulong stop_time= 0;
+
   DBUG_ENTER("Query_cache::send_result_to_client");
 
   /*
@@ -1069,7 +1076,29 @@ Query_cache::send_result_to_client(THD *
     }
   }
 
-  STRUCT_LOCK(&structure_guard_mutex);
+  stop_time= my_clock()+(ulong)lock_time_treshold*CLOCKS_PER_SEC;
+  while ((lock_status= pthread_mutex_trylock(&structure_guard_mutex)) == EBUSY
+         && spin_count < spin_treshold
+         && new_time < stop_time)
+  {
+    spin_count++;
+    if (spin_count%5)
+      new_time= my_clock();
+    pthread_yield();
+  }
+
+  if (lock_status != 0)
+  {
+    /*
+      Query cache is too busy doing something else.
+      Fall back on ordinary statement execution. We also mark this
+      query as unsafe to cache because otherwise this thread will
+      still be halted when the result set is stored to the cache.
+    */
+    thd->lex->safe_to_cache_query= FALSE; 
+    goto err;
+  }
+
   if (query_cache_size == 0 || flush_in_progress)
   {
     DBUG_PRINT("qcache", ("query cache disabled"));
Thread
bk commit into 5.0 tree (thek:1.2480) BUG#21074kpettersson3 Sep