From: Kristofer Pettersson
Date: November 13 2009 7:17pm
Subject: bzr commit into mysql-5.1-bugteam branch (kristofer.pettersson:3192)
 Bug#21074 Bug#39253
List-Archive: http://lists.mysql.com/commits/90392
X-Bug: 21074,39253
Message-Id: <0KT2006WXAXRXY20@fe-emea-10.sun.com>
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#At file:///Users/thek/Development/mysql-5.1-qc/ based on revid:kristofer.pettersson@stripped

 3192 Kristofer Pettersson	2009-11-13
      Bug#39253 Large query cache still freezes server after fix for bug #21074
      
      When the query cache is fragmented, the size of
      the free block lists in the memory bins grow which
      causes the query cache invalidation to become slow.
      
      Even if this is countered by defragmentation the
      hash_delete() operation still creates a bottle necks
      which can cause the server to hang for a short period
      of time depending on the size of the cache.
      
      This patch introduce a limit on the time the qc can
      block with a lock on SELECTs.
      
      Other operations which causes a change in the table
      data will still be blocked.
     @ sql/sql_cache.cc
        * Introduced a timeout value for the qc lock when entering send_result_to_client() and store_query() methods.
     @ sql/sql_cache.h
        * New signature for Query_cache::try_lock()

    modified:
      sql/sql_cache.cc
      sql/sql_cache.h
=== modified file 'sql/sql_cache.cc'
--- a/sql/sql_cache.cc	2009-10-16 10:29:42 +0000
+++ b/sql/sql_cache.cc	2009-11-13 19:17:40 +0000
@@ -421,16 +421,20 @@ TYPELIB query_cache_type_typelib=
   effect by another thread. This enables a quick path in execution to skip waits
   when the outcome is known.
 
+  The upper limit for time waited is set to one second.
+
   @return
    @retval FALSE An exclusive lock was taken
    @retval TRUE The locking attempt failed
 */
 
-bool Query_cache::try_lock(void)
+bool Query_cache::try_lock(bool use_timeout)
 {
   bool interrupt= FALSE;
   DBUG_ENTER("Query_cache::try_lock");
-
+  struct timespec waittime;
+  set_timespec_nsec(waittime,100000000L);  /* Wait for 0.1 sec */
+  
   pthread_mutex_lock(&structure_guard_mutex);
   while (1)
   {
@@ -456,7 +460,24 @@ bool Query_cache::try_lock(void)
     else
     {
       DBUG_ASSERT(m_cache_lock_status == Query_cache::LOCKED);
-      pthread_cond_wait(&COND_cache_status_changed, &structure_guard_mutex);
+      /*
+        To prevent send_result_to_client() and query_cache_insert() from
+        blocking execution for too long a timeout is put on the lock.
+      */
+      if (use_timeout)
+      {
+        int res= pthread_cond_timedwait(&COND_cache_status_changed,
+                                        &structure_guard_mutex,&waittime);
+        if (res == ETIMEDOUT)
+        {
+          interrupt= TRUE;
+          break;
+        }
+      }
+      else
+      {
+        pthread_cond_wait(&COND_cache_status_changed, &structure_guard_mutex);
+      }
     }
   }
   pthread_mutex_unlock(&structure_guard_mutex);
@@ -523,6 +544,7 @@ void Query_cache::lock(void)
 }
 
 
+
 /**
   Set the query cache to UNLOCKED and signal waiting threads.
 */
@@ -1190,8 +1212,14 @@ def_week_frmt: %lu, in_trans: %d, autoco
       A table- or a full flush operation can potentially take a long time to
       finish. We choose not to wait for them and skip caching statements
       instead.
+
+      In case the wait time can't be determined there is an upper limit which
+      causes try_lock() to abort with a time out.
+
+      The 'TRUE' parameter indicate that the lock is allowed to timeout
+
     */
-    if (try_lock())
+    if (try_lock(TRUE))
       DBUG_VOID_RETURN;
     if (query_cache_size == 0)
     {
@@ -1385,8 +1413,10 @@ Query_cache::send_result_to_client(THD *
     Try to obtain an exclusive lock on the query cache. If the cache is
     disabled or if a full cache flush is in progress, the attempt to
     get the lock is aborted.
+
+    The 'TRUE' parameter indicate that the lock is allowed to timeout
   */
-  if (try_lock())
+  if (try_lock(TRUE))
     goto err;
 
   if (query_cache_size == 0)

=== modified file 'sql/sql_cache.h'
--- a/sql/sql_cache.h	2009-06-16 08:34:47 +0000
+++ b/sql/sql_cache.h	2009-11-13 19:17:40 +0000
@@ -485,7 +485,7 @@ protected:
 			const char *name);
   my_bool in_blocks(Query_cache_block * point);
 
-  bool try_lock(void);
+  bool try_lock(bool use_timeout= FALSE);
   void lock(void);
   void lock_and_suspend(void);
   void unlock(void);


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