Increase value of thread_cache_size to 32

Added 5 minute timeout before automaticlally removing threads from thread
cache.

This solves a problem with jemalloc, which is slow with a small
thread cache and also makes thread_cache big enough that most users
doesn't have to touch it

mysql-test/r/mysqld--help.result

@@ -1068,7 +1068,8 @@ The following options may be given as the first argument:
  Decision to use in heuristic recover process. One of: 
  COMMIT, ROLLBACK
  --thread-cache-size=# 
- How many threads we should keep in a cache for reuse
+ How many threads we should keep in a cache for reuse.
+ These are freed after 5 minutes of idle time
  --thread-pool-idle-timeout=# 
  Timeout in seconds for an idle thread in the thread
  pool.Worker thread will be shut down after timeout
@@ -1426,7 +1427,7 @@ table-cache 431
 table-definition-cache 400
 table-open-cache 431
 tc-heuristic-recover COMMIT
-thread-cache-size 0
+thread-cache-size 151
 thread-pool-idle-timeout 60
 thread-pool-max-threads 1000
 thread-pool-oversubscribe 3

mysql-test/suite/sys_vars/r/thread_cache_size_basic.result

 SET @start_global_value = @@global.thread_cache_size;
 SELECT @start_global_value;
 @start_global_value
-0
+256
 select @@global.thread_cache_size;
 @@global.thread_cache_size
-0
+256
 select @@session.thread_cache_size;
 ERROR HY000: Variable 'thread_cache_size' is a GLOBAL variable
 show global variables like 'thread_cache_size';
 Variable_name	Value
-thread_cache_size	0
+thread_cache_size	256
 show session variables like 'thread_cache_size';
 Variable_name	Value
-thread_cache_size	0
+thread_cache_size	256
 select * from information_schema.global_variables where variable_name='thread_cache_size';
 VARIABLE_NAME	VARIABLE_VALUE
-THREAD_CACHE_SIZE	0
+THREAD_CACHE_SIZE	256
 select * from information_schema.session_variables where variable_name='thread_cache_size';
 VARIABLE_NAME	VARIABLE_VALUE
-THREAD_CACHE_SIZE	0
+THREAD_CACHE_SIZE	256
 set global thread_cache_size=1;
 select @@global.thread_cache_size;
 @@global.thread_cache_size
@@ -51,4 +51,4 @@ select @@global.thread_cache_size;
 SET @@global.thread_cache_size = @start_global_value;
 SELECT @@global.thread_cache_size;
 @@global.thread_cache_size
-0
+256

mysql-test/suite/sys_vars/t/thread_cache_size_basic-master.opt

+--max-connections=1024

sql/mysqld.cc

@@ -2995,6 +2995,7 @@ void unlink_thd(THD *thd)
 
 static bool cache_thread()
 {
+  struct timespec abstime;
   DBUG_ENTER("cache_thread");
 
   mysql_mutex_lock(&LOCK_thread_cache);
@@ -3013,8 +3014,21 @@ static bool cache_thread()
     PSI_THREAD_CALL(delete_current_thread)();
 #endif
 
+    set_timespec(abstime, THREAD_CACHE_TIMEOUT);
     while (!abort_loop && ! wake_thread && ! kill_cached_threads)
-      mysql_cond_wait(&COND_thread_cache, &LOCK_thread_cache);
+    {
+      int error= mysql_cond_timedwait(&COND_thread_cache, &LOCK_thread_cache,
+                                       &abstime);
+      if (error == ETIMEDOUT || error == ETIME)
+      {
+        /*
+          If timeout, end thread.
+          If a new thread is requested (wake_thread is set), we will handle
+          the call, even if we got a timeout (as we are already awake and free)
+        */
+        break;
+      }
+    }
     cached_thread_count--;
     if (kill_cached_threads)
       mysql_cond_signal(&COND_flush_thread_cache);

sql/sql_const.h

@@ -235,6 +235,8 @@
   that does not respond to "initial server greeting" timely
 */
 #define CONNECT_TIMEOUT		10
+ /* Wait 5 minutes before removing thread from thread cache */
+#define THREAD_CACHE_TIMEOUT	5*60
 
 /* The following can also be changed from the command line */
 #define DEFAULT_CONCURRENCY	10

sql/sys_vars.cc

@@ -3194,9 +3194,9 @@ static Sys_var_ulong Sys_table_cache_size(
 
 static Sys_var_ulong Sys_thread_cache_size(
        "thread_cache_size",
-       "How many threads we should keep in a cache for reuse",
+       "How many threads we should keep in a cache for reuse. These are freed after 5 minutes of idle time",
        GLOBAL_VAR(thread_cache_size), CMD_LINE(REQUIRED_ARG),
-       VALID_RANGE(0, 16384), DEFAULT(0), BLOCK_SIZE(1));
+       VALID_RANGE(0, 16384), DEFAULT(256), BLOCK_SIZE(1));
 
 #ifdef HAVE_POOL_OF_THREADS
 static bool fix_tp_max_threads(sys_var *, THD *, enum_var_type)