Increased back_log to 150, but not more than max_connections.

- This was done to get better performance when doing a lot of connections.
Ensure that thread_cache_size is not larger than max_connections (trivial optimizations).
Fixed that the --host_cache_size=# startup option works

mysql-test/r/variables.result:
  Increase back_log to 150
sql/hostname.cc:
  Fixed that the --host_cache_size=# startup option works
sql/mysqld.cc:
  Ensure that back_log and thread_cache_size is not set higher than max_connections (as this would not make any sense).
sql/sys_vars.cc:
  Increased back_log to 150

mysql-test/r/variables.result

@@ -1137,12 +1137,12 @@ ERROR HY000: Variable 'ft_stopword_file' is a read only variable
 #
 SHOW VARIABLES like 'back_log';
 Variable_name	Value
-back_log	50
+back_log	150
 SELECT @@session.back_log;
 ERROR HY000: Variable 'back_log' is a GLOBAL variable
 SELECT @@global.back_log;
 @@global.back_log
-50
+150
 SET @@session.back_log= 7;
 ERROR HY000: Variable 'back_log' is a read only variable
 SET @@global.back_log= 7;

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

 select @@global.back_log;
 @@global.back_log
-50
+150
 select @@session.back_log;
 ERROR HY000: Variable 'back_log' is a GLOBAL variable
 show global variables like 'back_log';
 Variable_name	Value
-back_log	50
+back_log	150
 show session variables like 'back_log';
 Variable_name	Value
-back_log	50
+back_log	150
 select * from information_schema.global_variables where variable_name='back_log';
 VARIABLE_NAME	VARIABLE_VALUE
-BACK_LOG	50
+BACK_LOG	150
 select * from information_schema.session_variables where variable_name='back_log';
 VARIABLE_NAME	VARIABLE_VALUE
-BACK_LOG	50
+BACK_LOG	150
 set global back_log=1;
 ERROR HY000: Variable 'back_log' is a read only variable
 set session back_log=1;

sql/hostname.cc

@@ -149,7 +149,7 @@ bool hostname_cache_init()
   Host_entry tmp;
   uint key_offset= (uint) ((char*) (&tmp.ip_key) - (char*) &tmp);
 
-  if (!(hostname_cache= new hash_filo(HOST_CACHE_SIZE,
+  if (!(hostname_cache= new hash_filo(host_cache_size,
                                       key_offset, HOST_ENTRY_KEY_SIZE,
                                       NULL, (my_hash_free_key) free,
                                       &my_charset_bin)))

sql/mysqld.cc

@@ -8943,6 +8943,13 @@ static int get_options(int *argc_ptr, char ***argv_ptr)
         max_binlog_size_var->option.def_value;
     }
   }
+
+  /* Ensure that some variables are not set higher than needed */
+  if (back_log > max_connections)
+    back_log= max_connections;
+  if (thread_cache_size > max_connections)
+    thread_cache_size= max_connections;
+  
   return 0;
 }
 

sql/sys_vars.cc

@@ -364,7 +364,7 @@ static Sys_var_ulong Sys_back_log(
        "MySQL can have. This comes into play when the main MySQL thread "
        "gets very many connection requests in a very short time",
        READ_ONLY GLOBAL_VAR(back_log), CMD_LINE(REQUIRED_ARG),
-       VALID_RANGE(1, 65535), DEFAULT(50), BLOCK_SIZE(1));
+       VALID_RANGE(1, 65535), DEFAULT(150), BLOCK_SIZE(1));
 
 static Sys_var_charptr Sys_basedir(
        "basedir", "Path to installation directory. All paths are "