InnoDB: Disable throttling against thread thrashing by default.

sql/ha_innodb.cc:
  Remove unused variable innobase_thread_concurrency.
  Add UNIV_LIKELY hints to srv_thread_concurrency tests
  and lower the limit from 500 to 20.
sql/ha_innodb.h:
  Remove unused variable innobase_thread_concurrency.
sql/mysqld.cc:
  Make innodb_thread_concurrency=20 by default (it was 8).

sql/ha_innodb.cc

@@ -98,8 +98,7 @@ long innobase_mirrored_log_groups, innobase_log_files_in_group,
      innobase_buffer_pool_awe_mem_mb,
      innobase_buffer_pool_size, innobase_additional_mem_pool_size,
      innobase_file_io_threads, innobase_lock_wait_timeout,
-     innobase_thread_concurrency, innobase_force_recovery,
-     innobase_open_files;
+     innobase_force_recovery, innobase_open_files;
 
 /* The default values for the following char* start-up parameters
 are determined in innobase_init below: */
@@ -279,7 +278,7 @@ innodb_srv_conc_enter_innodb(
 /*=========================*/
 	trx_t*	trx)	/* in: transaction handle */
 {
-	if (srv_thread_concurrency >= 500) {
+	if (UNIV_LIKELY(srv_thread_concurrency >= 20)) {
 
 		return;
 	}
@@ -296,7 +295,7 @@ innodb_srv_conc_exit_innodb(
 /*========================*/
 	trx_t*	trx)	/* in: transaction handle */
 {
-	if (srv_thread_concurrency >= 500) {
+	if (UNIV_LIKELY(srv_thread_concurrency >= 20)) {
 
 		return;
 	}

sql/ha_innodb.h

@@ -219,7 +219,7 @@ extern long innobase_log_file_size, innobase_log_buffer_size;
 extern long innobase_buffer_pool_size, innobase_additional_mem_pool_size;
 extern long innobase_buffer_pool_awe_mem_mb;
 extern long innobase_file_io_threads, innobase_lock_wait_timeout;
-extern long innobase_force_recovery, innobase_thread_concurrency;
+extern long innobase_force_recovery;
 extern long innobase_open_files;
 extern char *innobase_data_home_dir, *innobase_data_file_path;
 extern char *innobase_log_group_home_dir, *innobase_log_arch_dir;

sql/mysqld.cc

@@ -5170,7 +5170,7 @@ log and this option does nothing anymore.",
   {"innodb_thread_concurrency", OPT_INNODB_THREAD_CONCURRENCY,
    "Helps in performance tuning in heavily concurrent environments.",
    (gptr*) &srv_thread_concurrency, (gptr*) &srv_thread_concurrency,
-   0, GET_LONG, REQUIRED_ARG, 8, 1, 1000, 0, 1, 0},
+   0, GET_LONG, REQUIRED_ARG, 20, 1, 1000, 0, 1, 0},
   {"innodb_thread_sleep_delay", OPT_INNODB_THREAD_SLEEP_DELAY,
    "Time of innodb thread sleeping before joining InnoDB queue (usec). Value 0"
     " disable a sleep",