LP #1369373: minor changes to optimize innodb_kill_idle_transaction

**Reported in Launchpad by zhai weixiang last update 17-11-2016 07:28:29

If innodb_kill_idle_transaction is setting to a non-zero value , the monitor thread has to check the transaction list while holding trx_sys->mutex every one secound. This is reasonable if we set innodb_kill_idle_transaction to 1. But if innodb_kill_idle_transaction is larger than 1 , we can do some optimizations.

1. while scanning the trx list, pick out the oldest idle time ( that not need to be killed), called last_max_idle;
2.During next loop, if ++last_max_idle < last_max_idle, then we don't need to scan the trx list.

A simple patch (Not tested fully)
Index: srv/srv0srv.cc
===================================================================
— srv/srv0srv.cc (revision 7085)
+++ srv/srv0srv.cc (working copy)
@@ -2068,6 +2068,7 @@
/* the semaphore that is being waited for */
const void* sema = NULL;
const void* old_sema = NULL;
+ long long last_max_idle = 0;

ut_ad(!srv_read_only_mode);

@@ -2140,10 +2141,13 @@
old_sema = sema;
}

• if (srv_kill_idle_transaction && trx_sys) {
  + if (srv_kill_idle_transaction
  + && ((++last_max_idle) >= srv_kill_idle_transaction)
  + && trx_sys) {
  trx_t* trx;
  time_t now;
  rescan_idle:
  + last_max_idle = 0;
  now = time(NULL);
  mutex_enter(&trx_sys->mutex);
  trx = UT_LIST_GET_FIRST(trx_sys->mysql_trx_list);
  @@ -2158,12 +2162,16 @@
  if (trx->last_stmt_start != start_time) {
  trx->idle_start = now;
  trx->last_stmt_start = start_time;

• } else if (difftime(now, trx->idle_start)

• > srv_kill_idle_transaction) {

• /* kill the session */

• mutex_exit(&trx_sys->mutex);

• innobase_thd_kill(thd_id);

• goto rescan_idle;
  + } else {
  + longlong diff = difftime(now, trx->idle_start);
  + if (diff > srv_kill_idle_transaction) {
  + /* kill the session */
  + mutex_exit(&trx_sys->mutex);
  + innobase_thd_kill(thd_id);
  + goto rescan_idle;
  +
  + } else
  + last_max_idle = ut_max(last_max_idle, diff);
  }
  }
  trx = UT_LIST_GET_NEXT(mysql_trx_list, trx);