MDEV-31382 SET GLOBAL innodb_undo_log_truncate=ON has no effect on logically empty undo logs

innodb_undo_log_truncate_update(): A callback function. If SET GLOBAL innodb_undo_log_truncate=ON, invoke srv_wake_purge_thread_if_not_active(). srv_wake_purge_thread_if_not_active(): If innodb_undo_log_truncate=ON, always wake up the purge subsystem. srv_do_purge(): If the history is empty, invoke trx_purge_truncate_history() in order to free undo log pages. trx_purge_truncate_history(): If head.trx_no==0, consider the cached undo logs to be free. trx_purge(): Remove the parameter "bool truncate" and let the caller invoke trx_purge_truncate_history() directly. Reviewed by: Vladislav Lesin

Author: dr-m

storage/innobase/handler/ha_innodb.cc

@@ -19684,10 +19684,17 @@ static MYSQL_SYSVAR_ULONG(purge_rseg_truncate_frequency,
  " purge rollback segment(s) on every Nth iteration of purge invocation",
  NULL, NULL, 128, 1, 128, 0);
 
+static void innodb_undo_log_truncate_update(THD *thd, struct st_mysql_sys_var*,
+ void*, const void *save)
+{
+ if ((srv_undo_log_truncate= *static_cast<const my_bool*>(save)))
+ srv_wake_purge_thread_if_not_active();
+}
+
 static MYSQL_SYSVAR_BOOL(undo_log_truncate, srv_undo_log_truncate,
  PLUGIN_VAR_OPCMDARG,
  "Enable or Disable Truncate of UNDO tablespace.",
- NULL, NULL, FALSE);
+ NULL, innodb_undo_log_truncate_update, FALSE);
 
 static MYSQL_SYSVAR_LONG(autoinc_lock_mode, innobase_autoinc_lock_mode,
  PLUGIN_VAR_RQCMDARG | PLUGIN_VAR_READONLY,

storage/innobase/include/trx0purge.h

@@ -43,12 +43,19 @@ Remove the undo log segment from the rseg slot if it is too big for reuse.
 @param[in,out]	mtr	mini-transaction */
 void
 trx_purge_add_undo_to_history(const trx_t* trx, trx_undo_t*& undo, mtr_t* mtr);
+
+/**
+Remove unnecessary history data from rollback segments. NOTE that when this
+function is called, the caller (purge_coordinator_callback)
+must not have any latches on undo log pages!
+*/
+void trx_purge_truncate_history();
+
 /**
 Run a purge batch.
 @param n_tasks number of purge tasks to submit to the queue
-@param truncate whether to truncate the history at the end of the batch
 @return number of undo log pages handled in the batch */
-ulint trx_purge(ulint n_tasks, bool truncate);
+ulint trx_purge(ulint n_tasks);
 
 /** Rollback segements from a given transaction with trx-no
 scheduled for purge. */

storage/innobase/srv/srv0srv.cc

@@ -1346,17 +1346,14 @@ static tpool::waitable_task purge_coordinator_task
 static tpool::timer *purge_coordinator_timer;
 
 /** Wake up the purge threads if there is work to do. */
-void
-srv_wake_purge_thread_if_not_active()
+void srv_wake_purge_thread_if_not_active()
 {
-ut_ad(!srv_read_only_mode);
+ ut_ad(!srv_read_only_mode);
 
-if (purge_sys.enabled() && !purge_sys.paused()
- && trx_sys.rseg_history_len) {
-if(++purge_state.m_running == 1) {
-srv_thread_pool->submit_task(&purge_coordinator_task);
-}
-}
+ if (purge_sys.enabled() && !purge_sys.paused() &&
+ (srv_undo_log_truncate || trx_sys.rseg_history_len) &&
+ ++purge_state.m_running == 1)
+ srv_thread_pool->submit_task(&purge_coordinator_task);
 }
 
 /** @return whether the purge tasks are active */
@@ -1811,8 +1808,8 @@ static size_t srv_do_purge(ulint* n_total_purged)
 n_threads = n_use_threads = srv_n_purge_threads;
 srv_purge_thread_count_changed = 0;
 } else if (trx_sys.rseg_history_len > rseg_history_len
-  || (srv_max_purge_lag > 0
-&& rseg_history_len > srv_max_purge_lag)) {
+ || (srv_max_purge_lag > 0
+ && rseg_history_len > srv_max_purge_lag)) {
 
 /* History length is now longer than what it was
 when we took the last snapshot. Use more threads. */
@@ -1838,15 +1835,19 @@ static size_t srv_do_purge(ulint* n_total_purged)
 
 /* Take a snapshot of the history list before purge. */
 if (!(rseg_history_len = trx_sys.rseg_history_len)) {
-break;
+n_pages_purged = 0;
+goto truncate;
 }
 
-n_pages_purged = trx_purge(
-n_use_threads,
-!(++count % srv_purge_rseg_truncate_frequency)
-|| purge_sys.truncate.current
-|| (srv_shutdown_state != SRV_SHUTDOWN_NONE
- && srv_fast_shutdown == 0));
+n_pages_purged = trx_purge(n_use_threads);
+
+if (!(++count % srv_purge_rseg_truncate_frequency)
+ || purge_sys.truncate.current
+ || (srv_shutdown_state != SRV_SHUTDOWN_NONE
+&& srv_fast_shutdown == 0)) {
+truncate:
+trx_purge_truncate_history();
+}
 
 *n_total_purged += n_pages_purged;
 } while (n_pages_purged > 0 && !purge_sys.paused()

storage/innobase/trx/trx0purge.cc

@@ -522,10 +522,11 @@ __attribute__((optimize(0)))
 # endif
 #endif
 /**
-Removes unnecessary history data from rollback segments. NOTE that when this
-function is called, the caller must not have any latches on undo log pages!
+Remove unnecessary history data from rollback segments. NOTE that when this
+function is called, the caller (purge_coordinator_callback)
+must not have any latches on undo log pages!
 */
-static void trx_purge_truncate_history()
+void trx_purge_truncate_history()
 {
  ut_ad(purge_sys.head <= purge_sys.tail);
  purge_sys_t::iterator &head= purge_sys.head.trx_no
@@ -618,7 +619,7 @@ static void trx_purge_truncate_history()
  for (const trx_undo_t *undo= UT_LIST_GET_FIRST(rseg->undo_cached); undo;
  undo= UT_LIST_GET_NEXT(undo_list, undo))
  {
- if (head.trx_no < undo->trx_id)
+ if (head.trx_no && head.trx_no < undo->trx_id)
  goto not_free;
  else
  cached+= undo->size;
@@ -731,7 +732,7 @@ static void trx_purge_truncate_history()
  ut_ad(rseg->id == i);
  ut_ad(rseg->is_persistent());
  ut_ad(!rseg->trx_ref_count);
- ut_ad(rseg->needs_purge <= head.trx_no);
+ ut_ad(!head.trx_no || rseg->needs_purge <= head.trx_no);
  ut_d(const auto old_page= rseg->page_no);
 
  buf_block_t *rblock= trx_rseg_header_create(&space, i,
@@ -1235,9 +1236,8 @@ static void trx_purge_wait_for_workers_to_complete()
 /**
 Run a purge batch.
 @param n_tasks number of purge tasks to submit to the queue
-@param truncate whether to truncate the history at the end of the batch
 @return number of undo log pages handled in the batch */
-ulint trx_purge(ulint n_tasks, bool truncate)
+ulint trx_purge(ulint n_tasks)
 {
 que_thr_t*	thr = NULL;
 ulint	n_pages_handled;
@@ -1271,10 +1271,6 @@ ulint trx_purge(ulint n_tasks, bool truncate)
 
 trx_purge_wait_for_workers_to_complete();
 
-if (truncate) {
-trx_purge_truncate_history();
-}
-
 MONITOR_INC_VALUE(MONITOR_PURGE_INVOKED, 1);
 MONITOR_INC_VALUE(MONITOR_PURGE_N_PAGE_HANDLED, n_pages_handled);