MDEV-21751 innodb_fast_shutdown=0 can be unnecessarily slow

max out parallel purge worker tasks, on slow shutdown, to speedup

storage/innobase/include/srv0srv.h

@@ -757,6 +757,16 @@ srv_que_task_enqueue_low(
 /*=====================*/
 	que_thr_t*	thr);	/*!< in: query thread */
 
+/**
+Flag which is set, whenever innodb_purge_threads changes.
+It is read and reset in srv_do_purge().
+
+Thus it is Atomic_counter<int>, not bool, since unprotected
+reads are used. We just need an atomic with relaxed memory
+order, to please Thread Sanitizer.
+*/
+extern Atomic_counter<int> srv_purge_thread_count_changed;
+
 #ifdef UNIV_DEBUG
 /** @return whether purge or master task is active */
 bool srv_any_background_activity();
@@ -796,7 +806,7 @@ ulint srv_get_task_queue_length();
 void srv_purge_shutdown();
 
 /** Init purge tasks*/
-void srv_init_purge_tasks(uint n_max);
+void srv_init_purge_tasks();
 
 #ifdef UNIV_DEBUG
 /** Disables master thread. It's used by:

storage/innobase/include/trx0types.h

@@ -48,6 +48,8 @@ static const ulint TRX_SYS_SPACE = 0;
 /** Random value to check for corruption of trx_t */
 static const ulint TRX_MAGIC_N = 91118598;
 
+constexpr uint innodb_purge_threads_MAX= 32;
+
 /** Transaction execution states when trx->state == TRX_STATE_ACTIVE */
 enum trx_que_t {
 	TRX_QUE_RUNNING,		/*!< transaction is running */

storage/innobase/srv/srv0srv.cc

@@ -72,6 +72,7 @@ Created 10/8/1995 Heikki Tuuri
 #include "fil0fil.h"
 #include "fil0crypt.h"
 #include "fil0pagecompress.h"
+#include "trx0types.h"
 
 
 #include <my_service_manager.h>
@@ -2060,6 +2061,15 @@ static bool srv_task_execute()
 	return false;
 }
 
+std::mutex purge_thread_count_mtx;
+void srv_update_purge_thread_count(uint n)
+{
+	std::lock_guard<std::mutex> lk(purge_thread_count_mtx);
+	srv_n_purge_threads = n;
+	srv_purge_thread_count_changed = 1;
+}
+
+Atomic_counter<int> srv_purge_thread_count_changed;
 
 /** Do the actual purge operation.
 @param[in,out]	n_total_purged	total number of purged pages
@@ -2072,7 +2082,7 @@ static uint32_t srv_do_purge(ulint* n_total_purged)
 	static ulint	n_use_threads = 0;
 	static uint32_t	rseg_history_len = 0;
 	ulint		old_activity_count = srv_get_activity_count();
-	const ulint	n_threads = srv_n_purge_threads;
+	static ulint	n_threads = srv_n_purge_threads;
 
 	ut_a(n_threads > 0);
 	ut_ad(!srv_read_only_mode);
@@ -2088,7 +2098,20 @@ static uint32_t srv_do_purge(ulint* n_total_purged)
 	}
 
 	do {
-		if (trx_sys.rseg_history_len > rseg_history_len
+		if (UNIV_UNLIKELY(srv_purge_thread_count_changed)) {
+			/* Read the fresh value of srv_n_purge_threads, reset
+			the changed flag. Both variables are protected by
+			purge_thread_count_mtx.
+
+			This code does not run concurrently, it is executed
+			by a single purge_coordinator thread, and no races
+			involving srv_purge_thread_count_changed are possible.
+			*/
+
+			std::lock_guard<std::mutex> lk(purge_thread_count_mtx);
+			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)) {
 
@@ -2136,23 +2159,17 @@ static uint32_t srv_do_purge(ulint* n_total_purged)
 
 static std::queue<THD*> purge_thds;
 static std::mutex purge_thd_mutex;
-
-static void purge_create_background_thds(int n)
-{
-  THD *thd= current_thd;
-  std::unique_lock<std::mutex> lk(purge_thd_mutex);
-  while (n--)
-    purge_thds.push(innobase_create_background_thd("InnoDB purge worker"));
-  set_current_thd(thd);
-}
-
 extern void* thd_attach_thd(THD*);
 extern void thd_detach_thd(void *);
 
 THD* acquire_thd(void **ctx)
 {
 	std::unique_lock<std::mutex> lk(purge_thd_mutex);
-	ut_a(!purge_thds.empty());
+	if (purge_thds.empty()) {
+		THD* thd = current_thd;
+		purge_thds.push(innobase_create_background_thd("InnoDB purge worker"));
+		set_current_thd(thd);
+	}
 	THD* thd = purge_thds.front();
 	purge_thds.pop();
 	lk.unlock();
@@ -2251,10 +2268,8 @@ static void purge_coordinator_callback(void*)
   purge_state.m_running= 0;
 }
 
-void srv_init_purge_tasks(uint n_tasks)
+void srv_init_purge_tasks()
 {
-  purge_task_group.set_max_tasks(n_tasks - 1);
-  purge_create_background_thds(n_tasks);
   purge_coordinator_timer= srv_thread_pool->create_timer
     (purge_coordinator_timer_callback, nullptr);
 }
@@ -2310,6 +2325,7 @@ ulint srv_get_task_queue_length()
 void srv_purge_shutdown()
 {
 	if (purge_sys.enabled()) {
+		srv_update_purge_thread_count(innodb_purge_threads_MAX);
 		while(!srv_purge_should_exit()) {
 			ut_a(!purge_sys.paused());
 			srv_wake_purge_thread_if_not_active();

storage/innobase/srv/srv0start.cc

@@ -1962,8 +1962,7 @@ dberr_t srv_start(bool create_new_db)
 
 	if (!srv_read_only_mode && srv_operation == SRV_OPERATION_NORMAL
 	    && srv_force_recovery < SRV_FORCE_NO_BACKGROUND) {
-
-		srv_init_purge_tasks(srv_n_purge_threads);
+		srv_init_purge_tasks();
 		purge_sys.coordinator_startup();
 		srv_wake_purge_thread_if_not_active();
 		srv_start_state_set(SRV_START_STATE_PURGE);

storage/innobase/trx/trx0purge.cc

@@ -149,7 +149,7 @@ purge_graph_build()
 		NULL, NULL, QUE_FORK_PURGE, heap);
 	fork->trx = trx;
 
-	for (ulint i = 0; i < srv_n_purge_threads; ++i) {
+	for (auto i = innodb_purge_threads_MAX; i; i--) {
 		que_thr_t*	thr = que_thr_create(fork, heap, NULL);
 		thr->child = new(mem_heap_alloc(heap, sizeof(purge_node_t)))
 			purge_node_t(thr);