Add some variables to the generic threadpool, that could help to analyze

stalls etc better.

- thread_pool_exact_stats -  uses high precision timestamp for
the time when connection was added to the queue. This timestamp helps
calculating queuing time shown in I_S.THREADPOOL_QUEUES entries.

- If thread_pool_dedicated_listener is on, then each group will have its
own dedicated listener, that does not convert to worker.
With this variable on, the queueing time in I_S.THREADPOOL_QUEUES , and
actual queue size in I_S.THREADPOOOL_GROUPS will be more exact, since
IO request are immediately dequeued from poll, without delay.


Part of MDEV-19313.

mysql-test/main/mysqld--help.result

@@ -1320,6 +1320,11 @@ The following specify which files/extra groups are read (specified before remain
  --thread-cache-size=# 
  How many threads we should keep in a cache for reuse.
  These are freed after 5 minutes of idle time
+ --thread-pool-dedicated-listener 
+ If set to 1,listener thread will not pick up queries
+ --thread-pool-exact-stats 
+ If set to 1, provides better statistics in
+ information_schema threadpool tables
  --thread-pool-idle-timeout=# 
  Timeout in seconds for an idle thread in the thread
  pool.Worker thread will be shut down after timeout
@@ -1744,6 +1749,8 @@ tcp-keepalive-probes 0
 tcp-keepalive-time 0
 tcp-nodelay TRUE
 thread-cache-size 151
+thread-pool-dedicated-listener FALSE
+thread-pool-exact-stats FALSE
 thread-pool-idle-timeout 60
 thread-pool-max-threads 65536
 thread-pool-oversubscribe 3

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

@@ -5346,6 +5346,34 @@ NUMERIC_BLOCK_SIZE	NULL
 ENUM_VALUE_LIST	one-thread-per-connection,no-threads,pool-of-threads
 READ_ONLY	YES
 COMMAND_LINE_ARGUMENT	REQUIRED
+VARIABLE_NAME	THREAD_POOL_DEDICATED_LISTENER
+SESSION_VALUE	NULL
+GLOBAL_VALUE	OFF
+GLOBAL_VALUE_ORIGIN	COMPILE-TIME
+DEFAULT_VALUE	OFF
+VARIABLE_SCOPE	GLOBAL
+VARIABLE_TYPE	BOOLEAN
+VARIABLE_COMMENT	If set to 1,listener thread will not pick up queries
+NUMERIC_MIN_VALUE	NULL
+NUMERIC_MAX_VALUE	NULL
+NUMERIC_BLOCK_SIZE	NULL
+ENUM_VALUE_LIST	OFF,ON
+READ_ONLY	NO
+COMMAND_LINE_ARGUMENT	OPTIONAL
+VARIABLE_NAME	THREAD_POOL_EXACT_STATS
+SESSION_VALUE	NULL
+GLOBAL_VALUE	OFF
+GLOBAL_VALUE_ORIGIN	COMPILE-TIME
+DEFAULT_VALUE	OFF
+VARIABLE_SCOPE	GLOBAL
+VARIABLE_TYPE	BOOLEAN
+VARIABLE_COMMENT	If set to 1, provides better statistics in information_schema threadpool tables
+NUMERIC_MIN_VALUE	NULL
+NUMERIC_MAX_VALUE	NULL
+NUMERIC_BLOCK_SIZE	NULL
+ENUM_VALUE_LIST	OFF,ON
+READ_ONLY	NO
+COMMAND_LINE_ARGUMENT	OPTIONAL
 VARIABLE_NAME	THREAD_POOL_IDLE_TIMEOUT
 SESSION_VALUE	NULL
 GLOBAL_VALUE	60

sql/sys_vars.cc

@@ -3753,6 +3753,20 @@ static Sys_var_uint Sys_threadpool_threadpool_prio_kickup_timer(
   GLOBAL_VAR(threadpool_prio_kickup_timer), CMD_LINE(REQUIRED_ARG),
   VALID_RANGE(0, UINT_MAX), DEFAULT(1000), BLOCK_SIZE(1)
 );
+
+static Sys_var_mybool Sys_threadpool_exact_stats(
+  "thread_pool_exact_stats",
+  "If set to 1, provides better statistics in information_schema threadpool tables",
+  GLOBAL_VAR(threadpool_exact_stats), CMD_LINE(OPT_ARG), DEFAULT(FALSE),
+    NO_MUTEX_GUARD, NOT_IN_BINLOG
+);
+
+static Sys_var_mybool Sys_threadpool_dedicated_listener(
+  "thread_pool_dedicated_listener",
+  "If set to 1,listener thread will not pick up queries",
+  GLOBAL_VAR(threadpool_dedicated_listener), CMD_LINE(OPT_ARG), DEFAULT(FALSE),
+  NO_MUTEX_GUARD, NOT_IN_BINLOG
+);
 #endif /* HAVE_POOL_OF_THREADS */
 
 /**

sql/threadpool.h

@@ -28,6 +28,8 @@ extern uint threadpool_stall_limit;  /* time interval in 10 ms units for stall c
 extern uint threadpool_max_threads;  /* Maximum threads in pool */
 extern uint threadpool_oversubscribe;  /* Maximum active threads in group */
 extern uint threadpool_prio_kickup_timer;  /* Time before low prio item gets prio boost */
+extern my_bool threadpool_exact_stats; /* Better queueing time stats for information_schema, at small performance cost */
+extern my_bool threadpool_dedicated_listener; /* Listener thread does not pick up work items. */
 #ifdef _WIN32
 extern uint threadpool_mode; /* Thread pool implementation , windows or generic */
 #define TP_MODE_WINDOWS 0

sql/threadpool_common.cc

@@ -36,6 +36,8 @@ uint threadpool_max_threads;
 uint threadpool_oversubscribe;
 uint threadpool_mode;
 uint threadpool_prio_kickup_timer;
+my_bool threadpool_exact_stats;
+my_bool threadpool_dedicated_listener;
 
 /* Stats */
 TP_STATISTICS tp_stats;

sql/threadpool_generic.cc

@@ -744,7 +744,7 @@ static TP_connection_generic * listener(worker_thread_t *current_thread,
      more workers.
     */
     
-    bool listener_picks_event=is_queue_empty(thread_group);
+    bool listener_picks_event=is_queue_empty(thread_group) && !threadpool_dedicated_listener;
     queue_put(thread_group, ev, cnt);
     if (listener_picks_event)
     {
@@ -1068,7 +1068,7 @@ static void queue_put(thread_group_t *thread_group, TP_connection_generic *conne
 {
   DBUG_ENTER("queue_put");
 
-  connection->dequeue_time= pool_timer.current_microtime;
+  connection->enqueue_time= threadpool_exact_stats?microsecond_interval_timer():pool_timer.current_microtime;
   thread_group->queues[connection->priority].push_back(connection);
 
   if (thread_group->active_thread_count == 0)