MDEV-19312 Make throttling interval depend on thread_pool_stall_limit

A thread_pool_stall_limit which is smaller than default would result
in quicker creation of threads.

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

@@ -5466,7 +5466,7 @@ DEFAULT_VALUE	500
 VARIABLE_SCOPE	GLOBAL
 VARIABLE_TYPE	INT UNSIGNED
 VARIABLE_COMMENT	Maximum query execution time in milliseconds,before an executing non-yielding thread is considered stalled.If a worker thread is stalled, additional worker thread may be created to handle remaining clients.
-NUMERIC_MIN_VALUE	10
+NUMERIC_MIN_VALUE	1
 NUMERIC_MAX_VALUE	4294967295
 NUMERIC_BLOCK_SIZE	1
 ENUM_VALUE_LIST	NULL

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

@@ -37,7 +37,7 @@ Warnings:
 Warning	1292	Truncated incorrect thread_pool_stall_limit value: '-1'
 select @@global.thread_pool_stall_limit;
 @@global.thread_pool_stall_limit
-10
+1
 set global thread_pool_stall_limit=10000000000;
 Warnings:
 Warning	1292	Truncated incorrect thread_pool_stall_limit value: '10000000000'

mysql-test/suite/sys_vars/t/thread_pool_stall_limit_basic.test

 # uint global
---source include/not_windows.inc
 --source include/not_embedded.inc
 SET @start_global_value = @@global.thread_pool_stall_limit;
 

sql/sys_vars.cc

@@ -3733,7 +3733,7 @@ static Sys_var_uint Sys_threadpool_stall_limit(
  "If a worker thread is stalled, additional worker thread "
  "may be created to handle remaining clients.",
   GLOBAL_VAR(threadpool_stall_limit), CMD_LINE(REQUIRED_ARG),
-  VALID_RANGE(10, UINT_MAX), DEFAULT(500), BLOCK_SIZE(1),
+  VALID_RANGE(1, UINT_MAX), DEFAULT(DEFAULT_THREADPOOL_STALL_LIMIT), BLOCK_SIZE(1),
   NO_MUTEX_GUARD, NOT_IN_BINLOG, ON_CHECK(0), 
   ON_UPDATE(fix_threadpool_stall_limit)
 );

sql/threadpool.h

@@ -24,7 +24,7 @@ extern uint threadpool_min_threads;  /* Minimum threads in pool */
 extern uint threadpool_idle_timeout; /* Shutdown idle worker threads  after this timeout */
 extern uint threadpool_size; /* Number of parallel executing threads */
 extern uint threadpool_max_size;
-extern uint threadpool_stall_limit;  /* time interval in 10 ms units for stall checks*/
+extern uint threadpool_stall_limit;  /* time interval in milliseconds for stall checks*/
 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 */
@@ -36,6 +36,7 @@ extern uint threadpool_mode; /* Thread pool implementation , windows or generic
 #define TP_MODE_GENERIC 1
 #endif
 
+#define DEFAULT_THREADPOOL_STALL_LIMIT 500U
 
 struct TP_connection;
 extern void tp_callback(TP_connection *c);

sql/threadpool_generic.cc

@@ -28,6 +28,7 @@
 #include <time.h>
 #include <sql_plist.h>
 #include <threadpool.h>
+#include <algorithm>
 
 #ifdef HAVE_IOCP
 #define OPTIONAL_IO_POLL_READ_PARAM this
@@ -865,23 +866,24 @@ static int create_worker(thread_group_t *thread_group, bool due_to_stall)
  
  The actual values were not calculated using any scientific methods.
  They just look right, and behave well in practice.
- 
- TODO: Should throttling depend on thread_pool_stall_limit?
 */
+
+#define THROTTLING_FACTOR (threadpool_stall_limit/std::max(DEFAULT_THREADPOOL_STALL_LIMIT,threadpool_stall_limit))
+
 static ulonglong microsecond_throttling_interval(thread_group_t *thread_group)
 {
   int count= thread_group->thread_count;
   
-  if (count < 4)
+  if (count < 1+ (int)threadpool_oversubscribe)
     return 0;
-  
+
   if (count < 8)
-    return 50*1000; 
-  
+    return 50*1000*THROTTLING_FACTOR;
+
   if(count < 16)
-    return 100*1000;
-  
-  return 200*1000;
+    return 100*1000*THROTTLING_FACTOR;
+
+  return 200*100*THROTTLING_FACTOR;
 }