clarify --thread-pool-mode usage

mysql-test/main/mysqld--help,win.rdiff

@@ -1,4 +1,6 @@
-@@ -180,6 +180,7 @@
+--- a/mysql-test/main/mysqld--help.result
++++ b/mysql-test/main/mysqld--help.result
+@@ -180,6 +180,7 @@ The following specify which files/extra groups are read (specified before remain
   --console           Write error output on screen; don't remove the console
   window on windows.
   --core-file         Write core on crashes
@@ -6,7 +8,7 @@
   -h, --datadir=name  Path to the database root directory
   --date-format=name  The DATE format (ignored)
   --datetime-format=name 
-@@ -650,6 +651,7 @@
+@@ -649,6 +650,7 @@ The following specify which files/extra groups are read (specified before remain
   Use MySQL-5.6 (instead of MariaDB-5.3) format for TIME,
   DATETIME, TIMESTAMP columns.
   (Defaults to on; use --skip-mysql56-temporal-format to disable.)
@@ -14,7 +16,7 @@
   --net-buffer-length=# 
   Buffer length for TCP/IP and socket communication
   --net-read-timeout=# 
-@@ -1281,6 +1283,10 @@
+@@ -1280,6 +1282,10 @@ The following specify which files/extra groups are read (specified before remain
   Log slow queries to given log file. Defaults logging to
   'hostname'-slow.log. Must be enabled to activate other
   slow log options
@@ -25,7 +27,7 @@
   --socket=name       Socket file to use for connection
   --sort-buffer-size=# 
   Each thread that needs to do a sort allocates a buffer of
-@@ -1305,6 +1311,7 @@
+@@ -1304,6 +1310,7 @@ The following specify which files/extra groups are read (specified before remain
   deleting or updating every row in a table.
   --stack-trace       Print a symbolic stack trace on failure
   (Defaults to on; use --skip-stack-trace to disable.)
@@ -33,19 +35,20 @@
   --standard-compliant-cte 
   Allow only CTEs compliant to SQL standard
   (Defaults to on; use --skip-standard-compliant-cte to disable.)
-@@ -1380,6 +1387,11 @@
+@@ -1379,6 +1386,12 @@ The following specify which files/extra groups are read (specified before remain
   --thread-pool-max-threads=# 
   Maximum allowed number of worker threads in the thread
   pool
 + --thread-pool-min-threads=# 
 + Minimum number of threads in the thread pool.
 + --thread-pool-mode=name 
-+ Chose implementation of the threadpool. One of: windows, 
-+ generic
++ Chose implementation of the threadpool. Use 'windows'
++ unless you have a workload with a lot of concurrent
++ connections and minimal contention
   --thread-pool-oversubscribe=# 
   How many additional active worker threads in a group are
   allowed.
-@@ -1418,8 +1430,8 @@
+@@ -1417,8 +1430,8 @@ The following specify which files/extra groups are read (specified before remain
   automatically convert it to an on-disk MyISAM or Aria
   table.
   -t, --tmpdir=name   Path for temporary files. Several paths may be specified,
@@ -56,7 +59,7 @@
   --transaction-alloc-block-size=# 
   Allocation block size for transactions to be stored in
   binary log
-@@ -1634,6 +1646,7 @@
+@@ -1633,6 +1646,7 @@ myisam-sort-buffer-size 134216704
  myisam-stats-method NULLS_UNEQUAL
  myisam-use-mmap FALSE
  mysql56-temporal-format TRUE
@@ -64,7 +67,7 @@
  net-buffer-length 16384
  net-read-timeout 30
  net-retry-count 10
-@@ -1788,6 +1801,7 @@
+@@ -1787,6 +1801,7 @@ slave-transaction-retry-interval 0
  slave-type-conversions 
  slow-launch-time 2
  slow-query-log FALSE
@@ -72,7 +75,7 @@
  sort-buffer-size 2097152
  sql-mode STRICT_TRANS_TABLES,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION
  sql-safe-updates FALSE
-@@ -1814,6 +1828,8 @@
+@@ -1813,6 +1828,8 @@ thread-pool-dedicated-listener FALSE
  thread-pool-exact-stats FALSE
  thread-pool-idle-timeout 60
  thread-pool-max-threads 65536

sql/sys_vars.cc

@@ -3947,7 +3947,8 @@ static Sys_var_on_access_global<Sys_var_enum,
                                 PRIV_SET_SYSTEM_GLOBAL_VAR_THREAD_POOL>
 Sys_threadpool_mode(
   "thread_pool_mode",
-  "Chose implementation of the threadpool",
+  "Chose implementation of the threadpool. Use 'windows' unless you have a "
+  "workload with a lot of concurrent connections and minimal contention",
   READ_ONLY GLOBAL_VAR(threadpool_mode), CMD_LINE(REQUIRED_ARG),
   threadpool_mode_names, DEFAULT(TP_MODE_WINDOWS)
   );