how does aiomysql know to reuse a mysql pool?

in my programe,i made a global engine by store it into a list,

however when i use the ab to test a simple mysql proxy server with : ab -n 10000 -c 1000 http://localhost
and the proxy server has 2 worker on a 2cpu machine

i got a very strange result ,cause i set the connect pool (min 10 ,maxsize 20), and run the ab first ,i check the mysqldb the connection is 2, when i run it again and again ,i increase to 40 by the end ,and never grow up

i was wondering why is this result? how can i control all my connections?