pgbouncer issue

Guys can anyone please explain or point me to a link where i can understand this output for pgbouncer. What does each column of this table mean?

pgbouncer=# show mem;

     name     | size | used | free | memtotal

--------------+------+------+------+----------

 user_cache   |  184 |   12 |   77 |    16376

 db_cache     |  160 |    2 |  100 |    16320

 pool_cache   |  408 |    4 |   46 |    20400

 server_cache |  360 |  121 |  279 |   144000

 client_cache |  360 | 1309 |  291 |   576000

 iobuf_cache  | 2064 |    3 |  797 |  1651200

Thanks

Prabhjot