When you use memcached,the tokens will be saved in memcahced instead of database.And tokens need'nt write to database when using memcached.

Memcache in services
You want to configure memcache in  the services.  Specifically the authtoken section of your service refers to keystone middleware.  There you want to configure memcache so that keystone middleware makes use of memcache instead of talking to keystone for every request.

http://docs.openstack.org/developer/python-keystoneclient/middlewarearchitecture.html

Use memcache to cache the token in keystone server
  In keystone.conf there is cache settings.  enable global cache and then enable token cache and then configure the cache backend for dogpile. Dogphile cache backend can be  inmemory , memcache or mongo db.  BTW this is from icehouse release