If you do not flush old tokens from your database, it will be as big as Godzilla. You need to clear the tokens from the keystone database time to time. In Havana version, there is a new CLI command to flush the keystone tokens:

I suggest to put it in your crontab file for daily run. Add this line to /etc/crontab file in your Controller node which keystone server is installed:

This command is going to run everyday at 03:00 and clear your DB for old tokens.