Auth Cache for checkpassword

Hello,

I use the checkpassword driver for imap/pop authentication. To improve the performance I would like to enable authentication caching for it. However just adding the auth_cache_size and auth_cache_ttl doesn't work. No entries are cached. If I add a cache directive in the passdb, then the service doesnt start. I was wondering if the authentication caching is supported for the checkpassword driver. Please point me in the right direction.

Thanks in advance, Amol