How do you scale dovecot for good performance with Roundcube webmailer in front? (hitting limits without exhausting resources)

[Marc]

> 
> Generally speaking the setup performs OK, I wrote a JMeter Roundcube script to
> test the Roundcube. This includes actions like listing mails and fetching
> them. I am hitting a somehow dubious limit of 600 req/s though. (This includes
> all kinds of HTTP calls to Roundcube, not only the ones triggering IMAP). When
> the dovecot becomes unavailable, the performance without mails jumps to 7700
> req/s. Testing is done with 4 JMeter-servers and one client.
> 

Should you not be testing also dovecot performance directly? So you can see what is it's maximum. I can't imagine http interfaces are the bottle neck.