[Dovecot] Intermitent ldap auth problems benchmarking dovecot

[Timo Sirainen]

On Tue, 2011-04-12 at 09:15 +0000, Maria Arrea wrote:
> Hello
> 
>  We are using SLAMD (Distributed Load Generation Engine, www.slamd.com) to benchmark our dovecot server (ldap auth). We are simulating 2.000 simultaneous logins and 20% of them fail. We saw the following errors in the log:
> 
> 
> Apr 12 09:40:07 buzon dovecot: auth: Error: ldap(correo,192.168.4.153): Request queue is full (oldest added 1 secs ago)
>  Apr 12 09:40:07 buzon dovecot: auth: Error: ldap(correo,192.168.4.153): Request queue is full (oldest added 1 secs ago)
>  Apr 12 09:40:07 buzon dovecot: auth: Error: ldap(correo,192.168.4.153): Request queue is full (oldest added 1 secs ago)

Hmm. This is a hard coded limit. I didn't really think people wanted >1k
logins per second.. In src/auth/db-ldap.h:

/* Maximum number of requests in queue. After this new requests are
dropped. */
#define DB_LDAP_MAX_QUEUE_SIZE 1024

Maybe that limit should be dynamic. Like only >30sec old requests
actually get dropped when the limit gets over 1k.

>  We increased auth_worker_max_count from 350 to 3500 (10x increase). Now we see the following errors (still 20% of logins fail):

LDAP doesn't use auth workers, so that change shouldn't have affected
anything.