very slow queries sometimes with alias deref always and scope sub

["COMBES Julien - CETE Lyon/DI/ET/PAMELA" <julien.combes@i-carre.net>]

Hello,

I have a strange problem on all of my openldap servers (master and slaves).

In general, requests with alias always dereferenced and scope sub on an 
attribute indexed are working correctly and answers come  in a few 
milliseconds.

But sometimes , during some hours, on all my openldap servers, the same 
queries are taking a few seconds to be returned (between 10 to 45s). 
During this time all queries with alias never dereferenced are working 
correctly.

For the openldap master, which is only used to write entries and for 
slurpd replication, the load average is between 0.2 and 0.5 during the 
problem.
For others servers that have more requests, the load average is growing up.

I don't find the reasons which cause the problem.
All is coming back to normal performance after the night.

I use debian etch openldap package wich provide openldap 2.3.30. My 
directory has around 150000 entries. I use bdb database. I have somes 
tuning on DB_CONFIG that I could give, if you think that it could have 
an interest.

For exemple :
-> During problem :
  - with alias deref *never*
$ time ldapsearch -x -LLL -H ldap://master.my.domain -b 
"dc=my,dc=domain" -s sub -a never uid=jean.talus.i
<ldap answer>
real    0m0.007s
user    0m0.004s
sys     0m0.000s

  - with alias deref *always*
$ time ldapsearch -x -LLL -H ldap://master.my.domain -b 
"dc=my,dc=domain" -s sub -a always uid=jean.talus.i
<ldap answer>
real    0m47.225s
user    0m0.004s
sys     0m0.000s

$ time ldapsearch -x -LLL -H ldap://slave1.my.domain -b 
"dc=my,dc=domain" -s sub -a always uid=jean.talus.i
Time limit exceeded (3)
real    0m16.148s
user    0m0.000s
sys     0m0.004s

and so on on all my openldap slaves servers.

-> after the night, when it's working correctly again :
$ time ldapsearch -x -LLL -H ldap://master.my.domain -b 
"dc=my,dc=domain" -s sub -a always uid=jean.talus.i
<ldap answer>
real    0m0.016s
user    0m0.000s
sys     0m0.004s

$ time ldapsearch -x -LLL -H ldap://slave1.my.domain -b 
"dc=my,dc=domain" -s sub -a always uid=jean.talus.i
<ldap answer>
real    0m0.020s
user    0m0.000s
sys     0m0.004s

I don't find anything that help me on internet.
Do you know how can I correct this or what can it be the cause ?
Is there some docs that could help me to tune my openldap servers to 
correct this ?

Regards,
Julien