[Users] Performance troubleshooting

[Dimo]

Hi,
I am having performance issues with openser running on Fedora. Here is
the scenario:
I have a openser which is sending calls to two asterisks, doing radius
accounting and local syslog accounting, the database for user location
is a local MySQL and the asterisks are load-balanced through
dispatcher.
i have modules:
loadmodule "/usr/local/lib/openser/modules/mysql.so"
loadmodule "/usr/local/lib/openser/modules/sl.so"
loadmodule "/usr/local/lib/openser/modules/tm.so"
loadmodule "/usr/local/lib/openser/modules/rr.so"
loadmodule "/usr/local/lib/openser/modules/maxfwd.so"
loadmodule "/usr/local/lib/openser/modules/usrloc.so"
loadmodule "/usr/local/lib/openser/modules/registrar.so"
loadmodule "/usr/local/lib/openser/modules/nathelper.so"
loadmodule "/usr/local/lib/openser/modules/textops.so"
loadmodule "/usr/local/lib/openser/modules/xlog.so"
loadmodule "/usr/local/lib/openser/modules/avpops.so"
loadmodule "/usr/local/lib/openser/modules/avp_radius.so"
loadmodule "/usr/local/lib/openser/modules/acc.so"
loadmodule "/usr/local/lib/openser/modules/options.so"
loadmodule "/usr/local/lib/openser/modules/uri_db.so"
loadmodule "/usr/local/lib/openser/modules/auth.so"
loadmodule "/usr/local/lib/openser/modules/auth_db.so"
loadmodule "/usr/local/lib/openser/modules/auth_radius.so"
loadmodule "/usr/local/lib/openser/modules/dispatcher.so"

Now the problem is that if i use sipp to send INVITE with 10-20 call
attempts per seccond at one time ser just timeouts without answering
for 1-2 seconds and sipp has to retransmit the packets. The timeout
which sipp waits is 500ms so it should be enough.
I have something like on 500 call attempts i get 700 retransmits.

I will start stripping the config to troubleshoot but i was wondering
if you can give me some idea what the problem might be or some good
advice on troubleshooting, where should i look, and if you have any
bes practices to recommend.

BR,
Dimitar