Hi,

I am doing load testing with of Kamailio. I have sipp (version 3.3) as the load generator,Kamailio(4.2.7) as Load Balancer and Freeswitch (1.6.5) as Gateways.

I am using the following command to generate load 
./sipp  -sf  uac.xml -d 2000 -m 4000 -l 1000 -r 200 -trace_stat -i SIPP_IP -p 15060 --trace_err -trace_error_codes  -trace_calldebug -trace_screen KAM_IP

When the rate of generation is below 80 (-r 80), Kamailio behaves fine.But if we go beyond 80, Kamailio starts sending "500 Service Unavailable"

2016-01-12      11:18:15.375103 1452577695.375103: Aborting call on unexpected message for Call-Id '96-32493@SIPP_IP': while expecting '180' (index 2), received 'SIP/2.0 500 Service Unavailable^M
Via: SIP/2.0/UDP SIPP_IP:15060;branch=z9hG4bK-32493-96-0^M
From: sipp <sip:sipp@SIPP_IP:15060>;tag=96^M
To: sut <sip:service@KAM_IP:5060>;tag=55f576f507e822fa6633cf4bc22740e6-6660^M
Call-ID: 96-32493@SIPP_IP^M
CSeq: 1 INVITE^M
Server: kamailio (4.2.7 (x86_64/linux))^M
Content-Length: 0^M

With the above command, sipp output is 

----------------------------- Statistics Screen ------- [1-9]: Change Screen --
  Start Time             | 2016-01-12 11:26:07.723764 1452578167.723764
  Last Reset Time        | 2016-01-12 11:27:16.280901 1452578236.280901
  Current Time           | 2016-01-12 11:27:16.282737 1452578236.282737
-------------------------+---------------------------+--------------------------
  Counter Name           | Periodic value            | Cumulative value
-------------------------+---------------------------+--------------------------
  Elapsed Time           | 00:00:00:001000           | 00:01:08:558000
  Call Rate              |    0.000 cps              |   58.345 cps
-------------------------+---------------------------+--------------------------
  Incoming call created  |        0                  |        0
  OutGoing call created  |        0                  |     4000
  Total Call created     |                           |     4000
  Current Call           |        0                  |
-------------------------+---------------------------+--------------------------
  Successful call        |        0                  |     1192
  Failed call            |        0                  |     2808
-------------------------+---------------------------+--------------------------
  Response Time 1        | 00:00:00:000000           | 00:00:15:102000
  Call Length            | 00:00:00:000000           | 00:00:10:614000
------------------------------ Test Terminated --------------------------------

Kamailio is run with command /usr/local/sbin/kamailio -f /usr/local/etc/kamailio/kamailio.cfg -P /var/run/kamailio/kamailio.pid -m 512 -M 8 -u kamailio -g kamailio

version: kamailio 4.2.7 (x86_64/linux) 727746
flags: STATS: Off, USE_TCP, USE_TLS, USE_SCTP, TLS_HOOKS, USE_RAW_SOCKS, DISABLE_NAGLE, USE_MCAST, DNS_IP_HACK, SHM_MEM, SHM_MMAP, PKG_MALLOC, DBG_QM_MALLOC, USE_FUTEX, FAST_LOCK-ADAPTIVE_WAIT, USE_DNS_CACHE, USE_DNS_FAILOVER, USE_NAPTR, USE_DST_BLACKLIST, HAVE_RESOLV_RES
ADAPTIVE_WAIT_LOOPS=1024, MAX_RECV_BUFFER_SIZE 262144, MAX_LISTEN 16, MAX_URI_SIZE 1024, BUF_SIZE 65535, DEFAULT PKG_SIZE 8MB
poll method support: poll, epoll_lt, epoll_et, sigio_rt, select.
id: 727746
compiled on 15:59:03 Jan 11 2016 with gcc 4.6.3

System : Ubuntu 12.04.4 LTS
Linux devops10-60-20-169 3.11.0-15-generic #25~precise1-Ubuntu SMP Thu Jan 30 17:39:31 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux

I am not seeing any errors in Kamailio Logs.

Any pointers on how to get it to serve more calls/sec

Regards,
Sandeep

The most profound statements are often said in silence.
-Lynn Johnston