Performance de Nginx + uWSGI + Django bloquée à 100rq/s

J'ai configuré Nginx avec uWSGI et Django sur CentOS 6 x64 (3.06GHz i3 540, 4GB), qui devrait facilement gérer 2500 rq/s mais lorsque je lance un test ab ( ab -n 1000 -c 100 ) les performances s'arrêtent à 92 - 100 rq/s.

Nginx:
    user nginx;
    worker_processes 2;
    events {
        worker_connections 2048;
        use epoll;
    }

uWSGI:

    Emperor
    /usr/sbin/uwsgi --master --no-orphans --pythonpath /var/python --emperor /var/python/*/uwsgi.ini

[uwsgi]
socket = 127.0.0.2:3031
master = true
processes = 5

env = DJANGO_SETTINGS_MODULE=x.settings
env = HTTPS=on
module = django.core.handlers.wsgi:WSGIHandler()

disable-logging = true
catch-exceptions = false

post-buffering = 8192
harakiri = 30
harakiri-verbose = true

vacuum = true
listen = 500
optimize = 2

sysclt changes:
# Increase TCP max buffer size setable using setsockopt()
net.ipv4.tcp_rmem = 4096 87380 8388608
net.ipv4.tcp_wmem = 4096 87380 8388608

net.core.rmem_max = 8388608
net.core.wmem_max = 8388608
net.core.netdev_max_backlog = 5000
net.ipv4.tcp_max_syn_backlog = 5000
net.ipv4.tcp_window_scaling = 1
net.core.somaxconn = 2048

# Avoid a smurf attack
net.ipv4.icmp_echo_ignore_broadcasts = 1

# Optimization for port usefor LBs
# Increase system file descriptor limit
fs.file-max = 65535

Je l'ai fait sysctl -p pour activer les modifications.

Informations sur le serveur inactif :

top - 13:34:58 up 102 days, 18:35,  1 user,  load average: 0.00, 0.00, 0.00

Tasks: 118 total,   1 running, 117 sleeping,   0 stopped,   0 zombie

Cpu(s):  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st

Mem:   3983068k total,  2125088k used,  1857980k free,   262528k buffers

Swap:  2104504k total,        0k used,  2104504k free,   606996k cached

free -m

 total       used       free     shared    buffers     cached

Mem:          3889       2075       1814          0        256        592

-/+ buffers/cache:       1226       2663

Swap:         2055          0       2055

**During the test:**

top - 13:45:21 up 102 days, 18:46,  1 user,  load average: 3.73, 1.51, 0.58

Tasks: 122 total,   8 running, 114 sleeping,   0 stopped,   0 zombie

Cpu(s): 93.5%us,  5.2%sy,  0.0%ni,  0.2%id,  0.0%wa,  0.1%hi,  1.1%si,  0.0%st

Mem:   3983068k total,  2127564k used,  1855504k free,   262580k buffers

Swap:  2104504k total,        0k used,  2104504k free,   608760k cached

free -m

total       used       free     shared    buffers     cached

Mem:          3889       2125       1763          0        256        595

-/+ buffers/cache:       1274       2615

Swap:         2055          0       2055

iotop

30141 be/4 nginx       0.00 B/s    7.78 K/s  0.00 %  0.00 % nginx: wo~er process

Où se trouve le goulot d'étranglement ? Ou qu'est-ce que je fais mal ?