Below are some kernel tweaks that I use for CentOS 6.2 with a 10 GB NIC.
#Lower syn retry rates, default is 5
net.ipv4.tcp_synack_retries = 2
net.ipv4.tcp_syn_retries = 3
# Tune IPv6
net.ipv6.conf.default.router_solicitations = 0
net.ipv6.conf.default.accept_ra_rtr_pref = 0
net.ipv6.conf.default.accept_ra_pinfo = 0
net.ipv6.conf.default.accept_ra_defrtr = 0
net.ipv6.conf.default.autoconf = 0
net.ipv6.conf.default.dad_transmits = 0
net.ipv6.conf.default.max_addresses = 1
# Increase TCP max buffer size setable using setsockopt()
# default 4096 87380 4194304
net.ipv4.tcp_rmem = 4096 87380 8388608
net.ipv4.tcp_wmem = 4096 87380 8388608
# Increase Linux auto tuning TCP buffer limits
# min, default, and max number of bytes to use
# set max to at least 4MB, or higher if you use very high Bandwidth-delay product (BDP) paths
# Tcp Windows etc
# default 131071
net.core.rmem_max = 8388608
net.core.wmem_max = 8388608
# default 124928
net.core.rmem_default = 524287
net.core.wmem_default = 524287
# default net.core.netdev_max_backlog = 1000, set to 30000 for 10Gbit NICs
net.core.netdev_max_backlog = 32768
#
# default net.core.somaxconn = 128
net.core.somaxconn = 4096
# You might also try the following:
# default net.ipv4.tcp_max_syn_backlog = 1024
net.ipv4.tcp_max_syn_backlog = 4096
# metrics slows us down :)
net.ipv4.tcp_no_metrics_save = 1
# default net.ipv4.tcp_no_metrics_save = "32768 61000"
net.ipv4.ip_local_port_range = 1025 65535
# default tcp_fin_timeout = 60
net.ipv4.tcp_fin_timeout = 30
# default net.ipv4.tcp_keepalive_intvl = 75
net.ipv4.tcp_keepalive_intvl = 30
# default net.ipv4.tcp_max_tw_buckets = 180000
net.ipv4.tcp_max_tw_buckets = 1440000
# default net.ipv4.tcp_keepalive_time = 7200
net.ipv4.tcp_keepalive_time = 400
# default net.ipv4.tcp_keepalive_probe = 9
net.ipv4.tcp_keepalive_probes = 5
# default vm.swappiness = 60
vm.swappiness = 20
# ipcs -l
# default max seg size (kbytes) = 32768
kernel.shmmax = 500000000
# default max total shared memory (kbytes) = 8388608
kernel.shmall = 4000000000
# default max queues system wide = 1024
kernel.msgmni = 2048
net.ipv4.tcp_tw_recycle = 1
net.ipv4.tcp_max_syn_backlog = 4096
net.ipv4.conf.all.arp_filter = 1
net.core.netdev_max_backlog = 10000
Also set your per user limits higher in /etc/security/limits.conf:
* soft nofile 65536
* hard nofile 65536
* soft nproc 131072
* hard nproc 131072