Coalesce parameters and RX ring size

Please read through this very useful article :
http://netoptimizer.blogspot.dk/2014/06/pktgen-for-network-overload-testing.html

Coalesce parameters and RX ring size can have an impact on your IDS.
To see what are the coalesce parameters on the currently sniffing interface:

root@suricata:/var/log/suricata# ethtool -c eth3
Coalesce parameters for eth3:
Adaptive RX: off  TX: off
stats-block-usecs: 0
sample-interval: 0
pkt-rate-low: 0
pkt-rate-high: 0

rx-usecs: 1000
rx-frames: 0
rx-usecs-irq: 0
rx-frames-irq: 0

tx-usecs: 0
tx-frames: 0
tx-usecs-irq: 0
tx-frames-irq: 0

rx-usecs-low: 0
rx-frame-low: 0
tx-usecs-low: 0
tx-frame-low: 0

rx-usecs-high: 0
rx-frame-high: 0
tx-usecs-high: 0
tx-frame-high: 0

To change (try with different values) the coalesce parameter:

root@suricata:/var/log/suricata# ethtool -C eth3 rx-usecs 1
root@suricata:/var/log/suricata# ethtool -c eth3
Coalesce parameters for eth3:
Adaptive RX: off  TX: off
stats-block-usecs: 0
sample-interval: 0
pkt-rate-low: 0
pkt-rate-high: 0

rx-usecs: 1
rx-frames: 0
rx-usecs-irq: 0
rx-frames-irq: 0

tx-usecs: 0
tx-frames: 0
tx-usecs-irq: 0
tx-frames-irq: 0

rx-usecs-low: 0
rx-frame-low: 0
tx-usecs-low: 0
tx-frame-low: 0

rx-usecs-high: 0
rx-frame-high: 0
tx-usecs-high: 0
tx-frame-high: 0

Ring RX parameters on the network card play a role too:

root@suricata:~# ethtool -g eth3
Ring parameters for eth3:
Pre-set maximums:
RX:             4096
RX Mini:        0
RX Jumbo:       0
TX:            4096

Current hardware settings:
RX:             512
RX Mini:        0
RX Jumbo:       0
TX:             512

  To increase that to the max Pre-set RX:

root@suricata:~# ethtool -G eth3 rx 4096

To confirm:

root@suricata:~# ethtool -g eth3
Ring parameters for eth3:
Pre-set maximums:
RX:             4096
RX Mini:        0
RX Jumbo:       0
TX:             4096
Current hardware settings:
RX:             4096
RX Mini:        0
RX Jumbo:       0
TX:             512

Suggested approach - that worked best in my particular set up - for Suricata IDS/IPS deployment is to have the coalesce parameter to value 1 and increase the ring  RX size to the max available for that particular interface/card.

It is suggested that you try a few different scenarios with regards to the coalesce parameters in order to find the best combination that suits your needs.