Tuning nProbe Performance

As nProbe can be deployed on very different environments, it is necessary to tune it according to the network where is active. In order to achieve a good probe setup, it is necessary to understand how nProbe is working internally. Each captured packet is analyzed, associated to a flow, and stored onto a hash. Periodically, the hash is analyzed and expired flows are emitted. The hash size is static (-w flag) as this allows nProbe to: Allocate all the needed memory at startup (this is compulsory on embedded systems where memory is limited and it is necessary to know at startup whether a certain application can operate with the available resources). Avoid exhausting all the available memory in case of attacks that can produce several flows. Selecting the hash size is a matter of trade-off between efficiency (an efficient hash is at least 1/3 empty) and memory usage. This statement does not mean that a huge hash is always the solution as the flow export process can be slower (and more CPU cycles are needed) as a large hash needs to be explored.

On the other hand, the hash size is just a part of the problem. In fact, the hash fill percentage can be also controlled by other factors such as:

  • Reducing the flow lifetime (-t)
  • Reducing the maximum flow idle time (-d)
  • Increasing how often the hash is walked searching expired flows (-s)

nProbe allows users to ease the tuning process by printing the status of internal hashes using the –b flag. Users who experience severe nProbe performance problems, packet loss or high CPU usage, should start nProbe with –b in order to find out whether their probe setup is optimal.

High-Speed Flow Collection

When you need to collect flows at high speed with minimum CPU and memory requirements (this is the case whe collected flows are not stored into the cache but exported immediately), you hould consider cache bypassing. In particular this is achieved with the –collector-passthrough option. Please refer to the “nProbe Case Study” section of this manual for further information about this topic.