I’ve been doing some more experimentation with Linux QoS configurations using my ping-exp utility. Today I noticed that whenever I add a SFQ to the configuration there are large latency spikes. After a bit of digging it appears that these spikes happen when the SFQ changes its flow hash. This occurs every perturb interval as configured when the SFQ is created.
Below are the results from a couple experiments which show this behavior. For both experiments I had two outbound ping floods of MTU sized packets. This saturated the outbound link. The experiment itself pinged three other hosts. I made sure to use four distinct hosts (one for flood, three for the experiment) to avoid collisions in the SFQs flow hash.
The PNGs below are not ideal for detailed inspection of the graphs. However, you can also download the data files from the experiment and load them using ping-exp. This allows zooming in on the graph. See the links at the end.
The above graph is based on an experiment where the perturb value was set to five seconds. Although the large latency spikes do not occur at every five second interval, when they do occur they are on the five second grid.
The second experiment used a perturb time of twenty seconds. Again, the latency spikes do not occur every twenty seconds but they do occur on the twenty second grid.
During the experiment I ran a packet capture to make sure there wasn’t any activity that might skew the results. The amount of captured traffic was very small.
The network I performed this experiment on consists of a P3-450 Linux gateway where the QoS configuration is applied to the ppp0 device. The kernel version is 126.96.36.199-170.2.68.fc10.i686. A host behind the gateway was used to generate the ping floods and run ping-exp.
Configuration and data files