Julius Plenz – Blog

I'm currently working on a computer science project where we try to understand and possibly research solutions to the bufferbloat phenomenon. We created some simple RRD graphing automatism to better visualize the phenomenon.

In short – and most internet users would say this is perfectly normal behaviour – Bufferbloat describes that with high-speed uploads or downloads, network latency skyrockets. For my home router and a five-megabyte upload, it looks like this:

The grid intervals are in seconds and feature 10 data points corresponding to 10 pings in that second to a server (here 8.8.8.8). Lighter blue means further away from the median, which for clarity is displayed as a black line, too. – Thus you can see that the nearly constant ping time of ~20ms goes up to an unsteady ~140ms during the upload.

In the next-20120524 Kernel tree the codel and fq_codel queuing disciplines were made available. The CoDel implementation is based on this month's paper by van Jacobsen at al, which is definitely worth a read (and features good explanatory diagrams, too).

So I set out to try fq_codel locally first, that is: limiting my Wifi output rate to the supposed output rate of my cable modem and then re-do the same upload.

With tc-commands, this resolves to this:

IF=wlan0
tc qdisc del dev $IF root
tc qdisc add dev $IF root handle 1: htb
tc class add dev $IF parent 1: classid 1:1 htb rate 125kbps
tc qdisc add dev $IF parent 1:1 handle 10: fq_codel
tc filter add dev $IF protocol ip prio 1 u32 match ip dst 0.0.0.0/0 flowid 1:1

And guess what happens? The upload that took 45.7 seconds before now takes 46.9 seconds; but the median ping times are around 30ms as opposed to ~140ms. (Also, consider that the packet loss is down to 0% as opposed to 1.5% before.) So this is really nice:

I hope I can test this with my colleagues using a fresh CeroWRT install next week such that we can control all the parameters and do more accurate measurements.

Update: The default 13 parameter to the root handle HTB qdisc that was present in the original version of this post is unnecessary and was thus removed.