The CoDel AQM algorithm by Kathie Nichols and Van Jacobson provides us with an essential missing tool to control queues properly. This work is the culmination of their at three major attempts to solve the problems with AQM algorithms over the last 14 years.
Eric Dumazet wrote the codel queuing discipline (based on a quick prototype by Dave Täht, who spent the last year working 60 hour weeks on bufferbloat) which landed in net-next a week or two ago; yesterday, net-next was merged into the Linux mainline for inclusion in the next Linux release. Eric also implemented a fq_codel queuing discipline, combining fair queuing and CoDel (pronounced “coddle”), and it works very well. The CoDel implementation was dual licensed BSD/GPL to help the *BSD community. Eric and others have tested CoDel on 10G Ethernet interfaces; as expected, CoDel performance is good in what’s been tested to date.
Linux 3.5 will likely release in August. So it was less than a month from first access to the algorithm (which was formally published in the AQM Queue May 6) to Linux mainline; it should be about four total from availability of the algorithm to Linux release. Not bad at all.
Felix Fietkau merged both the codel and fq_codel into the OpenWrt mainline last week for release this summer. 37 architectures, 150 separate routing platforms, no waiting…
The final step should be to worry about all the details, and finally kill pfifo_fast once we’ve tested CoDel enough.
While I don’t think that this is the end of the story, fair queuing in combination with CoDel, and Tom Herbert’s great BQL work together go a very long way toward dealing with bufferbloat on Ethernet devices in Linux. I think there is more to be done, but we’re much/most of the way to what is possible.
Some Ethernet hardware (both NIC’s and many Ethernet switches) has embedded bufferbloat (in this case, large FIFO buffers) that software may not be able to easily avoid; as always you must test before you are sure you are bloat free! Unfortunately, we’ll see a lot of this: a very senior technologist of a major router vendor started muttering “line cards” in a troubled voice at an IETF meeting when he really grokked bufferbloat. Adding AQM, simple as CoDel is, as an afterthought may be very hard: do not infer from the speed of implementation in Linux on Ethernet that all operating systems, drivers, and hardware will it will be so easy or retrofit possible; for example, if your OS has no equivalent to BQL, you’ll have that work to do (and easily other work too).
Wireless is much more of a challenge than Ethernet for Linux, particularly 802.11n wireless; the buffering and queuing internal to these devices and drivers is much more complex, and the designers of those software stacks are still understanding the implications of AQM, particularly since the driver boundary has partitioned the buffering in unfortunate ways. I think it will be months, to even a year or two before those have good implementations that get us down to anything close to theoretical minimum latency. But running CoDel is likely a lot better than nothing anyway if you can: that’s certainly our CeroWrt/OpenWrt experience, and the tuning headaches Dave Täht was having trying to use RED go away. So give it a try….
Let me know of CoDel progress in other systems please!