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Got (Buffer) Bloat?
mtaht writes, "After a very intense month of development, the Bufferbloat project has announced the debloat-testing git kernel tree, featuring (via suggestion of Van Jacobson) a wireless network latency smashing algorithm, (called eBDP), the SFB and CHOKe packet schedulers, and a slew of driver level fixes that reduce network latency across the Linux kernel by over 2 orders of magnitude. Got Bloat?"
It's about the downside of memory becoming cheap causing latency problems with congestion control mechinisms that rely on the endpoints being able to inform the sender when it's sending too fast.
Jim Getty's research blog entry explains the problem in detail.
My understanding may not be correct but:
Bufferbloat (I first came across the term bufferbloat in this blog post by Jim Gettys) is the nickname that has been given to the high latency that can occur in modern network connections due to large buffers in the network. An example could be the way that a network game on one computer starts to stutter if another computer starts to use a protocol like bittorrent to transfer files on the same network connection.
The large buffers seem to have arisen from a desire to maximise download throughput regardless of network condition. This can give rise to the situation where small urgent packets are delayed because big packets (which perhaps should not have been sent) are queued up in front of them. The system sending the big packets is not told to stop sending them so quickly because its packets are being delivered...
The linked article sounds like people have modified the Linux kernel source to allow people who know how to compile their own kernels to test ideas people have had for reducing the bufferbloat effect on their hardware and to report back their results.
Does this help explain things a bit?
The core work where we saw latency under load drop by 2 orders of magnitude was in the wireless driver stack on Linux. Examples were the iwl driver (130ms to ~2), and the ath9k driver ( > 200ms to ~d) (and these numbers were for GOOD connections, at high wifi rates. You can get 3 orders of magnitude improvement if you are on a slow wifi connection.) There's a new rate sensitive algorithm for wireless (eBDP) that we are trying in this kernel tree. It's not fully baked yet. 802.11n wireless package aggregation is HARD. That said there's bloat in all the other wired drivers too. We are doing far too much uncontrolled buffering in the kernel - specifically the dma tx ring on many devices - for slower networks. As one example, A gigE interface, connnected to a 3Mbit cable modem - does bad, subtle, things to the stack.