[Codel] fq_codel : interval servo

Dave Taht dave.taht at gmail.com
Sun Sep 2 14:17:16 EDT 2012

On Sun, Sep 2, 2012 at 11:08 AM, Dave Taht <dave.taht at gmail.com> wrote:

In reviewing this mail I realized I used three different names
for tcp_limit_output_bytes, corrected below...

> Codel will push stuff down to, but not below, 5ms of latency (or
> target). In fq_codel you will typically end up with 1 packet outstanding in
> each active queue under heavy load. At 10Mbit it's pretty easy to
> have it strain mightily and fail to get to 5ms, particularly on torrent-like
> workloads.
> The "right" amount of host latency to aim for is ... 0, or as close to it as
> you can get.  Fiddling with codel target and interval on the host to
> get less host latency is well and good, but you can't get to 0 that way...
> The best queue on a host is no extra queue.
> I spent some time evaluating linux fq_codel vs the ns2 nfq_codel version I
> just got working. In 150 bidirectional competing streams, at 100Mbit,
> it retained about 30% less packets in queue (110 vs 140). Next up
> on my list is longer RTTs and wifi, but all else was pretty equivalent.
> The effects of fiddling with /proc/sys/net/ipv4/tcp_limit_output_bytes
> was even more remarkable. At 6000, I would get down to
> a nice steady 71-81 packets in queue on that 150 stream workload.
> So, I started thinking through and playing with how TSQ works:
> At one hop 100Mbit, with a BQL of 3000 and a tcp_limit_output_bytes of 6000,
> all offloads off, nfq_codel on both ends, I get single stream throughoutput
> of 92.85Mbit.  Backlog in qdisc is, 0.
> 2 netperf streams, bidirectional: 91.47 each, darn close to theoretical, less
> than one packet in the backlog.
> 4 streams backlogs a little over 3. (and sums to 91.94 in each direction)
> 8, backlog of 8. (optimal throughput)
> Repeating the 8 stream test with tcp_limit_output_bytes of 1500, I get
> packets outstanding of around 3, and optimal throughput. (1 stream test:
> 42Mbit throughput (obviously starved), 150 streams: 82...)
> 8 streams, limit set to 127k, I get 50 packets outstanding in the queue,
> and the same throughput. (150 streams, ~100)
> So I might argue that a more "right" number for tcp_limit_output_bytes is
> not 128k per TCP socket, but (BQL_limit*2/active_sockets), in conjunction
> with fq_codel. I realize that that raises interesting questions as to when
> to use TSO/GSO, and how to schedule tcp packet releases, and pushes
> the window reduction issue all the way up into the tcp stack rather
> than responding to indications from the qdisc... but it does
> get you closer to a 0 backlog in qdisc.
> And *usually* the bottleneck link is not on the host but on something
> inbetween, and that's where your signalling comes from, anyway.
> --
> Dave Täht
> http://www.bufferbloat.net/projects/cerowrt/wiki - "3.3.8-17 is out
> with fq_codel!"

Dave Täht
http://www.bufferbloat.net/projects/cerowrt/wiki - "3.3.8-17 is out
with fq_codel!"

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