[Bloat] [aqm] TSO sizing fixes and the new paced "fq" scheduler in Linux 3.12
luca.muscariello at orange.com
Wed Sep 25 11:38:23 EDT 2013
Le 25/09/2013 17:15, Eric Dumazet a écrit :
> On Tue, 2013-09-24 at 14:25 +0200, James Roberts wrote:
>> No one responded to Luca's Sept 1 comment (on the bloat list) that the
>> new code seems to do tail drop rather than longest queue drop.
>> If this is so, bandwidth sharing will not be fair since FQ alone is
>> not enough. This was shown in the previously cited Bell Labs
>> paper : http://ect.bell-labs.com/who/stiliadi/papers/jsac99.pdf. The
>> following table is copied from the paper.
> This paper assumes TCP stack can push cwin packets in the queue.
> We no longer have this behavior with linux.
> If you have drops on FQ, then you have a serious problem with your
> FQ is meant to be used on hosts, not on routers.
> For routers, fq_codel is fine.
> TCP Small Queues limits the number of packets in Qdisc for any tcp flow
> (2 packets). Default FQ setting allows 10000 packets.
> You can add tail drop on FQ if you really want, but I never had a single
> drop in my FQ settings, on 20Gbps links and thousands of flows.
> Therefore I did not add complexity to solve a non problem.
Then, I feel like FQ is a bad name to call this "newFQ".
It's an implementation of a fair TCP pacer. Which is very useful, but FQ
is kind of misleading, IMHO.
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