[Cake] Wifi Memory limits in small platforms

[Dave Taht]

It's very good to know how much folk have been struggling to keep
things from OOMing on 32MB platforms. I'd like to hope that the
unified memory management in cake (vs a collection of QoS qdiscs) and
the new fq_codel for wifi stuff (cutting it down to 1 alloc from four)
help, massively on this issue, but until today I was unaware of how
much the field may have been patching things out.

The default 32MB memory limits in fq_codel comes from the stressing
about 10GigE networking from google. 4MB is limit in openwrt,
which is suitable for ~1Gbit, and is sort of there  due to 802.11ac's
maximum (impossible to hit) of a txop that large.

Something as small as 256K is essentially about 128 full size packets
(and often, acks from an ethernet device's rx ring eat 2k).

The structure of the new fq_codel for wifi subsystem is "one in the
hardware, one ready to go, and the rest accumulating". I
typically see about 13-20 packets in an aggregate. 256k strikes me as
a bit small.

I haven't checked, but does this patch still exist in openwrt/dd-wrt?
It had helped a lot when under memory pressure from
a lot of small packets.

https://github.com/dtaht/cerowrt-3.10/blob/master/target/linux/generic/patches-3.10/657-qdisc_reduce_truesize.patch

Arguably this could be made more aggressive, but it massively reduced
memory burdens at the time I did it when
flooding the device, or having lots of acks, and while it cost cpu it
saved on ooming.

There's two other dubious things in the fq_codel for wifi stack
presently. Right now the codel target is set too high for p2p use
(20ms, where 6ms seems more right), and it also flips up to a really
high target and interval AND turns off ecn when there's more than a
few stations available (rather than "active") - it's an overly
conservative figure we used back when we had major issues with
powersave
and multicast that I'd hoped we could cut back to normal after we got
another round of research funding and feedback from the field (which
didn't happen, and we never got around to making it configurable, and
being 25x better than it was before seemed "enough")

I was puzzled at battlemesh as to why I had dropping at about 50ms
delay rather than ecn, and thought it was something
else, and this morning I'm thinking that folk have been reducing the
memlimit to 256k rather....