[Cake] Wifi Memory limits in small platforms

Toke Høiland-Jørgensen toke at redhat.com
Thu Aug 22 14:23:39 EDT 2019

Sebastian Gottschall <s.gottschall at newmedia-net.de> writes:

> Am 22.08.2019 um 19:03 schrieb Dave Taht:
>> Sebastian Gottschall <s.gottschall at newmedia-net.de> writes:
>>> Am 22.08.2019 um 15:15 schrieb 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.
>> I did kind of conflate "qos + fq_codel" vs wifi in this message. It
>> looks like yer staying with me.
>>>> Something as small as 256K is essentially about 128 full size packets
>>>> (and often, acks from an ethernet device's rx ring eat 2k).
>>> what i miss in mac80211 is the following option "fq_codel = off"
>>> its essential and i will definitly work on a patch to deal with this
>>> way for low memory 802.11n platforms.
>> Well, it would be my hope that turning it off would A) not help that
>> much on memory or cpu and B) show such a dramatic reduction in
>> multi-station performance that you'd immediately turn it on again.
> isnt it better to have a working platform with less performance than a 
> crashing platform with no performance?
> i mean i can user older mac80211 versions without that issue on a 
> typical nanostation 2/5 which is often used just as CPE device

So before the queueing patches to mac80211, the maximum packet queue
size for ath9k was 3MB in total, or 2.2MB if only a single AC was used
on the WiFi link (that's 128 packets in the driver + 1000 in the
pfifo_fast qdisc * 2074 bytes for the truesize of a full-size packet).
Whereas now the default is 4MB for a non-vht device. So it's not
actually that big of a difference, and as you've already discovered the
defaults can be changed.

Would it be helpful to add support for setting the memory limit in
hostapd (to avoid having to patch the kernel default)?

> but with current mac80211 versions (current means last 2-3 years). they 
> are just unstable and running out of memory after a while
> the only thing which helped was cutting of the memory limit of fq_codel 
> inside mac80211
> i also have another fancy testunit which is a linksys wrt400 with 32 mb 
> ram and 2 ath9k based wifi chipsets. no hope here fonr running stable
> for only 5 minutes even with a single connection under load (my crashing 
> test is running a hdtv iptv stream converted to unicast using a 
> stateless eoip tunnel)
>> I try to encourage folk to run the rtt_fair tests in flent when
>> twiddling with wifi. Those really shows how bad things are when you
>> don't have ATF + FQ + Per station aggregation and lots of
>> clients. Single threaded tests are misleading.
> i know but even single threaded tests arent working good on such 
> devices. so there is no need to talk about the benefits of atf,fq_codel etc.
> but there is need to talk about configurable use of it which also allows 
> to disable it if required.

Disabling the fq part won't actually gain you much in terms of memory
usage, though, as most of it is packet memory which is already

The one exception to this is the static overhead of 'struct fq_flow', of
which mac80211 currently allocates 4k. That's 300k of memory which is
currently not configurable. But that could be fixed :)


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