[Cerowrt-devel] Ubiquiti QOS

Sebastian Moeller moeller0 at gmx.de
Sun May 25 14:39:16 EDT 2014

Hi Dane,

On May 25, 2014, at 08:17 , Dane Medic <dm70dm at gmail.com> wrote:

> Is it true that devices with less than 64 MB can't handle QOS? -> https://lists.chambana.net/pipermail/commotion-dev/2014-May/001816.html

	I think this means that the commotion developers think that 64MB are required. But it does not sound like they have first hand experience so this is either hearsay, or commotions mesh networking is memory intensive. On the openwrt side there seems no documentation of minim ram requirements. Doing a quick back-of-the-envelop calculation here:
openWRT qos has 4 tiers which run fq_codel in both directions so we have 8 fq_codel instances, with each fq_codel having a limit of 10240 packets, so worst case we expect:

4 * 2 * 10240 = 81920 packets

at 1500bytes this equals

4 * 2 * 10240 * 1500 / (1024 * 1024) = 117.1875 MB

this indeed is a bit heavy on a 32MB router, but honestly 64MB will not really help you. Then again current openwrt has a limit off 800 instead of 10240 so we end up at a worst case of:

4 * 2 * 800 * 1500 / (1024 * 1024) = 9.1552734375 MB

which should still be possible with 32MB. (Note that typically fq_codel does not fill its queues up to limit, but it still would be bad if a router can easily be DOSed into OOM and rebooting…)

(For current cerowrt with simple.qos the worst case is:
(1001 * 4 + 1000 * 13 + 800 * 12) * 1500 / (1024 * 1024) = 38.0573272705 MB

yet this still works quite well on a 64MB device (only 4 of these queues are connected to the WAN interface though)

One of the bigger issues with devices with small RAM is that often they have relatively weak CPUs and I seem to recall that cerowrt tops out around 60 to 70 Mbit/sec (total for ingress and egress) due to its shaping performance.

So unless you want to run commotion you might want to ask on the openwrt list…

Best Regards


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