[Bloat] CPU consumption using TC-TBF and TC-POLICE to limit rate

[Jose Blanquicet]

Hi Jonathan,

Thanks for the response.

On Tue, May 26, 2020 at 2:50 PM Jonathan Morton <chromatix99 at gmail.com> wrote:
> > On 26 May, 2020, at 12:47 pm, Jose Blanquicet <blanquicet at gmail.com> wrote:
> >
> > We have an embedded system with limited CPU resources that acts as
> > gateway to provide Internet access from LTE to a private Wi-Fi
> > network. Our problem is that the bandwidth on LTE and Wi-Fi links is
> > higher than what the system is able to handle thus it reaches 100% of
> > CPU load when we perform a simple speed test from a device connected
> > to our Wi-Fi Hotspot.
> >
> > Therefore, we want to limit the bandwidth to avoid system gets
> > saturated is such use-case. To do so, we thought to use the QDISC-TBF
> > on the Wi-Fi interface. For instance, to have 10Mbps:
> >
> >    tc qdisc add dev wlan0 root tbf rate 10mbit burst 12500b latency 50ms
> >
> > It worked correctly and maximum rate was limited to 10Mbps. However,
> > we noticed that the CPU load added by the TBF was not negligible for
> > our system.
>
> Just how limited is the CPU on this device?  I have successfully shaped at several tens of Mbps on a Pentium-MMX, where the limiting factor may have been the PCI bus rather than the CPU itself.

We have just a percentage of an ARM Cortex A7 (1.2GHz) because the
rest is reserved for modem. We are now trying to optimize all the
applications in the system but LTE<->WIFI data transfer is indeed the
use-case that puts our system in crisis.

> Assuming your CPU is of that order of capability, I would suggest installing Cake using the out-of-tree build process, and the latest stable version of the iproute2 tools to configure it.  Start with:
>
>         git clone https://github.com/dtaht/sch_cake.git
>
> This provides a more efficient and more effective shaper than TBF, and a more effective AQM than a policer, and good flow-isolation properties, all in a single bundle that will be more efficient than running two separate components.
>
> Once installed, the following should set it up nicely for you:
>
>         tc qdisc replace dev wlan0 root cake bandwidth 10Mbit besteffort flows ack-filter
>
> Cake is considered quite a heavyweight solution, but very effective.  If it doesn't work well for this particular use case, it may be feasible to backport some more recent work which takes a simpler approach, though along similar lines.

Interesting, we will have a look at this and do some test to see how
it behaves in our "particular" environment.

Thanks,
Jose