This page describes the PID rate control algorithm for mac80211.

Overview

This is an implementation of a TX rate control algorithm that uses a PID controller. Given a target failed frames rate, the controller decides about TX rate changes to meet the target failed frames rate.

The controller basically computes the following:

adj = CP * err + CI * err_avg + CD * (err - last_err) * (1 + sharpening)

where values are as described below:

adj

adjustment value that is used to switch TX rate (see below)

err

current error: target vs. current failed frames percentage

last_err

last error

err_avg

average (i.e. poor man's integral) of recent errors

sharpening

non-zero when fast response is needed (i.e. right after association or no frames sent for a long time), heading to zero over time

CP

Proportional coefficient

CI

Integral coefficient

CD

Derivative coefficient

Once we have the adj value, we map it to a rate by means of a learning algorithm. This algorithm keeps the state of the percentual failed frames difference between rates. The behaviour of the lowest available rate is kept as a reference value, and every time we switch between two rates, we compute the difference between the failed frames each rate exhibited. By doing so, we compare behaviours which different rates exhibited in adjacent timeslices, thus the comparison is minimally affected by external conditions. This difference gets propagated to the whole set of measurements, so that the reference is always the same. Periodically, we normalize this set so that recent events weigh the most. By comparing the adj value with this set, we avoid pejorative switches to lower rates and allow for switches to higher rates if they behaved well.

Tuning

Currently, rc80211-pid is tunable via debugfs. We will need to switch to cfg80211, so that userspace tools such as iw can conveniently set rate control profiles such as tuning for better reliability, throughput or latency.

The parameters are:

target

target percentage for failed frames

sampling_period

error sampling interval in milliseconds

coeff_p

absolute value of the proportional coefficient

coeff_i

absolute value of the integral coefficient

coeff_d

absolute value of the derivative coefficient

smoothing_shift

absolute value of the integral smoothing factor (i.e. amount of smoothing introduced by the exponential moving average)

sharpen_factor

absolute value of the derivative sharpening factor (i.e. amount of emphasis given to the derivative term after low activity events)

sharpen_duration

duration of the sharpening effect after the detected low activity event, relative to sampling_period

norm_offset

amount of normalization periodically performed on the learnt rate behaviour values (lower means we should trust more what we learned about behaviour of rates, higher means we should trust more the natural ordering of rates)

fast_start

if true, push high rates right after initialization

Kerneldoc

PID rate control is handled through rc80211_pid.h, rc80211_pid_algo.c and rc80211_pid_debugfs.c. Here