Hi,
the patch below is a first attempt on the PID-controller approach for TX
rate control. It kind of works here, but I haven't spent much time
tuning the coefficients.
I wanted to share this at this early stage so you can experiment with
and comment.
Mattias
Signed-off-by: Mattias Nissler <mattias.nissler@xxxxxx>
---
net/mac80211/ieee80211_rate.h | 3 -
net/mac80211/rc80211_simple.c | 281 ++++++++++++++++++++++++++++-------------
2 files changed, 190 insertions(+), 94 deletions(-)
diff --git a/net/mac80211/ieee80211_rate.h b/net/mac80211/ieee80211_rate.h
index 2368813..9948d0f 100644
--- a/net/mac80211/ieee80211_rate.h
+++ b/net/mac80211/ieee80211_rate.h
@@ -18,9 +18,6 @@
#include "ieee80211_i.h"
#include "sta_info.h"
-#define RATE_CONTROL_NUM_DOWN 20
-#define RATE_CONTROL_NUM_UP 15
-
struct rate_control_extra {
/* values from rate_control_get_rate() to the caller: */
diff --git a/net/mac80211/rc80211_simple.c b/net/mac80211/rc80211_simple.c
index da72737..af6f8ff 100644
--- a/net/mac80211/rc80211_simple.c
+++ b/net/mac80211/rc80211_simple.c
@@ -20,52 +20,127 @@
#include "debugfs.h"
-/* This is a minimal implementation of TX rate controlling that can be used
- * as the default when no improved mechanisms are available. */
+/* This is an implementation of 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)
+ *
+ * where
+ * 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
+ * CP Proportional coefficient
+ * CI Integral coefficient
+ * CD Derivative coefficient
+ *
+ * CP, CI, CD are subject to careful tuning.
+ *
+ * The integral component uses a exponential moving average approach instead of
+ * an actual sliding window. Advantage is that we don't need to keep an array of
+ * the last N error values and computation is easier.
+ *
+ * Once we have the adj value, we need to map it to a TX rate to be selected.
+ * For now, we depend on the rates to be ordered in a way such that more robust
+ * rates (i.e. such that exhibit a lower framed failed percentage) come first.
+ * E.g. for the 802.11b/g case, we first have the b rates in ascending order,
+ * then the g rates. The adj simply decides the index of the TX rate in the list
+ * to switch to (relative to the current TX rate entry).
+ *
+ * Note that for the computations we use a fixed-point representation to avoid
+ * floating point arithmetic. Hence, all values are shifted left by
+ * RATE_CONTROL_ARITH_SHIFT.
+ */
+/* Sampling frequency for measuring percentage of failed frames. */
+#define RATE_CONTROL_INTERVAL (HZ / 1)
-#define RATE_CONTROL_EMERG_DEC 2
-#define RATE_CONTROL_INTERVAL (HZ / 20)
-#define RATE_CONTROL_MIN_TX 10
+/* Exponential averaging smoothness (used for I part of PID controller) */
+#define RATE_CONTROL_SMOOTHING_SHIFT 3
+#define RATE_CONTROL_SMOOTHING (1 << RATE_CONTROL_SMOOTHING_SHIFT)
-static void rate_control_rate_inc(struct ieee80211_local *local,
- struct sta_info *sta)
-{
- struct ieee80211_sub_if_data *sdata;
- struct ieee80211_hw_mode *mode;
- int i = sta->txrate;
- int maxrate;
+/* Fixed point arithmetic shifting amount. */
+#define RATE_CONTROL_ARITH_SHIFT 8
- sdata = IEEE80211_DEV_TO_SUB_IF(sta->dev);
- if (sdata->bss && sdata->bss->force_unicast_rateidx > -1) {
- /* forced unicast rate - do not change STA rate */
- return;
- }
+/* Fixed point arithmetic factor. */
+#define RATE_CONTROL_ARITH_FACTOR (1 << RATE_CONTROL_ARITH_SHIFT)
- mode = local->oper_hw_mode;
- maxrate = sdata->bss ? sdata->bss->max_ratectrl_rateidx : -1;
+/* Proportional PID component coefficient. */
+#define RATE_CONTROL_COEFF_P 15
+/* Integral PID component coefficient. */
+#define RATE_CONTROL_COEFF_I 10
+/* Derivative PID component coefficient. */
+#define RATE_CONTROL_COEFF_D 15
- if (i > mode->num_rates)
- i = mode->num_rates - 2;
+/* Target failed frames rate for the PID controller. NB: This effectively gives
+ * maximum failed frames percentage we're willing to accept. If communication is
+ * good, the controller will fail to adjust failed frames percentage to the
+ * target. This is intentional.
+ */
+#define RATE_CONTROL_TARGET_PF (20 << RATE_CONTROL_ARITH_SHIFT)
- while (i + 1 < mode->num_rates) {
- i++;
- if (sta->supp_rates & BIT(i) &&
- mode->rates[i].flags & IEEE80211_RATE_SUPPORTED &&
- (maxrate < 0 || i <= maxrate)) {
- sta->txrate = i;
- break;
- }
- }
-}
+struct sta_rate_control {
+ unsigned long last_change;
+ unsigned long last_sample;
+ u32 tx_num_failed;
+ u32 tx_num_xmit;
-static void rate_control_rate_dec(struct ieee80211_local *local,
- struct sta_info *sta)
+ /*
+ * Average failed frames percentage error (i.e. actual vs. target
+ * percentage), scaled by RATE_CONTROL_SMOOTHING. This value is a
+ * smoothed by using a exponentail weighted average technique:
+ *
+ * (SMOOTHING - 1) * err_avg_old + err
+ * err_avg = -----------------------------------
+ * SMOOTHING
+ *
+ * where err_avg is the new approximation, err_avg_old the previous one
+ * and err is the error w.r.t. to the current failed frames percentage
+ * sample. Note that the bigger SMOOTHING the more weight is given to
+ * the previous estimate, resulting in smoother behavior (i.e.
+ * corresponding to a longer integration window).
+ *
+ * For computation, we actually don't use the above formula, but this
+ * one:
+ *
+ * err_avg_scaled = err_avg_old_scaled - err_avg_old + err
+ *
+ * where:
+ * err_avg_scaled = err * SMOOTHING
+ * err_avg_old_scaled = err_avg_old * SMOOTHING
+ *
+ * This avoids floating point numbers and the per_failed_old value can
+ * easily be obtained by shifting per_failed_old_scaled right by
+ * RATE_CONTROL_SMOOTHING_SHIFT.
+ */
+ s32 err_avg_sc;
+
+ /* Last framed failes percentage sample */
+ u32 last_pf;
+
+ unsigned long avg_rate_update;
+ u32 tx_avg_rate_sum;
+ u32 tx_avg_rate_num;
+
+#ifdef CONFIG_MAC80211_DEBUGFS
+ struct dentry *tx_avg_rate_sum_dentry;
+ struct dentry *tx_avg_rate_num_dentry;
+#endif
+};
+
+
+static void rate_control_adjust_rate(struct ieee80211_local *local,
+ struct sta_info *sta, int adj)
{
struct ieee80211_sub_if_data *sdata;
struct ieee80211_hw_mode *mode;
- int i = sta->txrate;
+ int newidx = sta->txrate + adj;
+ int maxrate;
+ int back = (adj > 0) ? 1 : -1;
sdata = IEEE80211_DEV_TO_SUB_IF(sta->dev);
if (sdata->bss && sdata->bss->force_unicast_rateidx > -1) {
@@ -74,20 +149,29 @@ static void rate_control_rate_dec(struct ieee80211_local *local,
}
mode = local->oper_hw_mode;
- if (i > mode->num_rates)
- i = mode->num_rates;
+ maxrate = sdata->bss ? sdata->bss->max_ratectrl_rateidx : -1;
+
+ if (newidx < 0)
+ newidx = 0;
+ else if (newidx >= mode->num_rates)
+ newidx = mode->num_rates - 1;
+
+ while (newidx != sta->txrate) {
+ if (sta->supp_rates & BIT(newidx) &&
+ mode->rates[newidx].flags & IEEE80211_RATE_SUPPORTED &&
+ (maxrate < 0 || newidx <= maxrate)) {
+ sta->txrate = newidx;
+
+ printk(KERN_DEBUG "rate_control: new tx rate %d\n",
+ mode->rates[newidx].rate);
- while (i > 0) {
- i--;
- if (sta->supp_rates & BIT(i) &&
- mode->rates[i].flags & IEEE80211_RATE_SUPPORTED) {
- sta->txrate = i;
break;
}
+
+ newidx += back;
}
}
-
static struct ieee80211_rate *
rate_control_lowest_rate(struct ieee80211_local *local,
struct ieee80211_hw_mode *mode)
@@ -111,22 +195,6 @@ struct global_rate_control {
int dummy;
};
-struct sta_rate_control {
- unsigned long last_rate_change;
- u32 tx_num_failures;
- u32 tx_num_xmit;
-
- unsigned long avg_rate_update;
- u32 tx_avg_rate_sum;
- u32 tx_avg_rate_num;
-
-#ifdef CONFIG_MAC80211_DEBUGFS
- struct dentry *tx_avg_rate_sum_dentry;
- struct dentry *tx_avg_rate_num_dentry;
-#endif
-};
-
-
static void rate_control_simple_tx_status(void *priv, struct net_device *dev,
struct sk_buff *skb,
struct ieee80211_tx_status *status)
@@ -143,8 +211,20 @@ static void rate_control_simple_tx_status(void *priv, struct net_device *dev,
srctrl = sta->rate_ctrl_priv;
srctrl->tx_num_xmit++;
+
+ /* We count frames that totally failed to be transmitted as two bad
+ * frames, those that made it out but had some retries as one good and
+ * one bad frame.
+ */
+ if (status->excessive_retries) {
+ srctrl->tx_num_failed += 2;
+ srctrl->tx_num_xmit++;
+ } else if (status->retry_count) {
+ srctrl->tx_num_failed++;
+ srctrl->tx_num_xmit++;
+ }
+
if (status->excessive_retries) {
- srctrl->tx_num_failures++;
sta->tx_retry_failed++;
sta->tx_num_consecutive_failures++;
sta->tx_num_mpdu_fail++;
@@ -158,40 +238,59 @@ static void rate_control_simple_tx_status(void *priv, struct net_device *dev,
sta->tx_retry_count += status->retry_count;
sta->tx_num_mpdu_fail += status->retry_count;
- if (time_after(jiffies,
- srctrl->last_rate_change + RATE_CONTROL_INTERVAL) &&
- srctrl->tx_num_xmit > RATE_CONTROL_MIN_TX) {
- u32 per_failed;
- srctrl->last_rate_change = jiffies;
-
- per_failed = (100 * sta->tx_num_mpdu_fail) /
- (sta->tx_num_mpdu_fail + sta->tx_num_mpdu_ok);
- /* TODO: calculate average per_failed to make adjusting
- * parameters easier */
-#if 0
- if (net_ratelimit()) {
- printk(KERN_DEBUG "MPDU fail=%d ok=%d per_failed=%d\n",
- sta->tx_num_mpdu_fail, sta->tx_num_mpdu_ok,
- per_failed);
- }
-#endif
-
- /*
- * XXX: Make these configurable once we have an
- * interface to the rate control algorithms
+ /*
+ * Update PID controller state.
+ */
+ if (time_after(jiffies, srctrl->last_sample + RATE_CONTROL_INTERVAL)) {
+ u32 pf;
+ s32 err_avg;
+ s32 err_prop;
+ s32 err_int;
+ s32 err_der;
+ int adj;
+
+ srctrl->last_sample = jiffies;
+
+ /* If no frames were transmitted, we assume the old sample is
+ * still a good measurement and copy it.
*/
- if (per_failed > RATE_CONTROL_NUM_DOWN) {
- rate_control_rate_dec(local, sta);
- } else if (per_failed < RATE_CONTROL_NUM_UP) {
- rate_control_rate_inc(local, sta);
+ if (srctrl->tx_num_xmit == 0)
+ pf = srctrl->last_pf;
+ else {
+ pf = srctrl->tx_num_failed * 100 / srctrl->tx_num_xmit;
+ pf <<= RATE_CONTROL_ARITH_SHIFT;
+
+ srctrl->tx_num_xmit = 0;
+ srctrl->tx_num_failed = 0;
}
- srctrl->tx_avg_rate_sum += status->control.rate->rate;
- srctrl->tx_avg_rate_num++;
- srctrl->tx_num_failures = 0;
- srctrl->tx_num_xmit = 0;
- } else if (sta->tx_num_consecutive_failures >=
- RATE_CONTROL_EMERG_DEC) {
- rate_control_rate_dec(local, sta);
+
+ /* Compute the proportional, integral and derivative errors. */
+ err_prop = RATE_CONTROL_TARGET_PF - pf;
+
+ err_avg = srctrl->err_avg_sc >> RATE_CONTROL_SMOOTHING_SHIFT;
+ srctrl->err_avg_sc = srctrl->err_avg_sc - err_avg + err_prop;
+ err_int = srctrl->err_avg_sc >> RATE_CONTROL_SMOOTHING_SHIFT;
+
+ err_der = pf - srctrl->last_pf;
+ srctrl->last_pf = pf;
+
+ /* Compute the controller output. */
+ adj = (err_prop * RATE_CONTROL_COEFF_P
+ + err_int * RATE_CONTROL_COEFF_I
+ + err_der * RATE_CONTROL_COEFF_D)
+ >> (2 * RATE_CONTROL_ARITH_SHIFT);
+
+ printk(KERN_DEBUG "rate_control: sample %d "
+ "err_prop %d err_int %d err_der %d adj %d\n",
+ pf >> RATE_CONTROL_ARITH_SHIFT,
+ err_prop >> RATE_CONTROL_ARITH_SHIFT,
+ err_int >> RATE_CONTROL_ARITH_SHIFT,
+ err_der >> RATE_CONTROL_ARITH_SHIFT,
+ adj);
+
+ /* Change rate. */
+ if (adj)
+ rate_control_adjust_rate(local, sta, adj);
}
if (srctrl->avg_rate_update + 60 * HZ < jiffies) {
--
1.5.3.4
-
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