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mac80211/minstrel_ht: improve accuracy of throughput metric at high data rates

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At high data rates the average frame transmission durations are small
enough for rounding errors to matter, sometimes causing minstrel to use
slightly lower transmit rates than necessary.
To fix this, change the unit of the duration value to nanoseconds
instead of microseconds, and reorder the multiplications/divisions when
calculating the throughput metric so that they don't overflow or
truncate prematurely.
At 2-stream HT40 this makes TCP throughput a bit more stable.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
This commit is contained in:
Felix Fietkau 2013-03-02 21:20:12 +01:00 committed by Johannes Berg
parent 355199e02b
commit ed97a13c54
1 changed files with 13 additions and 10 deletions
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23
net/mac80211/rc80211_minstrel_ht.c
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@ -26,11 +26,11 @@
/* Number of symbols for a packet with (bps) bits per symbol */
#define MCS_NSYMS(bps) ((MCS_NBITS + (bps) - 1) / (bps))
/* Transmission time for a packet containing (syms) symbols */
/* Transmission time (nanoseconds) for a packet containing (syms) symbols */
#define MCS_SYMBOL_TIME(sgi, syms) \
(sgi ? \
((syms) * 18 + 4) / 5 : /* syms * 3.6 us */ \
(syms) << 2 /* syms * 4 us */ \
((syms) * 18000 + 4000) / 5 : /* syms * 3.6 us */ \
((syms) * 1000) << 2 /* syms * 4 us */ \
)
/* Transmit duration for the raw data part of an average sized packet */
@ -64,9 +64,9 @@
}
#define CCK_DURATION(_bitrate, _short, _len) \
(10 /* SIFS */ + \
(1000 * (10 /* SIFS */ + \
(_short ? 72 + 24 : 144 + 48 ) + \
(8 * (_len + 4) * 10) / (_bitrate))
(8 * (_len + 4) * 10) / (_bitrate)))
#define CCK_ACK_DURATION(_bitrate, _short) \
(CCK_DURATION((_bitrate > 10 ? 20 : 10), false, 60) + \
@ -211,7 +211,8 @@ static void
minstrel_ht_calc_tp(struct minstrel_ht_sta *mi, int group, int rate)
{
struct minstrel_rate_stats *mr;
unsigned int usecs = 0;
unsigned int nsecs = 0;
unsigned int tp;
mr = &mi->groups[group].rates[rate];
@ -221,10 +222,12 @@ minstrel_ht_calc_tp(struct minstrel_ht_sta *mi, int group, int rate)
}
if (group != MINSTREL_CCK_GROUP)
usecs = mi->overhead / MINSTREL_TRUNC(mi->avg_ampdu_len);
nsecs = 1000 * mi->overhead / MINSTREL_TRUNC(mi->avg_ampdu_len);
usecs += minstrel_mcs_groups[group].duration[rate];
mr->cur_tp = MINSTREL_TRUNC((1000000 / usecs) * mr->probability);
nsecs += minstrel_mcs_groups[group].duration[rate];
tp = 1000000 * ((mr->probability * 1000) / nsecs);
mr->cur_tp = MINSTREL_TRUNC(tp);
}
/*
@ -536,7 +539,7 @@ minstrel_calc_retransmit(struct minstrel_priv *mp, struct minstrel_ht_sta *mi,
mr->retry_updated = true;
group = &minstrel_mcs_groups[index / MCS_GROUP_RATES];
tx_time_data = group->duration[index % MCS_GROUP_RATES] * ampdu_len;
tx_time_data = group->duration[index % MCS_GROUP_RATES] * ampdu_len / 1000;
/* Contention time for first 2 tries */
ctime = (t_slot * cw) >> 1;