Unstable WiFi with mt76 on MT7628AN

Rafał Miłecki zajec5 at gmail.com
Sat Jul 10 09:53:41 PDT 2021 

On 10.07.2021 02:16, Rafał Miłecki wrote:
> So it seems there are two problems I managed to narrown down:
> 
> 1. On MT7603 "rx bitrate" of STA drops 72.2 → 6 MBit/s regularly
> 2. When catching up with traffic (after slow down) WiFi occasionally stalls

Today I decided to use my Netgear R6220's 5 GHz (MT7612EN).

On that chipset "rx bitrate" reported for my STA stays between 292 and
390 for most of them time BUT is also regularly drops to 6 Mbps:

 > while [ 1 ]; do date; iw dev wlan1 station dump | grep bitrate; sleep 1; done

Sat Jul 10 15:00:27 UTC 2021
         tx bitrate:     292.5 MBit/s VHT-MCS 7 80MHz VHT-NSS 1
         rx bitrate:     390.0 MBit/s VHT-MCS 9 80MHz VHT-NSS 1
Sat Jul 10 15:00:28 UTC 2021
         tx bitrate:     433.3 MBit/s VHT-MCS 9 80MHz short GI VHT-NSS 1
         rx bitrate:     390.0 MBit/s VHT-MCS 8 80MHz short GI VHT-NSS 1
Sat Jul 10 15:00:29 UTC 2021
         tx bitrate:     433.3 MBit/s VHT-MCS 9 80MHz short GI VHT-NSS 1
         rx bitrate:     325.0 MBit/s VHT-MCS 7 80MHz short GI VHT-NSS 1
Sat Jul 10 15:00:30 UTC 2021
         tx bitrate:     390.0 MBit/s VHT-MCS 9 80MHz VHT-NSS 1
         rx bitrate:     6.0 MBit/s
Sat Jul 10 15:00:32 UTC 2021
         tx bitrate:     292.5 MBit/s VHT-MCS 7 80MHz VHT-NSS 1
         rx bitrate:     6.0 MBit/s
Sat Jul 10 15:00:33 UTC 2021
         tx bitrate:     390.0 MBit/s VHT-MCS 8 80MHz short GI VHT-NSS 1
         rx bitrate:     325.0 MBit/s VHT-MCS 7 80MHz short GI VHT-NSS 1
Sat Jul 10 15:00:34 UTC 2021
         tx bitrate:     292.5 MBit/s VHT-MCS 7 80MHz VHT-NSS 1
         rx bitrate:     325.0 MBit/s VHT-MCS 7 80MHz short GI VHT-NSS 1
Sat Jul 10 15:00:35 UTC 2021
         tx bitrate:     390.0 MBit/s VHT-MCS 8 80MHz short GI VHT-NSS 1
         rx bitrate:     325.0 MBit/s VHT-MCS 7 80MHz short GI VHT-NSS 1
Sat Jul 10 15:00:36 UTC 2021
         tx bitrate:     390.0 MBit/s VHT-MCS 9 80MHz VHT-NSS 1
         rx bitrate:     325.0 MBit/s VHT-MCS 7 80MHz short GI VHT-NSS 1

Whenever it drops to 6 MBit/s I can see iperf slowing down from 150 -
180 Mb/s down to 100 - 130 Mb/s;

[  3] 19148.0-19149.0 sec  19.1 MBytes   160 Mbits/sec
[  3] 19149.0-19150.0 sec  21.9 MBytes   184 Mbits/sec
[  3] 19150.0-19151.0 sec  18.6 MBytes   156 Mbits/sec
[  3] 19151.0-19152.0 sec  21.2 MBytes   178 Mbits/sec
[  3] 19152.0-19153.0 sec  15.6 MBytes   131 Mbits/sec ← "iw" on MT7612EN reported 6 Mb/s "rx bitrate" at that moment
[  3] 19153.0-19154.0 sec  18.8 MBytes   157 Mbits/sec
[  3] 19154.0-19155.0 sec  20.0 MBytes   168 Mbits/sec
[  3] 19155.0-19156.0 sec  20.4 MBytes   171 Mbits/sec
[  3] 19156.0-19157.0 sec  19.6 MBytes   165 Mbits/sec
[  3] 19157.0-19158.0 sec  20.2 MBytes   170 Mbits/sec
[  3] 19158.0-19159.0 sec  21.6 MBytes   181 Mbits/sec

(...)

[  3] 19207.0-19208.0 sec  18.4 MBytes   154 Mbits/sec
[  3] 19208.0-19209.0 sec  18.1 MBytes   152 Mbits/sec
[  3] 19209.0-19210.0 sec  17.6 MBytes   148 Mbits/sec
[  3] 19210.0-19211.0 sec  18.6 MBytes   156 Mbits/sec
[  3] 19211.0-19212.0 sec  22.2 MBytes   187 Mbits/sec
[  3] 19212.0-19213.0 sec  15.1 MBytes   127 Mbits/sec ← "iw" on MT7612EN reported 6 Mb/s "rx bitrate" at that moment
[  3] 19213.0-19214.0 sec  16.9 MBytes   142 Mbits/sec
[  3] 19214.0-19215.0 sec  17.1 MBytes   144 Mbits/sec
[  3] 19215.0-19216.0 sec  20.2 MBytes   170 Mbits/sec
[  3] 19216.0-19217.0 sec  21.1 MBytes   177 Mbits/sec
[  3] 19217.0-19218.0 sec  21.6 MBytes   181 Mbits/sec

(...)

[  3] 19267.0-19268.0 sec  15.9 MBytes   133 Mbits/sec
[  3] 19268.0-19269.0 sec  18.2 MBytes   153 Mbits/sec
[  3] 19269.0-19270.0 sec  17.2 MBytes   145 Mbits/sec
[  3] 19270.0-19271.0 sec  17.0 MBytes   143 Mbits/sec
[  3] 19271.0-19272.0 sec  18.2 MBytes   153 Mbits/sec
[  3] 19272.0-19273.0 sec  15.2 MBytes   128 Mbits/sec ← "iw" on MT7612EN reported 6 Mb/s "rx bitrate" at that moment
[  3] 19273.0-19274.0 sec  14.0 MBytes   117 Mbits/sec ← "iw" on MT7612EN reported 6 Mb/s "rx bitrate" at that moment
[  3] 19274.0-19275.0 sec  18.9 MBytes   158 Mbits/sec
[  3] 19275.0-19276.0 sec  16.9 MBytes   142 Mbits/sec
[  3] 19276.0-19277.0 sec  18.6 MBytes   156 Mbits/sec
[  3] 19277.0-19278.0 sec  16.2 MBytes   136 Mbits/sec
[  3] 19278.0-19279.0 sec  23.9 MBytes   200 Mbits/sec
[  3] 19279.0-19280.0 sec  21.1 MBytes   177 Mbits/sec

(...)

[  3] 19297.0-19298.0 sec  21.6 MBytes   181 Mbits/sec
[  3] 19298.0-19299.0 sec  19.9 MBytes   167 Mbits/sec
[  3] 19299.0-19300.0 sec  22.9 MBytes   192 Mbits/sec
[  3] 19300.0-19301.0 sec  21.4 MBytes   179 Mbits/sec
[  3] 19301.0-19302.0 sec  19.6 MBytes   165 Mbits/sec

[  3] 19302.0-19303.0 sec  15.0 MBytes   126 Mbits/sec ← "iw" on MT7612EN reported 6 Mb/s "rx bitrate" at that moment
[  3] 19303.0-19304.0 sec  11.0 MBytes  92.3 Mbits/sec
[  3] 19304.0-19305.0 sec  11.2 MBytes  94.4 Mbits/sec

[  3] 19305.0-19306.0 sec  20.1 MBytes   169 Mbits/sec
[  3] 19306.0-19307.0 sec  18.4 MBytes   154 Mbits/sec
[  3] 19307.0-19308.0 sec  21.1 MBytes   177 Mbits/sec
[  3] 19308.0-19309.0 sec  22.6 MBytes   190 Mbits/sec
[  3] 19309.0-19310.0 sec  21.5 MBytes   180 Mbits/sec
[  3] 19310.0-19311.0 sec  20.0 MBytes   168 Mbits/sec

In 5 GHz case however mt76 successfully recovers from all those slow
downs. Traffic didn't stop for good not even once. I had it running
for over 5 hours:

[  3]  0.0-19330.1 sec   352 GBytes   157 Mbits/sec

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