Number of wireless clients in AP mode is very limited

[martignoni]

I'm reporting this on Jamesh's advice (see this forum discussion).

Describe the bug
On RPi 3B+, on can expect an optimistic max. of 20-30 devices connected simultaneously via Wi-Fi, even when no under-voltage event occurs.

To reproduce
Use case: the RPi serves as a wifi AP and a web server, that's all. No user is logged in the RPi in production use. An official Raspberry Foundation power supply and cable is used, so that no under-voltage event occurs.

Nginx and php-fpm are optimized so that memory is not a problem. Flooding the web server using ApacheBench shows that memory is not a problem: available memory remains at around 600M at any time, even with 100 concurrent http requests! Swap is never used.

The limitation experienced is uniquely caused by concurrent wifi connections to the AP (hostapd). For that reason, I'm suspecting it's a wifi driver issue.

Expected behaviour
More client devices than a mere 20 can be simultaneously connected to the RPi in AP mode.

Actual behaviour
When about 20 devices are connected to the RPi in AP mode, the RPi freezes.

System

• Model: Pi3B+
• OS and version: Raspberry Pi reference 2019-04-08
• Firmware: version 2e98b31d18547962e564bdf88e57b3df7085c29b (clean) (release) (start_cd)
• Kernel: Linux moodlebox 4.19.42-v7+ system freeze while unplug r8152 usb2network card #1219 SMP Tue May 14 21:20:58 BST 2019 armv7l GNU/Linux

Logs
Not applicable.

Additional context
This is the content of my /etc/hostapd/hostapd.conf:

# Set country code
country_code=US
# Name of the Wi-Fi interface
interface=wlan0
# Use the nl80211 driver
driver=nl80211
# Wi-Fi network name (SSID)
ssid2=4d6f6f646c65426f78
utf8_ssid=1
# Show or hide SSID
ignore_broadcast_ssid=0
# Use the 2.4GHz band
hw_mode=g
# The Wi-Fi channel
channel=11
# Enable 802.11n
ieee80211n=1
# Enable WMM
wmm_enabled=1
# Enable 40 MHz channels with short guard interval for 20 Mhz
#ht_capab=[MAX-AMSDU-3839][HT40+][SHORT-GI-20][SHORT-GI-40][DSSS_CCK-40]
ht_capab=[HT40][SHORT-GI-20][DSSS_CCK-40]
# Accept all MAC addresses
macaddr_acl=0
# Use WPA authentication
auth_algs=1
# Use WPA2
wpa=2
# Use a pre-shared key
wpa_key_mgmt=WPA-PSK
# The network passphrase
wpa_passphrase=moodlebox
# Use AES, instead of TKIP
rsn_pairwise=CCMP
# Enable hostapd_cli
ctrl_interface=/var/run/hostapd
ctrl_interface_group=0

[Choupinounours]

Mhhh I meet the same problem ! Could be really useful to fix it !

[marcusdiy]

I can connect only 9 devices with RPi 3B+ ...

[marcusdiy]

Seems like every time, before reaching the quota, there is like some kind undervoltage issue...

Jul 28 20:49:07 dnsmasq-dhcp[896]: DHCPACK(ap0) 10.3.141.110 xx:xx:xx:xx:xx:xx HUAWEI_P9_lite
Jul 28 20:49:12 spherae kernel: [ 440.556265] Under-voltage detected! (0x00050005)
Jul 28 20:49:16 spherae kernel: [ 444.716262] Voltage normalised (0x00000000)
Jul 28 20:49:21 dnsmasq-dhcp[896]: DHCPDISCOVER(ap0) xx:xx:xx:xx:xx:xx
Jul 28 20:49:21 dnsmasq-dhcp[896]: DHCPOFFER(ap0) 10.3.141.108 xx:xx:xx:xx:xx:xx
Jul 28 20:49:21 dnsmasq-dhcp[896]: DHCPREQUEST(ap0) 10.3.141.108 xx:xx:xx:xx:xx:xx
Jul 28 20:49:21 dnsmasq-dhcp[896]: DHCPACK(ap0) 10.3.141.108 xx:xx:xx:xx:xx:xx Galaxy-S7
Jul 28 20:50:48 spherae kernel: [ 536.727290] brcmfmac: send_key_to_dongle: wsec_key error (-52)

I'm using a duracell power bank rated for 2.4A, with no peripherals other than a LAN cable

[martignoni]

@marcusdiy You should absolutely be certain to use a adequate power supply and cable before any testing, so that not under-voltage issue occurs in these testing.

I'll edit my bug report to positively indicate that a limited number of clients occurs even when no under-voltage event occurs.

Edit: though, thank you for the confirmation that under-voltage has an drastic effect on the number of wireless clients

[christian-westphal]

RPI is a very nice plateform for a lot of "boxes" (moodlebox, piratebox etc) so this resolution willy be very usefull...

Thanks

[marcusdiy]

I were able to connect more than 20+ devices using ubuntu mate + snap wifi-ap
It is some kind of software limit that we got there...

Any news about this?

I manage to get exactly 19 connections on a Rpi 3b and 14 connections on a Rpi 3b+ with the following hostapd conf on Raspbian Stretch:

interface=wlan0
driver=nl80211
ssid=xxxx
hw_mode=g
channel=7
macaddr_acl=0
auth_algs=3

I will soon proceed to new tests on Raspbian Buster.

[pelwell]

Both 43438 and 43455 are designed for "mobile or handheld wireless system[s]". As such, 19 and 14 sounds pretty good. I suspect they both have relatively small RAMs that also need to contain the firmware, so resources are going to be limited.

[leond08]

any news about this?

[martignoni]

Some reports seem to indicate that the issue is worse on RPi 4 hardware, where max. number of wireless clients is sometimes less than 10!

Ref: https://discuss.moodlebox.net/d/198-wifi-client-advice/6

Same results here, 8 devices connected with a Rapsberry Pi 4 on Buster... but same with a 3b+
Having a few Rapsberry Pi in production for more than a year now with hostapd, the real dropdown in number of users connected simultaneously was seen by upgrading from Stretch to Buster.