15

I'm seeing very high RX dropped packets in the output of ifconfig: Thousands of packets per second, an order of magnitude more than regular RX packets.

wlan0     Link encap:Ethernet  HWaddr 74:da:38:3a:f4:bb  
          inet addr:192.168.99.147  Bcast:192.168.99.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:31741 errors:0 dropped:646737 overruns:0 frame:0
          TX packets:18424 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:90393262 (86.2 MiB)  TX bytes:2348219 (2.2 MiB)

I'm testing WiFi dongles. Both have this problem, and the one with the higher drop rate actually performs better in ping floods. The one with low dropped packets suffers from extreme Ping RTTs, while the other never skips a beat.

  • What does Linux consider a dropped packet?
  • Why am I seeing so many of them?
  • Why doesn't it seem to affect performance?

There are lots of questions around with answers that say a dropped packet could be one of the following but that doesn't help me very much, because those possibilities don't seem to make sense in this scenario.

  • The reason you are seeing dropped can be various. The most obvious one is that packets went through the hardware filtering, and still ended up not intended for the host. Multicast for instance. – ardje Jul 12 '18 at 11:44
11

Packet Dropped seen from ifconfig could be due to many reasons, you should dig deeper into NIC statistics to figure out real reason. Below are some general reasons

  • NIC ring buffers getting full and unable to cope-up with incoming bursts of traffic
  • CPU receiving NIC interrupts is very busy and unable to process
  • some cable/hardware/duplex issues
  • some bug in NIC driver

Look at ethtool -S wlan0, iwconfig wlan0 and look at /proc/net/wireless for any further information

  • iwconfig wlan0 does not mention anything related to dropped packets, and all of its information seems normal, ethtool -S wlan0 says no stats available, and /proc/net/wireless does not exist. – PythonNut May 26 '15 at 0:26
  • what distribution of Linux are you running and what card/driver? – VenkatC May 26 '15 at 0:49
  • I'm running Debian/Raspbian on a Raspberry Pi. The driver is Realtek Semiconductor Corp. RTL8188CUS 802.11n WLAN Adapter. – PythonNut May 26 '15 at 0:50
  • Thanks, could you post output of iwconfig here? The issue here could be specific to that driver. I have not worked with this card/driver,hence I could not provide any insight. but looking at the driver, some tuning related to power management has helped others achieve better performance .. for ex: like raspberrypi.stackexchange.com/questions/11713/… – VenkatC May 26 '15 at 2:02
  • Google points to some issues with driver github.com/OpenELEC/OpenELEC.tv/issues/3042 and some more useful info blog.stuffedcow.net/2014/04/rtl8192cu-and-linux-3-13-10 - it more sounds like driver related – VenkatC May 26 '15 at 2:16

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