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Tuesday, February 4, 2014

realtek RTL8188CUS slow on Raspberry Pi

I recently had an adventure troubleshooting slow wifi on one of my Raspberry Pi systems. No matter what I did, I could not get more than 57 kilobytes per second transfer speed to it.

First I tried different transfer methods (Samba and SSH) with no change, so it wasn't the transfer method.

Then I tested the storage. I changed to a fast USB hard drive, moved the USB hard drive around the USB hub in case of power/throughput issues, but was eventually satisfied it wasn't storage when I could get 22 megabyte throughput with a hdparm -t. I should have just done that first.

Next I suspected either USB or wifi.

A lot of forum posts suggest to try dwc_otg.speed=1 in cmdline.txt however that reduces the chipset to USB1 speeds (11Mb/sec) which wasn't a compromise I was willing to make. There was mention of something called FIQ from 2011/2012, but these improvements are included in the latest (2014-01-07) Raspbian image, so there's no need to tinker with FIQ anymore.

I wondered if it was the wifi signal, so I moved the Pi right next to the router, but no change. I plugged in an ethernet cable and the speed improved immensely. So it was either USB or wifi.

Searching around, I read many reports of people having problem with these Realtek RTL8188CUS (driver 8192cu) wifi dongles. There were several suggestions to make a file like /etc/modprobe.d/8192cu.conf and turn off the adaptor's power management features with the contents:

options 8192cu rtw_power_mgnt=0 rtw_enusbss=0

I tried this but still no luck, transfers still sat at 57kb/sec.

At this point I remembered my other Raspberry Pi was using a different USB wifi adaptor, an RaLink RT5370 (driver rt2800usb). I tried this dongle and the speed instantly improved. We can now rule out USB and wifi signal, and place the blame on the wifi adaptor.

I have two of these Realtek adaptors, a black one with EDUP on it, and a white one with COMFAST on it. Both produced the slow transfer speed, so it wasn't unique to this one adaptor.

As best I can figure out, either Realtek's driver or the implementation of the wifi hardware is rubbish, and there's no way it can be fixed.

I've ordered another RaLink dongle off eBay.


EpicLPer (Stefan K.) said...

"As best I can figure out, either Realtek's driver or the implementation of the wifi hardware is rubbish, and there's no way it can be fixed."

That's a kinda bad answer... but I guess true. Damn, I also searched hours now for a solution, why did I order this piece of shitty adapter in the first place :/

Jamie said...

At least Realtek wifi adaptors are only a few dollars off eBay. The Realtek seems to be a good choice, as it supports runnin as a wireless access point: