FS#2573 - BT Home Hub 5A xrx200 performance degradation caused by RPS XPS commit #7670
Known to affect BT Home Hub 5A. Other similar Lantiq xrx200 devices are likely to be affected too.
When OpenWrt 18.06 was released, it was observed that WAN to LAN throughput had suffered degradation. Maximum throughput dropping from 140 mbps to 80 mbps.
Some investigative testing was conducted when 18.06.1 was released.
It was discovered this commit was responsible for the fall in maximum throughput.
No one at the time thought it was a bug. In the UK, there is no support for VDSL vectoring, and so maximum DSL speeds is 80 Mbps which equates to 76 Mbps in real world speed tests through to the LAN ports.
Removing the above commit restored maximum possible throughput with 18.06.
mkresin recently took some time to look at this commit. Here are his comments:
First of all, it is about receive packet steering (rps) and transmit
Only cpu 0 should handle something: 00000001 (decimal 1)
I'm not yet sure what's the result of setting decimal 0.
The commit you already identified, changes the logic to not steer
Fun fact, the check which cpu/core handles the interrupts doesn't work
It seems to me, that on some targets this change increases the max.
As far as I remember, all that need to be done to get back the former
The same might be true for the wireless interfaces. Changing the values
The best would be if someone does some more tests to check which values
Following mkresin's suggestion, here are some test results:
The text was updated successfully, but these errors were encountered:
Fixed in master from 3rd March 2020
It will Not be backported to 19.07 releases.