I cannot explain why, but now, I am getting 38-40% CPU usage with fq_codel/simplest.qos shaping with 900000 kbit/s downstream and 24000 kbit/s upstream. Really good performance on the waveform bufferbloat test. Point is no where near saturation now. I did not change any SQM packages.
Hi,
this is my first post, so I'm sorry if this is the wrong place to ask but was this issue completely fixed by switching to the UE300 or is the change that's discussed here still needed?
I have a very similar setup, RPi4B with a UE300 currently running 21.02.0-rc4 and sqm-scripts 1.5.0-2. Since I specifically got it for running CAKE (for gaming) I'm not sure if I should wait until a fixed sqm-scripts package becomes available or upgrade to 21.02.0 stable and 1.5.1-1 right away.
I'd also volunteer to run some tests but I don't know if I'd be of much help since I'm not knowledgeable enough to just edit the files myself.
I have no explanation for the massive difference. The several things that changed including a new ethernet cable and physically relocating the RPi4 to a different room/plugging it into some pre-wired panels. I found cake on this hardware to be inferior to fq_codel/simplest in my case.
Cake does a bunch of stuff that the simplest script doesn't. If you don't need those things then the simplest script is fine. Mainly they have to do with load balancing across multiple IPs and such. For a relatively small deployment with fast internet it may be overkill.
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I ran some tests using cake/layer cake and cake/piece of cake on my RPi4.
I found nearly identical results using cake or fq_codel. Average of 2 runs presented in table. Also, despite maxing out one core, cake preformed likely within error of the others.
| sqm | CPU limited? | sqm settings | unloaded latency ms | download latency ms | upload latency ms | download Mbps | upload Mbps |
|---|---|---|---|---|---|---|---|
| none | no | 18 | 21 | 10 | 942.1 | 24.1 | |
| simplest | no | 900k/24k | 18 | 3 | 1 | 857.9 | 23 |
| cake/layer cake | yes | 900k/24k | 17 | 4 | 1 | 828.7 | 22.9 |
| cake/piece of cake | yes | 900k/24k | 19 | 2 | 0 | 838.7 | 22.7 |
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Please note that shaper settings are always gross rates, while speedtests report net throughput (also called goodput)
For a normal link layer (so not ATM) you can calculate the expected maximum goodput from shaper gross rate and per-paket_over-head setting as follows:
gross_shaper_rate * ((packet_size - Internet_layer_overhead - Transport_layer_overhead - Application_layer_overhead) / (packet_size + SQM_overhead))
Application_layer_overhead is often not well known, but for on-line speedtests the HTTP/HTTPS encapsulation will be relative small and can be ignored.
So in your case we fill in the numbers (partly by guess-work):
paket_size: for a speedtest it will be mostly as large as possible which means ~1500 Bytes
Internet_layer_overhead: for IPv4 that is 20B, for IPv6 40B
Transport_layer_overhead: for an online speedtest almost invariably TCP (until QUIC will take hold), probably without TCP options, but if there are options it most likely be TCP timestamps (rfc1323) 12B
Application_layer_overhead: relative little spread over a number of packets, so we just ignore that: "0B"
SQM_overhead: what ever you configured, and if you configured nothing it will be the kernel's default of 14B, since I have no clue what you configured, but simply assuming that on a Gbps-link your ISP uses true ethernet without a vlan tag, we just assume 38B
900 * ((1500 - 20 - 20 - 0)/(1500 + 38)) = 854.36 Mbps
Given the uncertainties in my calculations, all three results seem reasonably close to the theoretical limit.
I should note an observation: placing SQM on a bridge interface (br-wan) uses much more CPU than putting it on the physical interface (eth1). I get the same good results with much less CPU use.
RPi4 (cake/piece_of_cake.qos depicted in screenshot) routing gigalan traffic without breaking a sweat. Screenshot is the waveform bufferbloat test.
- Peak CPU usage was <64% (on single core)
- Average CPU usage during 22 sec download test was 42.5% (on single core)
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