SQM (Smart Queue Management)

OpenWrt has a package called SQM for mitigating bufferbloat, the undesirable latency that arises when your router buffers too much data.

Install the luci-app-sqm package (or sqm-scripts if you don't use LuCI) and read below.

SQM is an integrated system that performs per-packet/per flow network scheduling, active queue length management (AQM), traffic shaping/rate limiting, and QoS (prioritization). “Classic” QoS does prioritization only. “Classic” AQM manages queue lengths only.

Bufferbloat is most evident when the connection is heavily loaded with downloads or uploads which causes increased latency, or ping. It causes poor performance for VoIP and video chat, online games to lag, and makes people observe, “the Internet is not responsive.” This lag is mitigated with SQM and a small trade-off to peak throughput.

SQM is heavily CPU-based. Slower devices may not be able to keep up with your connection speed.

Before you can optimize your network, you need to know its current state. When your network is quiet, run a speed test to find your peak down/upload link speeds:

  • Run a speed test from Waveform or Speedtest. Both display the latency during download and upload traffic, and show your bufferbloat prior to enabling SQM.
  • If you are using this OpenWrt device as an Extender, Repeater or Bridge, test the bufferbloat of your upstream network device (OpenWrt or otherwise) and determine if an issue is present there first.

To enable and configure SQM in LuCI go to Network → SQM QoS. The default values will work, however you can improve performance by setting values specific to your internet connection described below:

  1. In the Basic Settings tab:
    • Check the Enable box
    • Set the Interface name to your wide area network (WAN) link. Interfaces are listed in the dropdown, or check Network → Interfaces to find the WAN port.
    • Set the Download and Upload speeds to 90% of what you measured in Preparation
  2. In the Queue Discipline tab:
    • Choose cake as the Queueing Discipline (or fq_codel, consider note 2)
    • Choose piece_of_cake.qos as the Queue Setup Script
    • Advanced Configuration may be left unchecked
  3. In the Link Layer Adaptation tab, select your link type (optional: set mpu see note 3):
    • For VDSL - Choose Ethernet, and set overhead 34 (or 26 if you're not using PPPoE) (mpu 68). If the link uses 100 Mbps ethernet, set overhead 42 (mpu 84).
    • For DSL of any other type - Choose ATM, and set overhead 44 (mpu 96)
    • For DOCSIS Cable - Choose Ethernet, and set overhead 22 (mpu 64). If you set the shaper rate to > 760 Mbps, set overhead 42 (mpu 84), because now 1 Gbps ethernet between modem and router affects the worst-case per-packet-overhead.
    • For Ethernet or Fiber to the premises - Choose Ethernet, and set overhead 44 (mpu 84)
    • If unsure, it's better to overestimate - Choose overhead 44 (mpu 96)
  4. Click Save & Apply.

That's it! You can confirm mitigation of bufferbloat by re-running the speedtest. Any increased ping during download/uploads will now be minimal.

As an example, the user below is running OpenWrt with kernel 5.15 on a WRT32X router. The internet is a DOCSIS cable modem with 300/20 Mbit service. Note this ISP is including ~15% over-provisioning on downloads. Speedtests were run before and after enabling SQM. SQM Cake was enabled with 90% dl/ul limits of baseline speedtest values. Increased latency under load dropped to zero, lower ping with no packet loss is observed during VoIP and online gaming during heavy internet usage. Link to user's speedtest results.

Bufferbloat Results - before and after SQM
QoS Download Upload Unloaded Ping DL Latency UL Latency Quality grade Bufferbloat grade
None 345 Mbits 20 Mbits 12 ms +22 ms +31 ms B B
SQM 312 Mbits 18 Mbits 12 ms +0 ms +0 ms A+ A+

1. The steps above will handle latency well with 85-95% limits on your peak throughput as a great starting point. But you can often improve speed and latency further via a couple tests to adjust the settings with these steps:

  • Increase the Download speed limit and retest until latency begins to increase, then go back to a slightly lower value.
  • Repeat the same for the Upload speed.
  • Use DSLReports Speedtest and/or Waveform's Blufferbloat test while configuring to achieve A+ quality and A+ bufferbloat grades when the optimal settings are found.
  • For DSL, the experiments above may produce Download and Upload values that are actually higher than the original speed test results. This is ok, the ATM framing bytes of a DSL link add an average of 9% overhead, and these settings simply tell SQM how to make up for that overhead.
  • For DOCSIS cable, Some cable providers trick speed tests by adding an extra 10-15% over-provisioning only for the first 10 seconds (so speed tests look better!).

2. Cake is often the preferred algorithm as it is robust and efficient in mitigating bufferbloat. However, fq_codel is often a faster, albeit less comprehensive option. One user found fq_codel gave about 15% higher throughput when CPU limited and this email thread showed similar results.

3. To set your link mpu (read SQM Details and SQM setting question) for efficiency improvements. Setting mpu will ensure rate shaping is correct for the smallest packets.

4. Check the FAQ and Troubleshooting SQM guides for more information. See also: SQM configuration for advanced options.

5. Consider Cake tuning parameters on the man page: https://man7.org/linux/man-pages/man8/tc-cake.8.html

6. To reach A+ score in Waveform's Blufferbloat test, it is required to disable Software and Hardware flow offloading - read post.

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  • Last modified: 2023/11/10 13:58
  • by palebloodsky