Byte Queue Limits

I see that one of my routers, a TP LINK Archer C7 V4 supports this but only in the overall CPU eth0. Why is this not implemented for lan or wan? Is it because they are vif devices? I'm confused about this. It seems like a great alternative to a software rate limiter.

LAN and WAN are bridges in a typical OpenWrt configuration. Byte-queue limits, at least as I understand it, apply to hardware transmit queues.

Changing the local hardware transmit-queue length doesn't really address the carrier-side bufferbloat issues, nor does it rate-limit the interface.

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Hmm. The way it's explained here:
https://www.bufferbloat.net/projects/bloat/wiki/BQL_enabled_drivers/

BQL is desirable for devices running at line rate. It is not required for soft rate shapers such as hfsc and htb, which are independent of the underlying wire rate.

Makes it seem as though if using a qdisc such as fq_codel + bql would give line rate performance without the need for a software rate shaper and significantly reduce bufferbloat at least on the tx side of things. As applied to the tx-ring-buffers.

https://lwn.net/Articles/469652/ as one reference as to what byte-queue limits do.

I don't think there's anything "magic" about changing the limit. It just lets you reduce the on-board latency to match the achievable link rate you have in terms of the nominal, queued packet size.

Correct me if I'm wrong, this is supposed to be a dynamic control of the buffers?

Yes, but this will only work if the bottleneck link is driven directly by the interface implementing BQL. If your router sports a DSL modem and is connected via this to the internet, BQL on that ptm/atm interface would make an active traffic shaper superfluous. Alas, no dsl/gsm/lte-modem has this and as far as I can tell no docsis modem either. So in short, if your uplink is true ethernet, all you need is fq_codel or "cake unlimited" and BQL and uplink bufferbloat will be mostly gone. For downstream your ISP would need BQL and fq_codel on its end.

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The keywords being "true ethernet" resonate. So ist das leben.

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