OpenBSD: PF queues break the 4 Gbps barrier

https://undeadly.org/cgi?action=article;sid=20260319125859

by defrost • 4 months ago

220 59 4 months ago

undeadly.org

bell-cot2• 4 months ago

"Values up to 999G are supported, more than enough for interfaces today and the future." - Article

"When we set the upper limit of PC-DOS at 640K, we thought nobody would ever need that much memory." - Bill Gates

WhyNotHugo• 4 months ago

Honestly, I'm really curious about this number. 10bits is 1024, so why 999G specifically?

elevation• 4 months ago

Looks like an arbitrary validation cap. By the time we're maxing out the 64-bit underlying representation we probably won't be using Ethernet any more.

abound• 4 months ago

Looking at the patch itself (linked in the article), the description has this:

We now support configuring bandwidth up to ~1 Tbps (overflow in m2sm at m > 2^40).

So I think that's it, 2^40 is ~1.099 trillion

throw0101d2• 4 months ago

"Values up to 999G are supported, more than enough for interfaces today and the future." - Article

Especially given that IEEE 802.3dj is working on 1.6T / 1600G, and is expected to publish the final spec in Summer/Autumn 2026:

* https://en.wikipedia.org/wiki/Terabit_Ethernet

Currently these interfaces are only on switches, but there are already NICs at 800G (P1800GO, Thor Ultra, ConnectX-8/9), so if you LACP/LAGG two together your bond is at 1600G.

arsome• 4 months ago

If you're moving those kind of speeds you're probably not doing packet filtering in software.

bitfilped• 4 months ago

Yes, we're already running 800G networks, so this phrasing seems really silly to me.

rayiner• 4 months ago

Can pf actually shape at speeds above 4 gbps?

daneel_w2• 4 months ago

With 7.9, shaping (read: bandwidth rate limiting) no longer tops out at 4 Gbps. PF could always process/transfer beyond 4 Gbps, presuming you had fast enough hardware to handle such bandwidth. The discussed limit was specific only to queues, when using such to shape traffic.

If you're asking about OpenBSD/PF's general network performance, it is finally performing acceptably since a couple of years back. You can easily saturate a 2.5 GbE interface with low-end hardware.

ralferoo• 4 months ago

In the days when even cheap consumer hardware ships with 2.5G ports, this number seems weirdly low. Does this mean that basically nobody is currently using OpenBSD in the datacentre or anywhere that might be expecting to handle 10G or higher per port, or is it just filtering that's an issue?

I'm not surprised that the issue exists as even 10 years ago these speeds were uncommon outside of the datacentre, I'm just surprised that nobody has felt a pressing enough need to fix this earlier in the previous few years.

citrin_ru• 4 months ago

AFAIK performance is not a priority for OpenBSD project - security is (and other related qualities like code which is easy to understand and maintain). FreeBSD (at least when I followed it several years ago) had better performance both for ipfw and its own PF fork (not fully compatible with OpenBSD one).

traceroute663• 4 months ago

AFAIK performance is not a priority for OpenBSD project - security is

TBF that was the case historically, but they have absolutely been putting in an effort into performance in their more recent releases.

Lots of stuff that used to be simply horrific on OpenBSD, such as multi-peer BGP full-table refreshes is SIGNIFICANTLY better in the last couple of years.

Clearly still not as good as FreeBSD, but compared to what it was...

TuxPowered• 4 months ago

not fully compatible with OpenBSD one

The OpenBSD NAT and scrub syntax, and af-to are available in FreeBSD 15.

toast04• 4 months ago

Does this mean that basically nobody is currently using OpenBSD in the datacentre or anywhere that might be expecting to handle 10G or higher per port, or is it just filtering that's an issue?

This looks like it only affects bandwidth limiting. I suspect it's pretty niche to use OpenBSD as a traffic shaper at 10G+, and if you did, I'd imagine most of the queue limits would tend toward significantly less than 4G.

IcePic• 4 months ago

One thing could also be that by the time you have 10GE uplinks, shaping is not as important.

When we had 512kbit links, prioritizing VOIP would be a thing, and for asymmetric links like 128/512kbit it was prudent to prioritize small packets (ssh) and tcp ACKs on the outgoing link or the downloads would suffer, but when you have 5-10-25GE, not being able to stick an ACK packet in the queue is perhaps not the main issue.

traceroute663• 4 months ago

Does this mean that basically nobody is currently using OpenBSD in the datacentre or anywhere

Half the problem is lack of proper drivers. I love OpenBSD but all the fibre stuff is just a bit half-baked.

For a long time OpenBSD didn't even have DOM (light-level monitoring etc.) exposed in its 1g fibre drivers. Stuff like that automatically kills off OpenBSD as a choice for datacentres where DOM stats are a non-negotiable hard requirement as they are so critical to troubleshooting.

OpenBSD finally introduced DOM stats for SFP somewhere around 2020–2021, but it doesn't always work, it depends if you have the right magic combination of SFP and card manufacturer. Whilst on FreeBSD it Just Works (TM).

And then overall, for higher speed optics, FreeBSD simply remains lightyears ahead (forgive the pun !). For example, Decisio make nice little router boxes with 10g SFP+ on them, FreeBSD has the drivers out-of-the-box, OpenBSD doesn't. And that's only an SFP+ example, its basically rolling-tumbleweed in a desert territory if you start venturing up to QSFP etc. ...

CursedSilicon• 4 months ago

How much work is it to port drivers between Free and Open BSD?

Someone2• 4 months ago

The article is about allowing bandwidth restrictions in bytes/second that are larger than 2³²-1, not about how fast pf can filter packets.

I guess few people with faster ports felt the need to limit bandwidth for a service to something that’s that large.

FTA:

“OpenBSD's PF packet filter has long supported HFSC traffic shaping with the queue rules in pf.conf(5). However, an internal 32-bit limitation in the HFSC service curve structure (struct hfsc_sc) meant that bandwidth values were silently capped at approximately 4.29 Gbps, ” the maximum value of a u_int ".

With 10G, 25G, and 100G network interfaces now commonplace, OpenBSD devs making huge progress unlocking the kernel for SMP, and adding drivers for cards supporting some of these speeds, this limitation started to get in the way. Configuring bandwidth 10G on a queue would silently wrap around, producing incorrect and unpredictable scheduling behaviour.

A new patch widens the bandwidth fields in the kernel's HFSC scheduler from 32-bit to 64-bit integers, removing this bottleneck entirely.”

nine_k2• 4 months ago

silently wrap around, producing incorrect and unpredictable

Now I'm more scared to use OpenBSD than I was a minute before.

I strongly prefer software that fails loudly and explicitly.

ffk• 4 months ago

A lot of the time once you get into multi-gig+ territory the answer isn't "make the kernel faster," it's "stop doing it in the kernel."

You end up pushing the hot path out to userland where you can actually scale across cores (DPDK/netmap/XDP style approaches), batch packets, and then DMA straight to and from the NIC. The kernel becomes more of a control plane than the data plane.

PF/ALTQ is very much in the traditional in-kernel, per-packet model, so it hits those limits sooner.

cperciva• 4 months ago

pushing the hot path out to userland where you can actually scale across cores

What sort of kernel do you have which can't scale across cores?

toast04• 4 months ago

The big things to avoid are crossing the user/kernel divide and communication across cores.

Staying in the kernel is approximately the same as bypassing the kernel (caveats apply); for a packet filtering / smoothing use case, I don't think kernel bypass is needed. You probably want to tune NIC hashing so that inbound traffic for a given shaping queue arrives in the same NIC rx queue; but you probably want that in a kernel bypass case as well. Userspace is certainly nicer during development, as it's easier to push changes, but in 2026, it feels like traffic shaping has pretty static requirements and letting the kernel do all the work feels reasonable to me.

Otoh, OpenBSD is pretty far behind the curve on SMP and all that (I think their PF now has support for SMP, but maybe it's still in development?; I'd bet there's lots of room to reduce cross core communication as well, but I haven't examined it). You can't pin userspace cores to cpus, I doubt their kernel datastructures are built to reduce communications, etc. Kernel bypass won't help as much as you would hope, if it's available, which it might not be, because you can't control the userspace to limit cross core communications.

Melatonic2• 4 months ago

Isnt OpenBSD mainly used for security testing or do I have it wrong? Would be surprised if it was used in production datacenter networking hardware at all. Seems like most people would use one of the proprietary implementations (which likely would include specific written drivers for that hardware) or something like FreeBSD

SoftTalker2• 4 months ago

It's widely used as a router, that's one of its primary uses. But not sure to what scale, likely at small orgs not at major ISPs.

But, OpenBSD is a project by and for its developers. They use it and develop it to do what they want; they don't really care what anyone else does or doesn't do with it.

lstodd• 4 months ago

You don't need 4gbps pf queues or even fiber on every single machine in a datacenter. So be surprised, it is used widely for its simplicity and reliability not to mention security compared to those proprietary implementations you speak of, may they rot in hell.

atmosx• 4 months ago

PF itself is not tailored towards ISPs and/or big orgs. IPFW (FreeBSD) is more powerful and flexible.

OpenBSD shines as a secure all-in-one router SOHO solution. And it’s great because you get all the software you need in the base system. PF is intuitive and easy to work with, even for non network gurus.

asmnzxklopqw• 4 months ago

OpenBSD was a great OS back in the late 90s and even early 2000s. In some cases it was competing neck to neck with Linux. Since then, well, Linux grew a lot and OpenBSD not so much. There are multiple causes for this, I will go only through a few: Linux has more support from the big companies; the huge difference in userbase numbers; Linux is more welcoming to new users. And the difference is only growing.

dim13• 4 months ago

"OpenBSD does not want to attract GNU newbies." misc@

And that's IMHO is a good thing.

zie2• 4 months ago

Linux is more welcoming to new users. And the difference is only growing.

Unless you want documentation. :) Linux documentation is ALL over the place, and it's hard to know if X docs apply to your particular mess.

On OpenBSD it's `man <thing>` and you get your docs. Yes man exists on Linux, but lots of tools, especially "new" stuff or linux specific stuff doesn't come with man pages and those that do have man pages, can be missing lots of stuff.

Systemd docs tend to be somewhat useless, missing all sorts of stuff. Troubleshooting systemd by reading the docs is usually a waste of time. Reading the source code or getting lucky with a web search is usually your only hope.

On FreeBSD you get `man <thing>` AND you get the FreeBSD handbook: https://docs.freebsd.org/en/books/handbook/

daneel_w2• 4 months ago

Not every in-and-out must pass through a queue in PF. The limitation specifically affected throughput of queues.

PunchyHamster2• 4 months ago

There is no reason to use OpenBSD (aside "we have OpenBSD nerds in staff" or I guess "we don't want to GPL our changes"), we had it ages ago (for the first of mentioned reasons) but even they dumped it once new server hardware wasn't supported.

chokan• 4 months ago

dsa

koala_man• 4 months ago

OpenBSD devs making huge progress unlocking the kernel for SMP

Isn't this anachronistic for 2026? Am I misunderstand what this means?

gigatexal• 4 months ago

It’s still single threaded. PF in FreeBSD is multithreaded. For home wan’s I’d be using openBSD. For anything else FreeBSD.

haunter• 4 months ago

My local fiber finally offers 4 Gbps connection but I’m not even sure what to use it for lol. I have 2 Gbps and that's more than enough already.

shpingbing• 4 months ago

I finally talked myself into going to 3Gbps (and working on internal network to 10). Internal transfer to NAS will be much faster, and downloading AI models should go from ~8 minutes to less than 3 minutes. Is it necessary? Not exactly. But super nice

darknavi• 4 months ago

I do nightly offsite mirroring (just to a cloud provider) and making that go faster and not cannibalize all of my throughput is nice.

razighter777• 4 months ago

I would love to use openbsd. I really wanna give it a try but the filesystem choices seem kinda meh. Are there any modern filesystems with good nvme and FDE support for openbsd.

OpenBSD: PF queues break the 4 Gbps barrier

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