kogus 2026-03-05 15:14 UTC link

Slightly tangential, but why is the first diagram duplicated at .1 opacity?

ratrocket 2026-03-05 15:17 UTC link

discussed in 2016: https://news.ycombinator.com/item?id=10872209 (53 comments)

lmz 2026-03-05 15:17 UTC link

Seems similar to the SEDA architecture https://en.wikipedia.org/wiki/Staged_event-driven_architectu...

bee_rider 2026-03-05 15:38 UTC link

> One thread per core, pinned (affinity) to separate CPUs, each with their own epoll/kqueue fd

> Each major state transition (accept, reader) is handled by a separate thread, and transitioning one client from one state to another involves passing the file descriptor to the epoll/kqueue fd of the other thread.

So this seems like a little pipeline that all of the requests go through, right? For somebody who doesn’t do server stuff, is there a general idea of how many stages a typical server might be able to implement? And does it create a load-balancing problem? I’d expect some stages to be quite cheap…

luizfelberti 2026-03-05 15:40 UTC link

A bit dated in the sense that for Linux you'd probably use io_uring nowadays, but otherwise it's a timeless design

Still, I'm conflicted on whether separating stages per thread (accept on one thread and the client loop in another) is a good idea. It sounds like the gains would be minimal or non-existent even in ideal circumstances, and on some workloads where there's not a lot of clients or connection churn it would waste an entire core for handling a low-volume event.

I'm open to contrarian opinions on this though, maybe I'm not seeing soemthing...

fao_ 2026-03-05 16:08 UTC link

this is more or less, in some way, what Erlang does and how Erlang is so easy to scale.

epicprogrammer 2026-03-05 16:11 UTC link

It’s an interesting throwback to SEDA, but physically passing file descriptors between different cores as a connection changes state is usually a performance killer on modern hardware. While it sounds elegant on a whiteboard to have a dedicated 'accept' core and a 'read' core, you end up trading a slightly simpler state machine for massive L1/L2 cache thrashing. Every time you hand off that connection, you immediately invalidate the buffers and TCP state you just built up. There’s a reason the industry largely settled on shared-nothing architectures like NGINX having a single pinned thread handle the entire lifecycle of a request keeps all that data strictly local to the CPU cache. When you're trying to scale, respecting data locality almost always beats pipeline cleanliness.

rot13maxi 2026-03-05 16:39 UTC link

i havent seen an sdf1.org url in a looooong time. lovely to see its still around

password4321 2026-03-05 17:27 UTC link

Always interesting to review the latest techempower web framework benchmarks, though it's been a year:

https://www.techempower.com/benchmarks/#section=data-r23&tes...

tecleandor 2026-03-05 15:33 UTC link

That plus the ellipsis makes me thing that it means the additional threads that would open for next connections...

eklavya 2026-03-05 16:01 UTC link

It is not a good idea, especially with the new chiplet/CCX processors.

raggi 2026-03-05 16:07 UTC link

It’s not a good idea and that’s where I’d really start with the dated commentary here rather than focusing on the polling mechanism. It depends on the application but if the buffers are large (>=64kb) such as a common TCP workload then uring won’t necessarily help that much. You’ll gain a lot of scalability regardless of polling mechanism by making sure you can utilize rss and xss optimizations.

jfindley 2026-03-05 16:14 UTC link

io_uring is in a curious place. Yes it does offer significant performance advantages, but it continues to be such a consistent source of bugs - many with serious security implications - that it's questionable if it's really worth using.

I do agree that it's a bit dated and today you'd do other things (notably SO_REUSEPORT), just feel that io_uring is a questionable example.

marcosdumay 2026-03-05 16:23 UTC link

> For somebody who doesn’t do server stuff, is there a general idea of how many stages a typical server might be able to implement?

On the HTTP server from the article, what I understood is that those 2 you are seeing are the ones you have. Or maybe 3, if disposing of things is slow.

I'm not sure what I prefer. On one hand, there's some expensive coordination for passing those file descriptors around. On the other hand, having some separate code bother with creating and closing the connections make it easier to focus on the actual performance issues where they appear, and create opportunity to dispatch work smartly.

Of course, you can go all the way in and make a green threads server where every bit of IO puts the work back on the queue. But you would use a single queue then, and dispatch the code that works on it. So you get more branching, but less coordination.

toast0 2026-03-05 17:01 UTC link

You could presumably have an acceptor thread per core, which passes the fds to core alligned next thread, etc.

That would get you the code simplicity benefits the article suggests, while keeping the socket bound to a single core, which is definitely needed.

Depending on if you actually need to share anything, you could do process per core, thread per loop, and you have no core to core communication from the usual workings of the process (i/o may cross though)

kev009 2026-03-05 18:34 UTC link

Well, kernels grown some support for steering accept() to worker thread directly. For instance SO_REUSE_PORT (Linux)/SO_REUSE_PORT_LB (FreeBSD).

vlovich123 2026-03-05 18:38 UTC link

While I agree that shared nothing wipes the pants performance-wise of shared state, surely the penalty you've outlined is only for super short lived connections?

For longer lived connections the cache is going to thrash on an inevitable context switch anyway (either do to needing to wait for more I/O or normal preemption). As long as processing of I/O is handled on a given core, I don't know if there is actually such a huge benefit. A single pinned thread for the entire lifecycle has the problem that you get latency bottlenecks under load where two CPU-heavy requests end up contending for the same core vs work stealing making use of available compute.

The ultimate benefit would be if you could arrange each core to be given a dedicated NIC. Then the interrupts for the NIC are arriving on the core that's processing each packet. But otherwise you're already going to have to wake up the NIC on a random core to do a cross-core delivery of the I/O data.

TLDR: It's super complex to get a truly shared nothing approach unless you have a single application and you correctly allocate the work. It's really hard to solve generically optimally for all possible combinations of request and processing patterns.

jauntywundrkind 2026-03-05 22:33 UTC link

In node.js I've seen time and time again some slow task that happens only every now and then, but which causes significant latency spikes. Having the one single event loop, with tasks big and small, from all stages of the processing pipeline mixed in, feels so crude. I really want a more sophisticated architecture where different stages of the execution can be managed independently.

I also want to mention that very very very few programs do, but io_uring does let you run multiple io_urings!! Your program can pick from which completion queue it wants to read, can put high priority tasks in a specific iou.

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