SBCL Fibers – Lightweight Cooperative Threads (opens in new tab)

(atgreen.github.io)

147 pointsanonzzzies11d ago35 comments

35 comments

256Kb stack per Fiber is still insane overhead compared to Actors. I guess if we survey programming community, I’d guesstimate that less than 2% of devs even know what the Actor model is, and an even smaller percentage have actually used it in production.

Any program that has at least one concurrent task that runs on a thread (naturally they’ll be more than one) is a perfect reason to switch to Actor programming model.

Even a simple print() function can see performance boost from running on a 2nd core. There is a lot of backround work to print text (parsing font metrics, indexing screen buffers, preparing scene graphs etc) and its really inefficient to block your main application while doing all this work while background cores sit idle. Yet most programmers dont know about this performance boost. Sad state of our education and the industry.

my-next-account11d ago

Actors are a model, I have no clue why you're saying that there is a particular memory cost to them on real hardware. To me, you can implement actors using fibers and a postbox.

I've no idea what the majority of programmers know or do not know about, but async logging isn't unknown and is supported by libraries like Log4j.

vitaminCPP10d ago

Yeah that was my also my thought.

I always understood that if you give a thread to each actor you get the "active object" design pattern.

zelphirkalt11d ago

I remember Joe Armstrong saying something like 2kB in his talks, for an Erlang process. That's 1/128 of 256kB.

1 more reply

atgreen11d ago

256k is just's just a placeholder for now. The default will get reduced as we get more experience with the draft implementation. The proposal isn't complete yet.

hrmtst9383710d ago

People fixate on stack size, but memory fragmentation is what bites as fiber counts grow, and actors dodge some of that at the cost of more message-passing overhead plus debugging hell once state gets hairy. Atomics or explicit channels cost cycles that never show up in naive benchmarks. If you need a million concurrent 'things' and they are not basically stateless, you're already in Erlang country, and the rest is wishful thinking.

tinco10d ago

What is more expensive, copying the message, or memory fencing it, or do you always need both in concurrent actors? Are you saying the message passing overhead is less than the cost of fragmented memory? I wouldn't have expected that.

1 more reply

praptak10d ago

The stack size is just mmapp-ed address space. It only needs backing memory for the pages actually used by the stack.

20k11d ago

Fibers are primarily when you have a problem which is easily expressible as thread-per-unit-of-work, but you want N > large. They can be useful for eg a job system as well, and in that case the primary advantage is the extremely low context switch time, as well as the manual yielding

There are lots of problems where I wouldn't recommend fibers though

mark_l_watson10d ago

I am excited by the proposal and early work. SBCL Common Lisp is my second most used programming language - glad to see useful extensions. Most of my recent experiments with SBCL involve tooling to be called by LLMs/agents and high speed tooling to provide LLMs/agents with better long term memory and context. Fibers will be useful for most of that work.

jjtheblunt10d ago

what's your first most used programming language?

mark_l_watson10d ago

Python because so much of my paid work in the last decade has been in deep learning.

nothrabannosir11d ago

I strongly recommend having a look at the mailing list to get some context:

https://sourceforge.net/p/sbcl/mailman/sbcl-devel/thread/CAF...

and

https://sourceforge.net/p/sbcl/mailman/sbcl-devel/thread/CAC...

This will certainly speak to some people taking part in some of the more controversial discussions taking place on HN recently, to put it mildly.

anonzzziesOP11d ago

Hmm, must have missed that ; tried to find. There was a SBCL discussion a few days ago but didn't read much controversial things in that? I'm a fanboy though so possibly i'm blind to these things.

ragall10d ago

I think he was referring to "Begone, slop men", which is the right answer to this.

beepbooptheory10d ago

Idk if I can quite place it but by the time it gets to, "I've created github issues for each section of your reviews.." in the second link its just so infuriating. Just want to shake them and say "for the love of god just talk to them"!

HexDecOctBin11d ago

Is there a similar document for the memory arena feature? I tried searching the official documentation, but found scant references and no instructions on how and when to use it.

dmpk2k11d ago

Huh, you're right.

Apparently it's still considered experimental (even though Google uses it in production) so it's not in the User Manual. There's this: https://github.com/sbcl/sbcl/blob/master/doc/internals-notes...

nesarkvechnep10d ago

Every move in the concurrency direction is good but I really wanted to see preemptive scheduling and Erlang-like processes.

justinhj11d ago

They should be called Anthony Green Threads. Seriously though, great to see.

matthewfcarlson11d ago

I personally like the name fiber better than green threads. But everywhere I’ve worked in user space cooperative threads, it’s always been green threads.

lll-o-lll11d ago

They are different things perhaps? Fibers imply strict cooperative behaviour; I have to explicitly “yield” to give the other fibers a go, green threads are just runtime managed threads?

varjag10d ago

Green threads are cooperative threads. Preemption requires ability to handle hardware interrupts, which are typically handled by the OS.

1 more reply

pestatije11d ago

SBCL - Steel Bank Common Lisp

theParadox4211d ago

I really thought this was gonna be a sick material science paper. Still cool though

lukasb11d ago

Serious question - I thought LLMs were bad at balancing parentheses?

cmrdporcupine10d ago

I had some ideas for extending the lem editor (emacs in common lisp) the other day and I am barely literate in Lisp. So I had Claude Code do it.

Fully awesome. No problems. A few paren issues, buit it seemed to not really struggle really. It produced working code. Was also really good at analyzing the lem codebase as well.

I even had it write an agentic coding tool in Common Lisp using the RLM ideas: https://alexzhang13.github.io/blog/2025/rlm/

Lisp is a natural fit for this kind of thing. And it worked well.

(I also suspect if parens were really a problem... there's room here for MCP or other tooling to help. Basically paredit but for agents)

atgreen11d ago

They are much better these days.

ivanb10d ago

Besides, one can easily code a skill+script for detecting the problem and suggesting fixes. In my anecdotal experience it cuts down the number of times dumber models walk in circle trying to balance parens.

j / k navigate · click thread line to collapse

35 comments

smallstepforman11d ago

Any program that has at least one concurrent task that runs on a thread (naturally they’ll be more than one) is a perfect reason to switch to Actor programming model.

my-next-account11d ago

Actors are a model, I have no clue why you're saying that there is a particular memory cost to them on real hardware. To me, you can implement actors using fibers and a postbox.

I've no idea what the majority of programmers know or do not know about, but async logging isn't unknown and is supported by libraries like Log4j.

vitaminCPP10d ago

Yeah that was my also my thought.

I always understood that if you give a thread to each actor you get the "active object" design pattern.

zelphirkalt11d ago

I remember Joe Armstrong saying something like 2kB in his talks, for an Erlang process. That's 1/128 of 256kB.

1 more reply

atgreen11d ago

256k is just's just a placeholder for now. The default will get reduced as we get more experience with the draft implementation. The proposal isn't complete yet.

hrmtst9383710d ago

tinco10d ago

1 more reply

praptak10d ago

The stack size is just mmapp-ed address space. It only needs backing memory for the pages actually used by the stack.

20k11d ago

There are lots of problems where I wouldn't recommend fibers though

mark_l_watson10d ago

jjtheblunt10d ago

what's your first most used programming language?

mark_l_watson10d ago

Python because so much of my paid work in the last decade has been in deep learning.

nothrabannosir11d ago

I strongly recommend having a look at the mailing list to get some context:

https://sourceforge.net/p/sbcl/mailman/sbcl-devel/thread/CAF...

and

https://sourceforge.net/p/sbcl/mailman/sbcl-devel/thread/CAC...

This will certainly speak to some people taking part in some of the more controversial discussions taking place on HN recently, to put it mildly.

anonzzziesOP11d ago

Hmm, must have missed that ; tried to find. There was a SBCL discussion a few days ago but didn't read much controversial things in that? I'm a fanboy though so possibly i'm blind to these things.

ragall10d ago

I think he was referring to "Begone, slop men", which is the right answer to this.

beepbooptheory10d ago

HexDecOctBin11d ago

Is there a similar document for the memory arena feature? I tried searching the official documentation, but found scant references and no instructions on how and when to use it.

dmpk2k11d ago

Huh, you're right.

Apparently it's still considered experimental (even though Google uses it in production) so it's not in the User Manual. There's this: https://github.com/sbcl/sbcl/blob/master/doc/internals-notes...

nesarkvechnep10d ago

Every move in the concurrency direction is good but I really wanted to see preemptive scheduling and Erlang-like processes.

justinhj11d ago

They should be called Anthony Green Threads. Seriously though, great to see.

matthewfcarlson11d ago

I personally like the name fiber better than green threads. But everywhere I’ve worked in user space cooperative threads, it’s always been green threads.

lll-o-lll11d ago

They are different things perhaps? Fibers imply strict cooperative behaviour; I have to explicitly “yield” to give the other fibers a go, green threads are just runtime managed threads?

varjag10d ago

Green threads are cooperative threads. Preemption requires ability to handle hardware interrupts, which are typically handled by the OS.

1 more reply

pestatije11d ago

SBCL - Steel Bank Common Lisp

theParadox4211d ago

I really thought this was gonna be a sick material science paper. Still cool though

lukasb11d ago

Serious question - I thought LLMs were bad at balancing parentheses?

cmrdporcupine10d ago

I had some ideas for extending the lem editor (emacs in common lisp) the other day and I am barely literate in Lisp. So I had Claude Code do it.

Fully awesome. No problems. A few paren issues, buit it seemed to not really struggle really. It produced working code. Was also really good at analyzing the lem codebase as well.

I even had it write an agentic coding tool in Common Lisp using the RLM ideas: https://alexzhang13.github.io/blog/2025/rlm/

Lisp is a natural fit for this kind of thing. And it worked well.

(I also suspect if parens were really a problem... there's room here for MCP or other tooling to help. Basically paredit but for agents)

atgreen11d ago

They are much better these days.

ivanb10d ago

j / k navigate · click thread line to collapse