Stream: wasmtime

Topic: Preemptive Threading for Wasmtime

Jerry Yang (Dec 12 2025 at 06:48):

Hi folks, my friend and I are working on supporting preemptive threading in Wasmtime (i.e., adding the scheduling part to time-slice running fibers and perform runtime-managed context switches, right now, round-robin only). This work is built directly on top of the existing cooperative threading context-switching work last month. Our current prototype is x86_64-only, since the runtime currently has full support only on x86_64.

We have an initial implementation exploring two interrupt mechanisms:

• epoch interrupt, driven by a ticker thread to trigger context switches with fixed time-elasped, and
• fuel metering, which basically works but currently still has some resources related issues around sync/async interactions that we are still debugging.

This has been a fun project, but we would like to sanity-check the direction with the Wasmtime team and the broader community:

• Is there any existing plan to have a proposal or design around preemptive threading?
• If not, would it be preferable for us to draft something and share it for discussion, and if so, should that discussion start on Zulip or GitHub?

This work is primarily exploratory and for fun, but we are also hoping to align on whether a proposal is plausible and, if so, to have a concrete target to guide further implementation.

Thanks!

Joel Dice (Dec 15 2025 at 14:23):

Sounds like a great project!

Jerry Yang said:

• Is there any existing plan to have a proposal or design around preemptive threading?

Not that I'm aware of -- at least not for the component model.

• If not, would it be preferable for us to draft something and share it for discussion, and if so, should that discussion start on Zulip or GitHub?

A PR to the component-model would probably be a good place for a draft if you've got something concrete to share. If you're still feeling out the design space, an issue on that repo and/or a discussion on Zulip would also be appropriate.

Joel Dice (Dec 15 2025 at 16:36):

The main question that comes to mind for me is: How might this be implemented in other runtimes which might not have an equivalent to epoch interruption or fuel metering?

Last updated: Jan 10 2026 at 20:04 UTC