Stream: wasmtime

Topic: Store state is non-virtualizable

Dan Gohman (Mar 18 2021 at 16:36):

Wasmtime now includes features to attach state to a store, which is useful, however host APIs which depend on this are inherently unvirtualizable.

Dan Gohman (Mar 18 2021 at 16:36):

I wonder if anyone has ideas about how best to communicate that this feature is available and useful, but it's important to be aware of the implications

Peter Huene (Mar 18 2021 at 17:10):

Could you expand upon how using state from a store vs. from a closure capture (i.e. with Func) impacts virtualization so that I can understand the implications? The biggest problem I see is that state in a store is shared between all instances of that store, but users can still link in a Func-based host function implementation at instantiation time when needed. At the bare minimum, we should be properly conveying to users that these Store functions mean every instance in the store has access to the state assuming they also share the same host function import.

Dan Gohman (Mar 18 2021 at 17:12):

That's a good point, that closure state is effectively in the same category.

Peter Huene (Mar 18 2021 at 17:12):

That is also to say the problem of shared state isn't new with config-defined host functions; I can define a Func with state and import that same function into multiple instances.

Peter Huene (Mar 18 2021 at 17:12):

yeah

Peter Huene (Mar 18 2021 at 17:13):

config-defined host functions definitely made it easier to implicitly share that state though

Peter Huene (Mar 18 2021 at 17:14):

so something that needs to be documented properly

Dan Gohman (Mar 18 2021 at 17:14):

Yeah. I'm starting to think about where and how we should document that

Peter Huene (Mar 18 2021 at 17:15):

The doc comments for the Store functions are woefully inadequate in conveying this :( that's my fault

Dan Gohman (Mar 18 2021 at 17:16):

No worries :-). We're all figuring this stuff out as we go here

Peter Huene (Mar 18 2021 at 17:20):

I'm happy to do a follow-up PR to document this properly; besides calling out that the state in the store may be accessible to all the instances in the store (I can go into lots of detail so there aren't surprises), are there other concerns relating to virtualization I should convey?

Dan Gohman (Mar 18 2021 at 17:26):

One question that I don't know the answer to yet is, "who needs to worry about virtualization?"

Dan Gohman (Mar 18 2021 at 17:27):

Otherwise, I think that's it.

Last updated: Nov 22 2024 at 17:03 UTC