Stream: git-wasmtime

Topic: wasmtime / PR #15049 Document how copy-on-write memory in...

Wasmtime GitHub notifications bot (Oct 28 2026 at 19:30):

yadava5 opened PR #15049 from yadava5:docs-fuel-cow-determinism to bytecodealliance:main:

Closes #14331.

As discussed in the issue, this documents that copy-on-write memory init changes how much fuel instantiation consumes from host to host, and that disabling memory_init_cow makes it the same everywhere. The note goes on Config::consume_fuel, with a pointer from Config::memory_init_cow, the "Deterministic Fuel" section, and the deterministic execution guide.

I measured it with the module from the issue (one memory, a data segment of 5 bytes) on GitHub's runners:

host	page size	CoW, Module::new	CoW, deserialize_file	no CoW
Linux x86_64	4 KiB	0	0	7
Linux aarch64	4 KiB	0	0	7
macOS x86_64	4 KiB	4097	0	7
macOS aarch64	16 KiB	16385	0	7
Windows x86_64	4 KiB	4097	4097	7

The probe is here.

One thing I noticed while measuring: when CoW is enabled but can't be used, the copy is charged for the whole image rounded up to a page, so 5 bytes cost a full page of fuel. I kept that out of this PR since it's a behavior change, but happy to look at it separately if it's worth changing.

I used Claude Code to measure the fuel numbers and check the doc links.

Wasmtime GitHub notifications bot (Oct 28 2026 at 19:30):

yadava5 requested fitzgen for a review on PR #15049.

Wasmtime GitHub notifications bot (Oct 28 2026 at 19:30):

yadava5 removed the review request for wasmtime-core-reviewers from PR #15049.

Wasmtime GitHub notifications bot (Oct 28 2026 at 19:30):

yadava5 requested wasmtime-core-reviewers for a review on PR #15049.

Wasmtime GitHub notifications bot (Oct 28 2026 at 19:30):

yadava5 requested wasmtime-default-reviewers for a review on PR #15049.

Wasmtime GitHub notifications bot (Oct 28 2026 at 21:44):

github-actions[bot] added the label wasmtime:api on PR #15049.

Wasmtime GitHub notifications bot (Oct 28 2026 at 21:44):

github-actions[bot] added the label wasmtime:config on PR #15049.

Wasmtime GitHub notifications bot (Oct 28 2026 at 21:44):

github-actions[bot] added the label wasmtime:docs on PR #15049.

Wasmtime GitHub notifications bot (Oct 28 2026 at 22:46):

github-actions[bot] commented on PR #15049:

Label Messager: wasmtime:config

It looks like you are changing Wasmtime's configuration options. Make sure to
complete this check list:

• [ ] If you added a new Config method, you wrote extensive documentation for
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  <details>

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  Example

  Optional example here.
  ```

  </details>

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  <details>

  For example, if you expose a new strategy for allocating the next instance
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  Often, all that is required of you is to ensure that there is a knob for this
  configuration option in [wasmtime_fuzzing::Config][fuzzing-config] (or one
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  Rarely, this may require authoring a new fuzz target to specifically test this
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  </details>

• [ ] If you are enabling a configuration option by default, make sure that it
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[fuzzing-config]: https://github.com/bytecodealliance/wasmtime/blob/ca0e8d0a1d8cefc0496dba2f77a670571d8fdcab/crates/fuzzing/src/generators.rs#L182-L194
[fuzzing-docs]: https://docs.wasmtime.dev/contributing-fuzzing.html

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Last updated: Oct 31 2026 at 23:03 UTC