tracing: significantly improve code generation at trace points by jstarks · Pull Request #3398 · tokio-rs/tracing

jstarks · 2025-10-28T17:01:46Z

ValueSets contain both a FieldSet reference and a slice of (&Field, Option<&dyn Value>) pairs. In cases where ValueSets are generated via documented interfaces (specifically, tracing::event! and other macros), the Field references are redundant, because the ValueSet contains a value slot for every field (either a value or None), in the correct order.

As a result, the code generated by the macros is terrible--it must put a Field on the stack for each field--that's 32 bytes per field! This is a lot of work for apparently no purpose at runtime, and it can't be moved into a read-only data section since it's intermixed with dynamic data.

Fix this by adding a variant of ValueSet that skips the Field references, knowing that it represents the full set of fields. Keep the old kind of ValueSet, too--it's still needed by Span::record, by old versions of crates, and potentially by third-party crates using undocumented methods such as FieldSet::value_set.

In some adhoc tests on x86_64 Linux, this reduces the code size as follows:

One-field event: 258 bytes to 189 bytes, 25% reduction.
Five-field event: 638 bytes to 276 bytes, 57% reduction.
In a larger project with lots of events, ~5% reduction in .text section.

jstarks · 2025-10-28T18:27:05Z

Test and clippy failures are pre-existing. But the test failure is a little alarming since it's in the area that I'm changing. Looking into it.

jstarks · 2025-10-28T19:13:40Z

I see the test bug. Someone regressed the test last year while cleaning up warnings. I will open a separate PR to fix this.

jstarks · 2025-10-28T19:17:17Z

#3399

tracing-core/src/field.rs

hds

Thank you for your PR!

Question on the naming. But otherwise this looks good.

tracing-core/src/field.rs

`ValueSet`s contain both a `FieldSet` reference and a slice of (`&Field`, `Option<&dyn Value>`) pairs. In cases where `ValueSet`s are generated via documented interfaces (specifically, `tracing::event!` and other macros), the `Field` references are redundant, because the `ValueSet` contains a value slot for every field (either a value or `None`), in the correct order. As a result, the code generated by the macros is terrible--it must put a `Field` on the stack for each field--that's 32 bytes per field! This is a lot of work for apparently no purpose at runtime, and it can't be moved into a read-only data section since it's intermixed with dynamic data. Fix this by adding a variant of `ValueSet` that skips the `Field` references, knowing that it represents the full set of fields. Keep the old kind of `ValueSet`, too--it's still needed by `Span::record`, by old versions of crates, and potentially by third-party crates using undocumented methods such as `FieldSet::value_set`. In some adhoc tests on x86_64 Linux, this reduces the code size as follows: * One-field event: 258 bytes to 189 bytes, 25% reduction. * Five-field event: 638 bytes to 276 bytes, **57%** reduction. * In a larger project with lots of events, ~5% reduction in .text section.

hds · 2025-11-19T15:09:38Z

I've rebased and fixed some conflicts introduced by #3382.

hds

Thank you for this PR! It's a great idea!

# 0.1.35 (November 24, 2025) ### Added - Switch to unconditional `no_std` ([#3323]) - Improve code generation at trace points significantly ([#3398]) ### Fixed - Add missing `dyn` keyword in `Visit` documentation code sample ([#3387]) ### Documented - Add favicon for extra pretty docs ([#3351]) [#3323]: https://github.com/tokio-rs/tracing/pull/#3323 [#3351]: https://github.com/tokio-rs/tracing/pull/#3351 [#3387]: https://github.com/tokio-rs/tracing/pull/#3387 [#3398]: https://github.com/tokio-rs/tracing/pull/#3398

# 0.1.42 (November 30, 2025) ### Added - **attributes**: Support constant expressions as instrument field names ([#3158]) - **core**: Improve code generation at trace points significantly ([#3398]) ### Fixed - Fix perf regression when `release_max_level_*` not set ([#3373]) - Use imported instead of fully qualified path ([#3374]) - Make `valueset` macro sanitary ([#3382]) ### Documented - **core**: Add missing `dyn` keyword in `Visit` documentation code sample ([#3387]) [#3158]: https://github.com/tokio-rs/tracing/pull/#3158 [#3373]: https://github.com/tokio-rs/tracing/pull/#3373 [#3374]: https://github.com/tokio-rs/tracing/pull/#3374 [#3382]: https://github.com/tokio-rs/tracing/pull/#3382 [#3387]: https://github.com/tokio-rs/tracing/pull/#3387 [#3398]: https://github.com/tokio-rs/tracing/pull/#3398

# 0.1.35 (November 26, 2025) ### Added - Switch to unconditional `no_std` ([#3323]) - Improve code generation at trace points significantly ([#3398]) ### Fixed - Add missing `dyn` keyword in `Visit` documentation code sample ([#3387]) ### Documented - Add favicon for extra pretty docs ([#3351]) [#3323]: https://github.com/tokio-rs/tracing/pull/#3323 [#3351]: https://github.com/tokio-rs/tracing/pull/#3351 [#3387]: https://github.com/tokio-rs/tracing/pull/#3387 [#3398]: https://github.com/tokio-rs/tracing/pull/#3398

# 0.1.42 (November 26, 2025) ### Important The [`Span::record_all`] method has been removed from the documented API. It was always unsuable via the documented API as it requried a `ValueSet` which has no publically documented constructors. The method remains, but should not be used outside of `tracing` macros. ### Added - **attributes**: Support constant expressions as instrument field names ([#3158]) - Add `record_all!` macro for recording multiple values in one call ([#3227]) - **core**: Improve code generation at trace points significantly ([#3398]) ### Changed - `tracing-core`: updated to 0.1.35 ([#3414]) - `tracing-attributes`: updated to 0.1.31 ([#3417]) ### Fixed - Fix "name / parent" variant of `event!` ([#2983]) - Remove 'r#' prefix from raw identifiers in field names ([#3130]) - Fix perf regression when `release_max_level_*` not set ([#3373]) - Use imported instead of fully qualified path ([#3374]) - Make `valueset` macro sanitary ([#3382]) ### Documented - **core**: Add missing `dyn` keyword in `Visit` documentation code sample ([#3387]) [#2983]: https://github.com/tokio-rs/tracing/pull/#2983 [#3130]: https://github.com/tokio-rs/tracing/pull/#3130 [#3158]: https://github.com/tokio-rs/tracing/pull/#3158 [#3227]: https://github.com/tokio-rs/tracing/pull/#3227 [#3373]: https://github.com/tokio-rs/tracing/pull/#3373 [#3374]: https://github.com/tokio-rs/tracing/pull/#3374 [#3382]: https://github.com/tokio-rs/tracing/pull/#3382 [#3387]: https://github.com/tokio-rs/tracing/pull/#3387 [#3398]: https://github.com/tokio-rs/tracing/pull/#3398 [#3414]: https://github.com/tokio-rs/tracing/pull/#3414 [#3417]: https://github.com/tokio-rs/tracing/pull/#3417 [`Span::record_all`]: https://docs.rs/tracing/0.1.41/tracing/struct.Span.html#method.record_all

jstarks requested review from a team and hawkw as code owners October 28, 2025 17:01

jstarks force-pushed the full_valueset branch from 1f6c9aa to 48ce86e Compare November 8, 2025 03:28

martin-g reviewed Nov 11, 2025

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hds reviewed Nov 16, 2025

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jstarks added 3 commits November 19, 2025 15:46

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hds force-pushed the full_valueset branch from fb68372 to 2262dd7 Compare November 19, 2025 14:47

hds approved these changes Nov 19, 2025

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hds merged commit 4bf3fef into tokio-rs:main Nov 19, 2025
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conradludgate added a commit to conradludgate/tracing that referenced this pull request Nov 28, 2025

restore original valueset from before tokio-rs#3398

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conradludgate added a commit to conradludgate/tracing that referenced this pull request Nov 28, 2025

restore original valueset from before tokio-rs#3398

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hds mentioned this pull request Dec 2, 2025

tracing: fix record_all panic #3432

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jstarks mentioned this pull request Dec 16, 2025

Add tracing support microsoft/litebox#558

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claude bot mentioned this pull request Mar 10, 2026

deps(deps): bump tracing from 0.1.41 to 0.1.44 d-matsui/rustile#73

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tracing: significantly improve code generation at trace points#3398

tracing: significantly improve code generation at trace points#3398
hds merged 3 commits intotokio-rs:mainfrom
jstarks:full_valueset

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