Increasing Rust’s codegen-unit count can give LLVM more crate work to process in parallel and may shorten compilation, but it can also make the resulting program slower. There is no universally fastest value: check whether code generation is actually limiting your build, then compare settings against the same workload and machine.
What codegen units change
The codegen-units setting controls the maximum number of units into which rustc splits a crate for code generation. LLVM can process multiple units in parallel. More units may reduce compile time, but can produce slower generated code; using one unit may improve runtime performance while making compilation slower. The Rust Project describes this as a tradeoff, not a guaranteed speedup for a particular value or project: rustc Book: Codegen Options.
Check the profile and its default first
Cargo’s documented defaults differ by profile: incremental builds use 256 codegen units, while non-incremental builds use 16. Cargo’s default dev profile enables incremental compilation; the default release profile does not. These are configuration defaults, not benchmark results. Confirm your active profile and any overrides before adding a setting. See the Cargo Book: Profiles.
| Profile/workload | Documented default | What to consider |
|---|---|---|
Default dev profile |
Incremental compilation enabled; 256 codegen units | Relevant to typical development builds and recompilation. |
Default release profile |
Incremental compilation disabled; 16 codegen units | Relevant to optimized builds; assess runtime performance as well as build time. |
The values are documented in the Cargo profile reference. A project can override profile options, so the defaults do not establish what a particular build is using.
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Measure the build before tuning
- Record the conditions. Note the Rust toolchain, target, active profile, incremental setting, machine, and exact Cargo command. Keep them unchanged when comparing settings. Compiler options can vary by toolchain;
rustc -C helplists options supported by the installed compiler. - Capture a baseline. Run the actual workload with Cargo’s
--timingsoption. Record whether it is a clean build or an incremental rebuild; compare those cases separately. - Inspect where time goes. Cargo’s timing report includes total and codegen time by compilation unit, plus concurrency information. It does not expose all compiler-internal concurrency. Use it to see whether code generation is a meaningful part of the delay and whether dependency ordering is a constraint. Details are in Cargo’s build-timings documentation.
- Change only the relevant profile. Put the setting in the workspace-root
Cargo.tomlunder the profile being tested, such as[profile.dev]or[profile.release]. For example:
[profile.dev]
codegen-units = 256This merely makes the documented default for the default dev profile explicit; it is not automatically an improvement. To test another setting, replace 256 with a positive integer and rebuild under the same conditions.
- Compare both outcomes. Repeat the same workload and compare compile time, runtime performance of the generated program, and any relevant artifact or debugging requirements. Do not assume that a faster build is a better result if its runtime cost is unacceptable.
Make sure Cargo is reading the setting you changed
Cargo reads profile definitions from the workspace root manifest; profile settings written in dependency manifests are ignored. A Cargo configuration file or environment variable can also override a manifest setting. The environment-variable form for this option is CARGO_PROFILE_<name>_CODEGEN_UNITS, where <name> corresponds to the profile. Check these sources if changing the manifest appears to have no effect. See the Cargo profile reference.
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Incremental compilation and codegen units are related to different parts of the build experience: incremental compilation is intended to reuse saved information on recompilation, while codegen units affect how a crate’s code-generation work is divided. Treat developer iteration and optimized release builds as separate workloads rather than assuming a setting that helps one will help the other.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.If codegen units are not the bottleneck
More codegen parallelism will not necessarily help if most of the delay is elsewhere. Cargo’s build-performance guidance recommends looking for slow dependencies and features, duplicate versions of the same crate, large crates, and crates that hold up many dependent compilations. Depending on what the timing data shows, reducing slow or duplicated dependencies, splitting an oversized crate, or improving a crate on the dependency critical path may be more relevant than changing codegen units. See Cargo’s build-performance guide.
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