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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesIncreasing Rust’s codegen-units can give LLVM more crate work to process in parallel, which may reduce compile time—but can also make the resulting program slower. There is no universally fastest value. First check the active Cargo profile and use cargo build --timings to see whether code generation is a meaningful bottleneck; then compare settings under the same build conditions.
What codegen units change
codegen-units sets the maximum number of code-generation units into which rustc splits a crate. LLVM can process multiple units in parallel. More units may reduce compilation time, but may produce slower generated code; setting the count to 1 may improve runtime performance while making compilation slower. The Rust Project describes this as a tradeoff, not a guaranteed speedup for a particular project: rustc codegen options.
Check your profile’s current setting first
Cargo’s documented defaults already use different codegen-unit counts for the two common profiles. The Rust Project’s Cargo documentation, accessed October 4, 2026, lists 256 for incremental builds and 16 for non-incremental builds. These are defaults, not measured performance results: Cargo profiles.
| Typical workload | Profile and documented defaults | What to consider |
|---|---|---|
| Developer builds and iteration | dev: incremental compilation is enabled by default; 256 codegen units. |
Incremental compilation reuses saved information on recompilation. Measure rebuilds as well as clean builds if both matter to your workflow. |
| Optimized builds | release: incremental compilation is disabled by default; 16 codegen units. |
Consider runtime performance as well as build time before increasing the count. |
Defaults and available compiler options can vary with the toolchain. Record the compiler version and, if needed, inspect options supported by your installed compiler with rustc -C help. Cargo’s profile reference documents both the defaults and profile behavior.
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Measure before changing the value
Capture a representative baseline
Record the Rust toolchain, target, active profile, incremental setting, machine, and exact build command. Run the workload you want to improve, distinguishing a clean build from an incremental rebuild; they can have different bottlenecks and should not be compared as if they were the same test.
Inspect Cargo’s timing report
Run Cargo with --timings, for example cargo build --timings (or add the flag to the equivalent command you normally use). The report includes total and codegen time per compilation unit and concurrency information. It does not expose all compiler-internal concurrency, so use it to locate likely bottlenecks rather than as a complete account of LLVM’s work. See the Rust Project’s Cargo build timings documentation.
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If codegen takes little of the time, changing its parallelism may not help. Cargo’s build-performance guidance suggests looking for slow dependencies, unnecessary features, duplicate versions of a crate, large crates, and crates that block many dependent compilations: Optimizing build performance.
Change the setting in the workspace root
Put profile settings in the root workspace’s Cargo.toml, under the profile that matches the workload. For example, this makes the documented development-profile default explicit; it does not by itself improve performance:
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[profile.dev]
codegen-units = 256To test a different setting, replace 256 with a positive integer and rebuild under the same conditions. Define [profile.release] instead when tuning release builds. Cargo ignores profile definitions in dependency manifests, so editing a dependency’s Cargo.toml is not the way to change the workspace build profile.
Also check whether a Cargo config file or environment variable is overriding the manifest. Cargo documents CARGO_PROFILE_<name>_CODEGEN_UNITS as the environment-variable form for a profile’s setting; the relevant name corresponds to the profile, such as DEV or RELEASE. Consult the Cargo profile reference when resolving which setting is active.
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Compare settings fairly, then decide
- Choose a value to test in the profile used by the measured workload; do not assume the dev profile’s setting applies to release.
- Repeat the same build command with the same toolchain, target, machine, and incremental mode. Compare clean builds with clean builds and incremental rebuilds with incremental rebuilds.
- Use the timings report to compare total build time and codegen time, while accounting for dependency critical paths and the report’s limits on internal concurrency details.
- Check generated-program performance as well as compile time, and consider any project requirements around debugging or artifact size.
- Keep the setting only if the measured tradeoff helps your actual workload; otherwise restore the prior value and investigate the bottleneck the timing report identifies.
Official documentation establishes the direction of the tradeoff, but does not identify a value that will compile every Rust project faster or promise a particular gain. Choose based on repeated, comparable measurements rather than a universal recommendation.
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