Feldera drastically reduced Rust compile times for a project with over a thousand crates from 30 minutes to 2 minutes by strategically leveraging sccache. They initially tried using a shared volume for the sccache directory but encountered performance issues. The solution involved setting up a dedicated, high-performance sccache server, accessed by developers via SSH, which dramatically improved cache hit rates and reduced compilation times. Additionally, they implemented careful dependency management, reducing unnecessary rebuilds by pinning specific crate versions in a lockfile and leveraging workspaces to manage the many inter-related crates effectively.
The blog post "Cutting down Rust compile times from 30 to 2 minutes with one thousand crates" by Aleksey Kladov details the author's journey in significantly reducing the compilation time of a large Rust project, specifically a monorepo containing over one thousand crates. Initially, the project suffered from extremely long compile times, reaching up to 30 minutes, severely hindering developer productivity and the feedback loop.
The author systematically investigated and addressed several bottlenecks to achieve this drastic improvement. A key issue was the overuse of procedural macros. While powerful, procedural macros can significantly increase compile times as they execute arbitrary Rust code during compilation. Kladov discovered numerous instances where functions were unnecessarily annotated with procedural macros, leading to redundant computations. By refactoring these instances and replacing them with simpler function calls where possible, he eliminated unnecessary macro expansions, dramatically reducing compile time.
Another substantial contributor to slow compilation was the frequent recompilation of unchanged code. The author identified that the build system's caching mechanism wasn't effectively utilized. Through careful examination of the dependency graph and the build process, he pinpointed inefficiencies in how crate metadata was handled, leading to spurious recompilations. By optimizing the build configuration and ensuring the build system correctly recognized unchanged dependencies, the author drastically reduced redundant compilation work.
Furthermore, the author leveraged sccache, a compiler cache, to further accelerate the build process. sccache effectively caches compilation artifacts, allowing the build system to reuse previously compiled code, even across different machines or build environments. This caching strategy significantly reduced the need to recompile unchanged code, especially during incremental builds.
Beyond these primary optimizations, the author explored and implemented several other smaller improvements. These included careful consideration of dependency management, ensuring that only necessary dependencies were included, and optimizing the build system's configuration to leverage available resources effectively. The combined effect of these optimizations resulted in a remarkable reduction in compile time, from an initial 30 minutes down to approximately 2 minutes, representing a significant improvement in developer productivity and workflow. The author concludes by emphasizing the importance of continuously profiling and optimizing the build process in large Rust projects to maintain efficient compilation times as the project grows and evolves.
Summary of Comments ( 48 )
https://news.ycombinator.com/item?id=43715235
HN commenters generally praise the author's work in reducing Rust compile times, while also acknowledging that long compile times remain a significant issue for the language. Several point out that the demonstrated improvement is largely due to addressing a specific, unusual dependency issue (duplicated crates) rather than a fundamental compiler speedup. Some express hope that the author's insights, particularly around dependency management, will contribute to future Rust development. Others suggest additional strategies for improving compile times, such as using sccache and focusing on reducing dependencies in the first place. A few commenters mention the trade-off between compile time and runtime performance, suggesting that Rust's speed often justifies the longer compilation.
The Hacker News post discussing the blog post "Cutting down Rust compile times from 30 to 2 minutes with one thousand crates" has a substantial number of comments exploring various aspects of Rust compilation speed, dependency management, and the author's approach to optimization.
Several commenters express skepticism about the author's claim of 30-minute compile times, suggesting this is an unusually high figure even for large Rust projects. They question the initial project setup and dependencies that could lead to such lengthy compilations. Some speculate about the potential impact of excessive dependencies, the use of build scripts, or inefficiently structured code.
A recurring theme is the comparison between Rust's compilation times and those of other languages. Commenters discuss the trade-offs between compile-time checks and runtime performance, with some arguing that Rust's robust type system and safety guarantees contribute to longer compilation times. Others point out that while Rust compilation can be slow, the resulting binaries are often highly optimized and performant.
Several commenters delve into the technical details of the author's optimization strategies, including the use of workspaces, dependency management tools like Cargo, and the benefits of incremental compilation. There's discussion around the impact of different dependency structures on compile times, and the potential for further optimization through techniques like caching and pre-built dependencies.
Some commenters offer alternative approaches to improving Rust compilation speed, such as using sccache (a shared compilation cache) or employing different linker strategies. They also discuss the role of hardware, particularly CPU and disk speed, in influencing compilation times.
A few commenters share their own experiences with Rust compilation times, offering anecdotal evidence of both successes and challenges in optimizing large projects. They highlight the ongoing efforts within the Rust community to improve compilation speed and the importance of tools and techniques for managing dependencies effectively.
Finally, there's some discussion about the overall developer experience with Rust, with some commenters acknowledging the frustration of slow compile times, while others emphasize the advantages of Rust's safety and performance characteristics.