bazel-zig-cc and llvm --------------------- bazel-zig-cc has a performance issue when compiling many files. This repository reproduces that. The test -------- Compiles 64 small binaries with [bazel-zig-cc][1] and [llvm14][2]. The tests were run on an x86_64 8-core machine running Ubuntu 22.04. Results ------- Benchmark 1: bazel build --spawn_strategy=local //... Time (mean ± σ): 4.813 s ± 0.140 s [User: 0.015 s, System: 0.009 s] Range (min … max): 4.529 s … 5.007 s 10 runs Benchmark 2: bazel build --extra_toolchains=@llvm_toolchain//:cc-toolchain-x86_64-linux --spawn_strategy=local //... Time (mean ± σ): 6.921 s ± 0.238 s [User: 0.013 s, System: 0.011 s] Range (min … max): 6.513 s … 7.279 s 10 runs Benchmark 3: bazel build --extra_toolchains=@llvm_toolchain_with_sysroot//:cc-toolchain-x86_64-linux --spawn_strategy=local //... Time (mean ± σ): 9.336 s ± 0.347 s [User: 0.013 s, System: 0.012 s] Range (min … max): 8.968 s … 10.220 s 10 runs Benchmark 4: bazel build --platforms=@zig_sdk//libc_aware/platform:linux_amd64_gnu.2.28 --spawn_strategy=local //... Time (mean ± σ): 9.311 s ± 0.190 s [User: 0.014 s, System: 0.010 s] Range (min … max): 9.050 s … 9.667 s 10 runs Benchmark 5: bazel build --experimental_reuse_sandbox_directories //... Time (mean ± σ): 4.726 s ± 0.051 s [User: 0.011 s, System: 0.011 s] Range (min … max): 4.653 s … 4.793 s 10 runs Benchmark 6: bazel build --extra_toolchains=@llvm_toolchain//:cc-toolchain-x86_64-linux --experimental_reuse_sandbox_directories //... Time (mean ± σ): 7.674 s ± 0.370 s [User: 0.012 s, System: 0.012 s] Range (min … max): 7.000 s … 8.492 s 10 runs Benchmark 7: bazel build --extra_toolchains=@llvm_toolchain_with_sysroot//:cc-toolchain-x86_64-linux --experimental_reuse_sandbox_directories //... Time (mean ± σ): 13.098 s ± 0.329 s [User: 0.018 s, System: 0.006 s] Range (min … max): 12.365 s … 13.510 s 10 runs Benchmark 8: bazel build --platforms=@zig_sdk//libc_aware/platform:linux_amd64_gnu.2.28 --experimental_reuse_sandbox_directories //... Time (mean ± σ): 13.465 s ± 0.292 s [User: 0.014 s, System: 0.012 s] Range (min … max): 13.033 s … 13.897 s 10 runs Benchmark 9: bazel build --spawn_strategy=sandboxed //... Time (mean ± σ): 4.766 s ± 0.179 s [User: 0.014 s, System: 0.008 s] Range (min … max): 4.553 s … 5.224 s 10 runs Benchmark 10: bazel build --extra_toolchains=@llvm_toolchain//:cc-toolchain-x86_64-linux --spawn_strategy=sandboxed //... Time (mean ± σ): 8.666 s ± 0.116 s [User: 0.014 s, System: 0.009 s] Range (min … max): 8.504 s … 8.883 s 10 runs Benchmark 11: bazel build --extra_toolchains=@llvm_toolchain_with_sysroot//:cc-toolchain-x86_64-linux --spawn_strategy=sandboxed //... Time (mean ± σ): 33.827 s ± 0.630 s [User: 0.019 s, System: 0.010 s] Range (min … max): 32.121 s … 34.412 s 10 runs Warning: Statistical outliers were detected. Consider re-running this benchmark on a quiet PC without any interferences from other programs. It might help to use the '--warmup' or '--prepare' options. Benchmark 12: bazel build --platforms=@zig_sdk//libc_aware/platform:linux_amd64_gnu.2.28 --spawn_strategy=sandboxed //... Time (mean ± σ): 20.396 s ± 0.330 s [User: 0.016 s, System: 0.011 s] Range (min … max): 19.966 s … 20.939 s 10 runs Summary 'bazel build --experimental_reuse_sandbox_directories //...' ran 1.01 ± 0.04 times faster than 'bazel build --spawn_strategy=sandboxed //...' 1.02 ± 0.03 times faster than 'bazel build --spawn_strategy=local //...' 1.46 ± 0.05 times faster than 'bazel build --extra_toolchains=@llvm_toolchain//:cc-toolchain-x86_64-linux --spawn_strategy=local //...' 1.62 ± 0.08 times faster than 'bazel build --extra_toolchains=@llvm_toolchain//:cc-toolchain-x86_64-linux --experimental_reuse_sandbox_directories //...' 1.83 ± 0.03 times faster than 'bazel build --extra_toolchains=@llvm_toolchain//:cc-toolchain-x86_64-linux --spawn_strategy=sandboxed //...' 1.97 ± 0.05 times faster than 'bazel build --platforms=@zig_sdk//libc_aware/platform:linux_amd64_gnu.2.28 --spawn_strategy=local //...' 1.98 ± 0.08 times faster than 'bazel build --extra_toolchains=@llvm_toolchain_with_sysroot//:cc-toolchain-x86_64-linux --spawn_strategy=local //...' 2.77 ± 0.08 times faster than 'bazel build --extra_toolchains=@llvm_toolchain_with_sysroot//:cc-toolchain-x86_64-linux --experimental_reuse_sandbox_directories //...' 2.85 ± 0.07 times faster than 'bazel build --platforms=@zig_sdk//libc_aware/platform:linux_amd64_gnu.2.28 --experimental_reuse_sandbox_directories //...' 4.32 ± 0.08 times faster than 'bazel build --platforms=@zig_sdk//libc_aware/platform:linux_amd64_gnu.2.28 --spawn_strategy=sandboxed //...' 7.16 ± 0.15 times faster than 'bazel build --extra_toolchains=@llvm_toolchain_with_sysroot//:cc-toolchain-x86_64-linux --spawn_strategy=sandboxed //...' | Command | Mean [s] | Min [s] | Max [s] | Relative | |:-----------------------------------------------------------------------------------------------------------------------------------------|---------------:|--------:|--------:|------------:| | `bazel build --experimental_reuse_sandbox_directories //...` | 4.726 ± 0.051 | 4.653 | 4.793 | 1.00 | | `bazel build --spawn_strategy=sandboxed //...` | 4.766 ± 0.179 | 4.553 | 5.224 | 1.01 ± 0.04 | | `bazel build --spawn_strategy=local //...` | 4.813 ± 0.140 | 4.529 | 5.007 | 1.02 ± 0.03 | | `bazel build --extra_toolchains=@llvm_toolchain//:cc-toolchain-x86_64-linux --spawn_strategy=local //...` | 6.921 ± 0.238 | 6.513 | 7.279 | 1.46 ± 0.05 | | `bazel build --extra_toolchains=@llvm_toolchain//:cc-toolchain-x86_64-linux --experimental_reuse_sandbox_directories //...` | 7.674 ± 0.370 | 7.000 | 8.492 | 1.62 ± 0.08 | | `bazel build --extra_toolchains=@llvm_toolchain//:cc-toolchain-x86_64-linux --spawn_strategy=sandboxed //...` | 8.666 ± 0.116 | 8.504 | 8.883 | 1.83 ± 0.03 | | `bazel build --platforms=@zig_sdk//libc_aware/platform:linux_amd64_gnu.2.28 --spawn_strategy=local //...` | 9.311 ± 0.190 | 9.050 | 9.667 | 1.97 ± 0.05 | | `bazel build --extra_toolchains=@llvm_toolchain_with_sysroot//:cc-toolchain-x86_64-linux --spawn_strategy=local //...` | 9.336 ± 0.347 | 8.968 | 10.220 | 1.98 ± 0.08 | | `bazel build --extra_toolchains=@llvm_toolchain_with_sysroot//:cc-toolchain-x86_64-linux --experimental_reuse_sandbox_directories //...` | 13.098 ± 0.329 | 12.365 | 13.510 | 2.77 ± 0.08 | | `bazel build --platforms=@zig_sdk//libc_aware/platform:linux_amd64_gnu.2.28 --experimental_reuse_sandbox_directories //...` | 13.465 ± 0.292 | 13.033 | 13.897 | 2.85 ± 0.07 | | `bazel build --platforms=@zig_sdk//libc_aware/platform:linux_amd64_gnu.2.28 --spawn_strategy=sandboxed //...` | 20.396 ± 0.330 | 19.966 | 20.939 | 4.32 ± 0.08 | | `bazel build --extra_toolchains=@llvm_toolchain_with_sysroot//:cc-toolchain-x86_64-linux --spawn_strategy=sandboxed //...` | 33.827 ± 0.630 | 32.121 | 34.412 | 7.16 ± 0.15 | Explanation: * `--spawn_strategy=local` is the baseline. This is the theoretically fastest approach (no sandbox), but unfit for production. * `--spawn_strategy=sandboxed` is the default for Bazel. This is what happens if one does not pass any arguments. * `--experimental_reuse_sandbox_directories` is an optimization which is meaningful on sandboxes with many files. As of Bazel 6 this has been [promoted to stable][3], so safe to use and, as one can see in the numbers, recommended with both bazel-zig-cc and a nontrivial sysroot. As a result, this is the most important benchmark to look at. Extracting the fastest meaningful benchmarks in all categories: 'bazel build --experimental_reuse_sandbox_directories //...' ran 2.77 ± 0.08 times faster than 'bazel build --extra_toolchains=@llvm_toolchain_with_sysroot//:cc-toolchain-x86_64-linux --experimental_reuse_sandbox_directories //...' 2.85 ± 0.07 times faster than 'bazel build --platforms=@zig_sdk//libc_aware/platform:linux_amd64_gnu.2.28 --experimental_reuse_sandbox_directories //...' This demonstrates that **adding a hermetic toolchain to a project caused 177% slowdown on this project**. Note that we are not counting `llvm_toolchain//...` as a real toolchain -- one needs a sysroot to compile anything meaningful anyway (which zig bundles with the compiler). Once we have determined that a hermetic C++ toolchain is required, bazel-zig-cc is about 2.8% slower than its contender plain llvm14. Flame graphs and discussion --------------------------- Flame graphs in results/. Only of historical interest, because the biggest performance issues have been resolved with a combination of zig and bazel-zig-cc changes. [1]: https://sr.ht/~motiejus/bazel-zig-cc [2]: https://github.com/grailbio/bazel-toolchain [3]: https://github.com/bazelbuild/bazel/issues/16138