63 lines
2.0 KiB
Markdown
63 lines
2.0 KiB
Markdown
bazel-zig-cc and llvm
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---------------------
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bazel-zig-cc has a performance issue when compiling many files. This repository
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reproduces that.
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The test
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--------
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Compiles 64 small binaries with [bazel-zig-cc][1] and [llvm14][2]. The tests
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were run on an x86_64 8-core machine running Ubuntu 22.04.
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Steps to reproduce
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------------------
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Baseline is llvm14: 12-13 seconds:
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```
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bazel --batch clean; bazel --batch build --extra_toolchains=@llvm_toolchain//:cc-toolchain-x86_64-linux ...
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INFO: Elapsed time: 12.454s, Critical Path: 1.11s
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```
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zig cc without the sandbox. Which means different invocations of `zig cc` will
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see that all files in `zig_lib_dir` are the same file. 17 seconds:
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```
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bazel --batch clean; bazel --batch build --spawn_strategy=local --platforms=@zig_sdk//libc_aware/platform:linux_amd64_gnu.2.28 ...
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INFO: Elapsed time: 17.021s, Critical Path: 1.67s
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```
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zig cc plain: 61 seconds. Sandbox is on a real disk, which means it will take
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even longer to re-hash all its dependencies:
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```
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bazel --batch clean; bazel --batch build --platforms=@zig_sdk//libc_aware/platform:linux_amd64_gnu.2.28 ...
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INFO: Elapsed time: 61.128s, Critical Path: 8.35s
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```
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zig cc with hardlinks: `--config=hermetic-sandbox`. This uses hardlinks to
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zig_sdk instead of symlinks:
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```
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bazel --batch clean; bazel --batch build --config=hermetic-sandbox --platforms=@zig_sdk//libc_aware/platform:linux_amd64_gnu.2.28 ...
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INFO: Elapsed time: 45.886s, Critical Path: 9.56s
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```
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llvm with sysroot (this sysroot is about half the size of zig):
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```
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bazel --batch clean; bazel --batch build --config=hermetic-sandbox --extra_toolchains=@llvm_toolchain_with_sysroot//:cc-toolchain-x86_64-linux ...
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INFO: Elapsed time: 25.644s, Critical Path: 3.03s
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```
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Flame graphs and discussion
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---------------------------
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Flame graphs are in results/. As of 2022-12-11 most of the overhead comes from
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creating and deleting the sandboxes. Time for a new sandboxfs!
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[1]: https://sr.ht/~motiejus/bazel-zig-cc
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[2]: https://github.com/grailbio/bazel-toolchain
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