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