perf: trim glibc malloc arena after module loading#32662
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bartlomieju merged 4 commits intomainfrom Mar 17, 2026
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On Linux, glibc's malloc doesn't release freed memory back to the OS, causing RSS to be 3-5x higher than on Windows for the same workload. This is especially noticeable when loading large TypeScript codebases (e.g. generated OpenAPI clients) where module compilation creates heavy allocation churn. Call `malloc_trim(0)` after the main/side module loading phase completes to force glibc to release unused heap pages back to the OS. Closes #25722 Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
The `libc` crate was used in `worker.rs` but not declared as a dependency of `deno_lib`, causing build failures on Linux. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
littledivy
approved these changes
Mar 17, 2026
Co-authored-by: Divy <dj.srivastava23@gmail.com> Signed-off-by: Bartek Iwańczuk <biwanczuk@gmail.com>
- Fix indentation of #[cfg] attribute (3 spaces → 4) - Use consistent cfg guard in execute_side_module: target_env = "gnu" is needed since malloc_trim is glibc-specific Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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Summary
malloc_trim(0)on Linux after the main/side module loading phase completes, forcing glibc to release unused heap pages back to the OSmalloc_trimreclaims thisRoot cause
glibc's malloc doesn't automatically return freed memory to the OS — it keeps freed pages in its arena for future reuse. On Windows, the NT heap manager coalesces and releases freed memory much more aggressively. After TypeScript compilation/module loading (which creates many temporary allocations for AST nodes, source maps, etc.), glibc holds onto all that memory even though it's been freed.
Why this is safe
malloc_trimis a standard glibc function, safe to call at any time#[cfg(target_os = "linux")]— no-op on other platformsDENO_USR2_MEMORY_TRIMmechanism already uses the same call, just triggered externally via SIGUSR2Closes #25722
Test plan
cargo check -p deno)cargo test specs)Deno.memoryUsage().rssmalloc_trimcall is cfg-gated to Linux only)🤖 Generated with Claude Code