perf: Use Arc<str> for task dependency hashes to avoid heap clones#11962
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anthonyshew merged 12 commits intomainfrom Feb 22, 2026
Merged
perf: Use Arc<str> for task dependency hashes to avoid heap clones#11962anthonyshew merged 12 commits intomainfrom
Arc<str> for task dependency hashes to avoid heap clones#11962anthonyshew merged 12 commits intomainfrom
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…very Three targeted optimizations to the turbo run hot path: 1. Engine builder: Cache turbo.json chain per package and move the visited check before the expensive task_definition() call. The chain only depends on the package name, so multiple tasks in the same package reuse the cached result. 2. Task visitor: Defer env() computation to non-dry-run branches. The execution environment is unused during dry runs, avoiding per-task RwLock acquisition and env var map cloning. 3. find_untracked_files: Replace Mutex<Vec> with per-thread local buffers flushed via mpsc channel on drop, eliminating per-file mutex contention in the parallel walker.
Parallelize several sequential phases of turbo run's pre-execution pipeline: dependency resolution, turbo.json loading, and task summary construction. Also reduce per-call allocation overhead in the task hash tracker and gix index classification.
…parallelize-hot-path # Conflicts: # crates/turborepo-scm/src/repo_index.rs
…parallelize-hot-path
Adds profiling visibility to functions that were invisible in --profile output: TLS initialization, rayon-spawned hash tasks, Visitor constructor, Engine scheduler, and per-task cache phases.
…/add-profiling-spans
…/add-profiling-spans
Store task hashes as Arc<str> in TaskHashTrackerState instead of String. In calculate_dependency_hashes, cloning an Arc<str> is a ref count bump instead of a heap allocation. For the api monorepo (1687 tasks, ~3 deps each), this eliminates ~5000 String heap allocations per run.
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Arc<str> for task dependency hashes to avoid heap clones
…Summary Removes the .to_string() conversion in the HashTrackerInfo::hash() trait impl by changing the trait return type from Option<String> to Option<Arc<str>>. Also changes SharedTaskSummary.hash to Arc<str> so the Arc flows all the way to serialization without any String allocation.
![vercel[bot]](https://avatars.githubusercontent.com/in/8329?s=60&v=4){kind=small}
The Arc<str> fields in TaskHashTrackerState and SharedTaskSummary require serde's "rc" feature for Serialize. Previously this worked via transitive feature activation; make it explicit so it doesn't break if the transitive path changes.
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## Release v2.8.11-canary.22 Versioned docs: https://v2-8-11-canary-22.turborepo.dev ### Changes - release(turborepo): 2.8.11-canary.21 (#11961) (`83774bc`) - perf: Use `Arc<str>` for task dependency hashes to avoid heap clones (#11962) (`56329a6`) --------- Co-authored-by: Turbobot <turbobot@vercel.com>
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Summary
Arc<str>inTaskHashTrackerStateinstead ofString, so thatcalculate_dependency_hashesclones a ref-counted pointer (atomic increment) instead of heap-allocating a newStringfor each dependency hash lookup.Why
queue_taskis called once per task in the topological dispatch loop. For each task,calculate_dependency_hasheslooks up every dependency's hash and clones it into aVec. On large monorepos likeapi(1687 tasks, ~3 deps each), that's ~5000Stringheap allocations per run — all for 16-char hex strings that are already stored in the tracker and never mutated.Switching to
Arc<str>makes each "clone" a pointer-width copy + atomic ref count increment instead of a heap allocation + memcpy.Testing
Arc<str>derefs tostr).task_hashable_multiple_dependency_hashestest with a pinned hash value to guard against serialization regressions.queue_taskself-time dropped from ~208ms to ~193ms.