[Yuruzuu]

Problem

Creating a worktree on a Mac spends much of its CPU in the kernel, and more cores make that worse. On a 14-core Mac, a 29k-file checkout used about 9.4 s of system CPU to finish in 1.69 s wall. #28727 reports 30.8 s of system CPU on an M3 Ultra.

Change

createWorktree in GitVcsDriverCore.ts passes checkout.workers=0 when the user has no value set, and Git reads 0 as every core. On macOS the default is now min(cores, 6). A sweep of 0, 2, 4, 6, 8 and 10 workers made 6 the fastest: 2 and 4 were slower on wall time, and 8 used more CPU and was slightly slower than 6.

A user's own checkout.workers still wins, including an explicit 0.

It is macOS-only by choice. I had no Linux or Windows machine to measure, and #11633 measured a speedup from workers=0 on 12-vCPU Ubuntu, so nothing shows those platforms need this. Clients and providers are untouched: this is server-side Git.

#28727 is complementary (an opt-in external tool) and edits the same call, so whichever lands second needs a small rebase. Its cache keys on checkout.* config. I did not test whether passing 6 rather than 0 changes its cache hits.

Scope and approval

This qualifies for the small, focused fix exception in CONTRIBUTING.md. It adds no feature, setting or contract. It tunes an internal Git performance value on macOS, and the checkout result is the same.

Verification

14-core Mac (10P+4E), macOS 27.2, APFS, Git 2.54.0. Rounds alternate in random order. A/A runs identical code twice, to show noise. Other agents were running throughout, and load was 15 to 21 during the driver runs. This covers createWorktree in Node, not app latency.

createWorktree, median ms [range], one warm-up dropped:

Input	Before (A)	Before (A/A)	After	Runs
50 files	50 [48-59]	51 [47-72]	51 [48-82]	12
300 files	80 [77-82]	80 [77-98]	66 [63-79]	12
3,000 files	200 [190-251]	196 [189-213]	146 [139-182]	12
29k files, pass 1	1911 [1643-2284]	1944 [1650-2147]	1812 [1333-2413]	12
29k files, pass 2	1965 [1759-2693]	2083 [1764-2774]	1692 [1487-2350]	15
29k files, pass 3	2094 [1742-2605]	1899 [1743-2721]	1758 [1448-4254]	15

On 29k files under this load, the wall gain is 5 to 15%, and the A/A gap reaches 195 ms, so the gain is near the noise. Below 100 files Git checks out serially, so the 50-file case ties, as expected.

Raw git worktree add, 29k files, 14 rounds, in the calmest run (load about 11 to 13):

Workers	Wall	System CPU
0 (A)	1.69 s [1.66-1.71]	9.41 s [8.85-9.63]
0 (A/A)	1.68 s [1.64-1.78]	9.43 s [8.78-9.65]
6	1.31 s [1.25-1.39]	4.44 s [4.29-4.68]
8	1.39 s [1.31-1.56]	5.83 s [5.67-6.01]

Halved system CPU held in every run, including under the heaviest load (8.4 s to 5.0 s).

Tests: vp test run src/vcs/GitVcsDriverCore.test.ts: 228 of 229 pass. The defaults checkout workers on darwin rows fail with the old default. The one failure, keeps generated worktree folders short without changing long branch names, fails the same way on unmodified upstream/main (/private/var vs /var). vp run check:changed: format, lint, knip and typecheck pass; its test step reports that same failure.

Not measured: Linux, Windows, other core counts including the M3 Ultra, or a full thread start in a real client.

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[macroscopeapp]

Approvability

Verdict: Needs Human Approval

Macroscope's review found this PR requires human approval — New macOS worktrees default to at most six checkout workers, while explicit Git configuration remains respected. Human review is required for this product-default change and its performance tradeoff.

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[coderabbitai]

No actionable comments were generated in the recent review.

Walkthrough

The Git VCS driver selects a default checkout worker count by platform. macOS uses up to six workers based on available parallelism; other platforms use "0". Configured checkout.workers values remain unchanged.

Changes: Checkout worker defaults

Layer / File(s)	Summary
Define and test platform defaults apps/server/src/vcs/GitVcsDriverCore.ts, apps/server/src/vcs/GitVcsDriverCore.test.ts	Adds defaultCheckoutWorkers, which returns up to six workers on macOS and "0" on other platforms. Tests cover the platform-specific results.
Apply defaults during worktree creation apps/server/src/vcs/GitVcsDriverCore.ts, apps/server/src/vcs/GitVcsDriverCore.test.ts	Worktree creation uses the platform default when checkout.workers is unset. Tests cover empty and explicit configuration values.

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Refactor

Possibly related issues

• pingdotgg/t3code#21328 identifies checkout.workers=0 as the default in createWorktree. This change modifies the macOS default; the issue’s index-refresh problem remains separate.

Suggested reviewers: hieu-lee

Merge Risk: Minimal · up to 4e60c

The macOS worker default is covered while explicit configuration remains preserved, with no remaining merge-blocking risk evidenced.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title clearly and concisely describes the main change: reducing CPU use and improving worktree creation time on macOS.
Description check		The description includes all required sections: Problem, Change, Scope and approval, and Verification. It explains the behavior, scope, measurements, test results, known failure, and unmeasured cases.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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Recent review info

Run configuration

• Source mode: Auto
• Repository: pingdotgg/t3code
• Branch: main
• File: .coderabbit.config.ts
• Commit: 61c3794
• Layers: Repository TS config
• Review profile: CHILL
• Plan: Advanced
• Run ID: 83564f59-f713-47e6-a95f-efb3fdebbeb3

Commits

Reviewing files that changed from the base of the PR and between 061ce88 and 4e60ccb.

Files selected for processing: 1

• apps/server/src/vcs/GitVcsDriverCore.test.ts

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[Yuruzuu]

Rebased onto upstream/main at 61c3794 and pushed 061ce88 with an explicit force-with-lease. The remote branch still contained only Yuruzuu's original commit before the push.

The driver merged automatically, retaining #29115's repository-discovery changes. The test conflict was between adjacent additions; both the upstream hook-locale regression and this PR's checkout-worker cases are retained. The PR's checkout behavior is unchanged by this rebase, so the existing benchmark stands; I did not rerun it.

Validation after refreshing frozen-lockfile dependencies:

• GitVcsDriverCore tests: 237 passed, one failed.
• vp run check:changed: format, lint, both Knip stages, GraphQL generation, and all three affected typechecks passed; tests had 238 passes and the same one failure.
• The failure is keeps generated worktree folders short without changing long branch names: Git reports /private/var while the assertion expects /var. Temporarily restoring both changed files to upstream/main reproduced the same failure. Rebased files are restored and the working tree is clean.

There are no unresolved review threads or actionable CodeRabbit findings. Macroscope's Needs Human Approval remains for a maintainer to decide.

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Follow-up Comments

Read host parallelism through the HostProcess environment.

4222-4222: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Read host parallelism through the HostProcess environment.

makeGitVcsDriverCore reads other host state, such as HostProcess.Platform and HostProcess.HomeDirectory, with yield*. This call reads NodeOS.availableParallelism() directly. Tests can therefore not supply a core count, and the darwin default test expects the real host value.

Expose the core count as a HostProcess reference, like Platform. Read it with yield* during construction and pass it to defaultCheckoutWorkers. The pure helper can keep its cores parameter.

Open the source location

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Review comment at @apps/server/src/vcs/GitVcsDriverCore.ts at line 4222:
Update makeGitVcsDriverCore to read the available core count through a
HostProcess reference using yield*, then pass that value to
defaultCheckoutWorkers instead of calling NodeOS.availableParallelism()
directly; keep the helper’s cores parameter unchanged.

• Fix comments: Fix CodeRabbit comments on this PR

[Yuruzuu]

Reviewed the follow-up suggestion to add a HostProcess core-count reference. Leaving that refactor out of this fix: defaultCheckoutWorkers already accepts an explicit core count, with table cases for macOS counts below/at/above the cap and for Linux/Windows. The integration assertion deliberately uses the actual host value, matching the production call. I found no incorrect behavior requiring a new shared reference in this PR.

[Yuruzuu]

Closing this myself. After measuring, the saving is too small for users to notice, so it isn't worth a reviewer's time. Thanks to anyone who looked at it.