Carry the scheduler's memory between CI runners

@vzn/vx-schedule-history starts the longest chain first, using task
times from past runs. A fresh CI runner has no past runs, so the first
run on every runner fell back to a guess.

One file

// vx.workspace.ts
import { scheduleHistoryPlugin } from '@vzn/vx-schedule-history'

export default {
  plugins: [scheduleHistoryPlugin({ file: '.vx-timings.json' })],
}

vx reads the file before it orders the run and rewrites it after, with
each task's median time:

{
  "version": 1,
  "tasks": {
    "app#build": 41200,
    "app#test": 18350,
    "docs#build": 29010
  }
}

Cache that one file in CI and the next runner starts with the times the
last one learned:

- uses: actions/cache@v4
  with:
    path: .vx-timings.json
    key: vx-timings-${{ github.run_id }}
    restore-keys: vx-timings-

Source of a task's time	Wins over
Runs on this machine	the file and assume
The file	assume
assume	nothing

What it saves

A graph where order matters: eight packages whose build, check and
pack chain unlocks more work (0.5 s each), and three slow e2e tasks
(10 s each) that nothing depends on. Every task runs; only the order
changes.

Runner	Wall time (min of 3)
Cold, no file	12.09 s
Cold, with the file	10.68 s
Full local history	10.64 s

With no times, the slow tasks start last. The file starts them first,
cutting 12% off the wall time and matching a machine with its full
history. The lower bound here is 10.5 s.

Setup: linux x64, 4 cores, 4 workers, sleep tasks, vx 0.0.634. Run it
yourself with bun packages/vx-bench/timings-file-bench.ts.

When one task is the whole wall time, order cannot help. vx's own CI job
is like that: one 194 s test suite set its time with and without the
file (194.07 s and 194.60 s).

A run that executed nothing and restored only tasks the file already
times leaves the file alone, so an all-cached CI run pays no extra history
read. Runs that use no history plugin do no extra work.

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Originally published on the vx blog. vx is an MIT task runner and build cache for JS monorepos: vznjs.github.io/vx · GitHub.

_{Written with AI assistance.}