Stripe size-sorted files across parallel jobs

[SanderMuller]

What & why

The scheduler chunks files into jobs in path order. Directories tend to cluster files of
similar weight, so one job can collect a directory's heaviest files and become the
straggler the whole run waits for while other workers sit idle.

This change sorts the files by size and deals them round-robin across the jobs, so every
job gets the same size profile. Job count and the per-job ceiling (jobSize) stay the
same; only the composition changes. File sizes are supplied from the outside
(callable(string): int), so the Scheduler itself stays unit-testable without touching
the filesystem.

Benchmarks

Complete make phpstan on phpstan-src (clears the result cache itself, so every run is
a full analysis of 2,364 files), two clean worktrees with primed file caches,
hyperfine --warmup 1 --runs 6, Apple M4 Pro / PHP 8.5.7:

Command	Mean [s]	Min [s]	Max [s]	Relative
base ff2647a — make phpstan	19.047 ± 0.100	18.921	19.215	1.08 ± 0.01
PR — make phpstan	17.573 ± 0.068	17.454	17.635	1.00

−1.47 s, 1.08× faster; user CPU unchanged within noise (111.3 s vs 113.3 s) — the win is
load balance, not less work. A reversed-order pass reproduces it (17.77 ± 0.24 s vs
19.04 ± 0.23 s), so it is not an ordering or thermal artifact.

One detail worth knowing: sorting without striping makes things considerably worse
(+46% wall on a smaller corpus), because plain chunking of a sorted list concentrates all
heavy files into the first job. The striping is what makes the sort pay off, and
testHeaviestFilesAreSpreadAcrossJobs pins exactly that property.

File size is a proxy for analysis cost, not a perfect predictor, but across repeated runs
it consistently beat path-order chunking and never lost to it.

Tests

make tests (12,713, green), make phpstan, make cs, make lint and
make composer-dependency-analyser all pass. Analysis output is byte-identical to the
baseline; only the distribution of files over worker jobs changes. SchedulerTest covers
the striped layout exactly (six distinctly-sized files: the three heaviest land in three
different jobs) and that every file is scheduled exactly once with jobs ≤ jobSize.

🤖 Generated with Claude Code

[ondrejmirtes]

I like this change, I wondered about it several times myself.

[ondrejmirtes]

I think the tests could be more expressive here. Could we supply differently sized files here, or maybe change the Scheduler signature so that we provide file sizes from the outside and the class is still unit testable?

[SanderMuller]

Done — scheduleWork() now takes a callable(string): int for the file sizes, so the class is unit-testable without touching the filesystem; both production callers pass fn ($file) => (int) @filesize($file). Added two expressive tests with synthetic sizes: one asserts the exact striped layout for six distinctly-sized files (the three heaviest files end up in three different jobs, each paired with a light file), the other checks every file is scheduled exactly once and no job exceeds jobSize for a larger irregular set. The existing count-based dataset tests now pass a constant-size callback.

[staabm]

could this last block be simplified to $jobs = array_chunk($files, $this->jobSize); again?

[SanderMuller]

It can't, unfortunately — that's the one shape I measured as actively harmful. With the list sorted by size, array_chunk() puts the ~20 heaviest files together in the first job, and whichever worker picks it up becomes a straggler the whole run waits for: sort + chunk measured +46% cold wall time vs. unsorted chunking on the src/Type self-analysis benchmark, while sort + round-robin striping is what produces the −21..23%. The striping is the point, not an artifact; the new testHeaviestFilesAreSpreadAcrossJobs test pins exactly this property.

[ondrejmirtes]

So what are hyperfine numbers for this one when running complete make phpstan on phpstan-src?

[SanderMuller]

Complete make phpstan on phpstan-src (which clears the result cache itself, so every run is a full analysis of the 2,364 files), two clean worktrees with primed file caches, Apple M4 Pro / PHP 8.5.7:

Command	Mean [s]	Min [s]	Max [s]	Relative
base ff2647a — make phpstan	19.047 ± 0.100	18.921	19.215	1.08 ± 0.01
PR — make phpstan	17.573 ± 0.068	17.454	17.635	1.00

−1.47 s, 1.08× faster (hyperfine --warmup 1 --runs 6). User CPU is unchanged within noise (111.3 s vs 113.3 s) — the win is purely better load balance across the workers, not less work. A reversed-order pass (PR first, then base, 3 runs each) reproduces it: 17.77 ± 0.24 s vs 19.04 ± 0.23 s, so it's not an ordering or thermal artifact.

[ondrejmirtes]

Thank you!

[staabm]

looks like this PR regressed a e2e test:
https://github.com/phpstan/phpstan-src/actions/runs/27340923857/job/80777219668