Take first task group for further execution by zetanumbers · Pull Request #154419 · rust-lang/rust

zetanumbers · 2026-03-26T11:03:18Z

I thought that storing a first group of tasks for immediate execution instead of pushing and immediately poping it from rayon's local task queue in par_slice would avoid overwhelming work stealing potentially blocking the original thread. So I've implemented this change.

8 threads benchmarks:

Benchmark	baseline~~9	new~take-first-group~1
Benchmark	Time	Time	%
🟣 hyper:check	0.1110s	0.1086s	💚 -2.13%
🟣 hyper:check:initial	0.1314s	0.1298s	💚 -1.23%
🟣 hyper:check:unchanged	0.0771s	0.0755s	💚 -2.14%
🟣 clap:check	0.3787s	0.3757s	-0.80%
🟣 clap:check:initial	0.4680s	0.4564s	💚 -2.48%
🟣 clap:check:unchanged	0.2337s	0.2301s	💚 -1.52%
🟣 syn:check	0.4321s	0.4265s	💚 -1.31%
🟣 syn:check:initial	0.5586s	0.5401s	💚 -3.31%
🟣 syn:check:unchanged	0.3434s	0.3429s	-0.14%
🟣 regex:check	0.2755s	0.2661s	💚 -3.40%
🟣 regex:check:initial	0.3350s	0.3347s	-0.11%
🟣 regex:check:unchanged	0.1851s	0.1832s	💚 -1.01%
Total	3.5296s	3.4695s	💚 -1.70%
Summary	1.0000s	0.9837s	💚 -1.63%

rustbot · 2026-03-26T11:03:23Z

r? @jieyouxu

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lqd · 2026-03-26T11:34:20Z

I wonder what variance, and noise level, you're seeing on your benchmarking machine? BTZ, does this remove some small overhead a few times, or does it translate to good results on bigger benchmarks as well?

jieyouxu · 2026-03-26T11:48:00Z

@rustbot reroll

zetanumbers · 2026-03-26T11:53:26Z

I wonder what variance, and noise level, you're seeing on your benchmarking machine? BTZ, does this remove some small overhead a few times, or does it translate to good results on bigger benchmarks as well?

Here's baseline compiler running against itself:

Benchmark	baseline~~9	baseline~~9
Benchmark	Time	Time	%
🟣 hyper:check	0.1143s	0.1157s	💔 1.25%
🟣 hyper:check:initial	0.1405s	0.1420s	💔 1.08%
🟣 hyper:check:unchanged	0.0805s	0.0797s	💚 -1.02%
🟣 clap:check	0.3914s	0.3909s	-0.14%
🟣 clap:check:initial	0.4765s	0.4823s	💔 1.21%
🟣 clap:check:unchanged	0.2345s	0.2329s	-0.68%
🟣 syn:check	0.4328s	0.4303s	-0.58%
🟣 syn:check:initial	0.5820s	0.5731s	💚 -1.53%
🟣 syn:check:unchanged	0.3695s	0.3696s	0.02%
🟣 regex:check	0.2668s	0.2720s	💔 1.95%
🟣 regex:check:initial	0.3224s	0.3289s	💔 2.02%
🟣 regex:check:unchanged	0.1901s	0.1846s	💚 -2.92%
Total	3.6015s	3.6021s	0.02%
Summary	1.0000s	1.0006s	0.06%

I have run these benchmarks on various changes before and never seen all greens like above.

compiler/rustc_data_structures/src/sync/parallel.rs

rustbot · 2026-03-27T10:39:12Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

Zoxc · 2026-03-30T01:35:54Z

I wasn't able to reproduce the improvements. Perhaps different scheduling on Windows is the cause? The change seems unlikely to be a regression anyway.

Results with 7 threads:

Benchmark	Before	Before		After		Before	Before		After		Before	Before		After
Benchmark	Time	Time	%	Time	%	Physical Memory	Physical Memory	%	Physical Memory	%	Committed Memory	Committed Memory	%	Committed Memory	%
🟣 clap:check	0.3963s	0.3965s	0.04%	0.3979s	0.40%	204.25 MiB	204.22 MiB	-0.01%	204.14 MiB	-0.05%	276.78 MiB	276.56 MiB	-0.08%	276.79 MiB	0.00%
🟣 hyper:check	0.1318s	0.1308s	-0.80%	0.1309s	-0.69%	127.65 MiB	127.66 MiB	0.01%	127.60 MiB	-0.03%	195.84 MiB	195.82 MiB	-0.01%	195.83 MiB	-0.00%
🟣 regex:check	0.2722s	0.2726s	0.13%	0.2726s	0.15%	167.22 MiB	167.32 MiB	0.06%	167.29 MiB	0.04%	227.83 MiB	227.85 MiB	0.01%	227.92 MiB	0.04%
🟣 syn:check	0.5073s	0.5070s	-0.06%	0.5060s	-0.27%	197.98 MiB	198.04 MiB	0.03%	198.09 MiB	0.06%	259.91 MiB	260.00 MiB	0.03%	260.03 MiB	0.04%
Total	1.3077s	1.3068s	-0.07%	1.3074s	-0.02%	697.10 MiB	697.23 MiB	0.02%	697.12 MiB	0.00%	960.37 MiB	960.23 MiB	-0.01%	960.57 MiB	0.02%
Summary	1.0000s	0.9983s	-0.17%	0.9990s	-0.10%	1 byte	1.00 bytes	0.02%	1.00 bytes	0.00%	1 byte	1.00 bytes	-0.01%	1.00 bytes	0.02%

Zoxc · 2026-03-30T02:48:56Z

I did a benchmark run with 7 threads in a Linux VM and that does look like an improvement:

Benchmark	Before	Before		After
Benchmark	Time	Time	%	Time	%
🟣 regex:check	0.2905s	0.2895s	-0.33%	0.2877s	-0.95%
🟣 hyper:check	0.1317s	0.1307s	-0.76%	0.1295s	💚 -1.71%
🟣 clap:check	0.4345s	0.4347s	0.06%	0.4299s	💚 -1.05%
🟣 syn:check	0.5256s	0.5248s	-0.15%	0.5209s	-0.90%
Total	1.3823s	1.3798s	-0.18%	1.3680s	💚 -1.03%
Summary	1.0000s	0.9970s	-0.30%	0.9885s	💚 -1.15%

nnethercote · 2026-03-31T00:20:51Z

@bors try @rust-timer queue

Take first task group for further execution

rust-bors · 2026-03-31T02:29:00Z

☀️ Try build successful (CI)
Build commit: 7162208 (7162208e3ee809a53abb3682431abfd4dd7bf537, parent: cf7da0b7277cad05b79f91b60c290aa08a17a6f0)

rust-timer · 2026-03-31T03:10:24Z

Finished benchmarking commit (7162208): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.4%	[-0.6%, -0.1%]	2
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 2.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.4%	[1.6%, 3.1%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.4%	[1.6%, 3.1%]	2

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 483.836s -> 484.409s (0.12%)
Artifact size: 394.90 MiB -> 394.83 MiB (-0.02%)

nnethercote · 2026-03-31T05:23:39Z

@bors r+ rollup=maybe

rust-bors · 2026-03-31T05:23:43Z

📌 Commit 576a727 has been approved by nnethercote

It is now in the queue for this repository.

…ethercote Take first task group for further execution Continuing from rust-lang#153768 (comment). I thought that storing a first group of tasks for immediate execution instead of pushing and immediately poping it from rayon's local task queue in par_slice would avoid overwhelming work stealing potentially blocking the original thread. So I've implemented this change. 8 threads benchmarks: <table><tr><td rowspan="2">Benchmark</td><td colspan="1">baseline~~9</th><td colspan="2">new~take-first-group~1</th></tr><tr><td align="right">Time</td><td align="right">Time</td><td align="right">%</th></tr><tr><td>🟣 hyper:check</td><td align="right">0.1110s</td><td align="right">0.1086s</td><td align="right">💚 -2.13%</td></tr><tr><td>🟣 hyper:check:initial</td><td align="right">0.1314s</td><td align="right">0.1298s</td><td align="right">💚 -1.23%</td></tr><tr><td>🟣 hyper:check:unchanged</td><td align="right">0.0771s</td><td align="right">0.0755s</td><td align="right">💚 -2.14%</td></tr><tr><td>🟣 clap:check</td><td align="right">0.3787s</td><td align="right">0.3757s</td><td align="right"> -0.80%</td></tr><tr><td>🟣 clap:check:initial</td><td align="right">0.4680s</td><td align="right">0.4564s</td><td align="right">💚 -2.48%</td></tr><tr><td>🟣 clap:check:unchanged</td><td align="right">0.2337s</td><td align="right">0.2301s</td><td align="right">💚 -1.52%</td></tr><tr><td>🟣 syn:check</td><td align="right">0.4321s</td><td align="right">0.4265s</td><td align="right">💚 -1.31%</td></tr><tr><td>🟣 syn:check:initial</td><td align="right">0.5586s</td><td align="right">0.5401s</td><td align="right">💚 -3.31%</td></tr><tr><td>🟣 syn:check:unchanged</td><td align="right">0.3434s</td><td align="right">0.3429s</td><td align="right"> -0.14%</td></tr><tr><td>🟣 regex:check</td><td align="right">0.2755s</td><td align="right">0.2661s</td><td align="right">💚 -3.40%</td></tr><tr><td>🟣 regex:check:initial</td><td align="right">0.3350s</td><td align="right">0.3347s</td><td align="right"> -0.11%</td></tr><tr><td>🟣 regex:check:unchanged</td><td align="right">0.1851s</td><td align="right">0.1832s</td><td align="right">💚 -1.01%</td></tr><tr><td>Total</td><td align="right">3.5296s</td><td align="right">3.4695s</td><td align="right">💚 -1.70%</td></tr><tr><td>Summary</td><td align="right">1.0000s</td><td align="right">0.9837s</td><td align="right">💚 -1.63%</td></tr></table>

…uwer Rollup of 12 pull requests Successful merges: - #154419 (Take first task group for further execution) - #154569 (Fix type alias where clause suggestion spacing issue) - #154617 (Update flate2 users to use zlib-rs) - #154618 (Fix AtomicPtr::update's cfg gate) - #154620 (stabilize new Range type and iterator) - #151932 (refactor: remove `Adjust::ReborrowPin`) - #153980 (refactor: move doc(rust_logo) check to parser) - #154134 (fix: guard paren-sugar pretty-printing on short trait args) - #154270 (Create `Ty` type alias in `rustc_type_ir`) - #154580 (Split AttributeParserError Diagnostic implementation into subfunctions) - #154606 (misc test cleanups) - #154612 (Add a test for a now fixed ICE with `offset_of!()`)

Rollup merge of #154419 - zetanumbers:take-first-group, r=nnethercote Take first task group for further execution Continuing from #153768 (comment). I thought that storing a first group of tasks for immediate execution instead of pushing and immediately poping it from rayon's local task queue in par_slice would avoid overwhelming work stealing potentially blocking the original thread. So I've implemented this change. 8 threads benchmarks: <table><tr><td rowspan="2">Benchmark</td><td colspan="1">baseline~~9</th><td colspan="2">new~take-first-group~1</th></tr><tr><td align="right">Time</td><td align="right">Time</td><td align="right">%</th></tr><tr><td>🟣 hyper:check</td><td align="right">0.1110s</td><td align="right">0.1086s</td><td align="right">💚 -2.13%</td></tr><tr><td>🟣 hyper:check:initial</td><td align="right">0.1314s</td><td align="right">0.1298s</td><td align="right">💚 -1.23%</td></tr><tr><td>🟣 hyper:check:unchanged</td><td align="right">0.0771s</td><td align="right">0.0755s</td><td align="right">💚 -2.14%</td></tr><tr><td>🟣 clap:check</td><td align="right">0.3787s</td><td align="right">0.3757s</td><td align="right"> -0.80%</td></tr><tr><td>🟣 clap:check:initial</td><td align="right">0.4680s</td><td align="right">0.4564s</td><td align="right">💚 -2.48%</td></tr><tr><td>🟣 clap:check:unchanged</td><td align="right">0.2337s</td><td align="right">0.2301s</td><td align="right">💚 -1.52%</td></tr><tr><td>🟣 syn:check</td><td align="right">0.4321s</td><td align="right">0.4265s</td><td align="right">💚 -1.31%</td></tr><tr><td>🟣 syn:check:initial</td><td align="right">0.5586s</td><td align="right">0.5401s</td><td align="right">💚 -3.31%</td></tr><tr><td>🟣 syn:check:unchanged</td><td align="right">0.3434s</td><td align="right">0.3429s</td><td align="right"> -0.14%</td></tr><tr><td>🟣 regex:check</td><td align="right">0.2755s</td><td align="right">0.2661s</td><td align="right">💚 -3.40%</td></tr><tr><td>🟣 regex:check:initial</td><td align="right">0.3350s</td><td align="right">0.3347s</td><td align="right"> -0.11%</td></tr><tr><td>🟣 regex:check:unchanged</td><td align="right">0.1851s</td><td align="right">0.1832s</td><td align="right">💚 -1.01%</td></tr><tr><td>Total</td><td align="right">3.5296s</td><td align="right">3.4695s</td><td align="right">💚 -1.70%</td></tr><tr><td>Summary</td><td align="right">1.0000s</td><td align="right">0.9837s</td><td align="right">💚 -1.63%</td></tr></table>

…uwer Rollup of 12 pull requests Successful merges: - rust-lang/rust#154419 (Take first task group for further execution) - rust-lang/rust#154569 (Fix type alias where clause suggestion spacing issue) - rust-lang/rust#154617 (Update flate2 users to use zlib-rs) - rust-lang/rust#154618 (Fix AtomicPtr::update's cfg gate) - rust-lang/rust#154620 (stabilize new Range type and iterator) - rust-lang/rust#151932 (refactor: remove `Adjust::ReborrowPin`) - rust-lang/rust#153980 (refactor: move doc(rust_logo) check to parser) - rust-lang/rust#154134 (fix: guard paren-sugar pretty-printing on short trait args) - rust-lang/rust#154270 (Create `Ty` type alias in `rustc_type_ir`) - rust-lang/rust#154580 (Split AttributeParserError Diagnostic implementation into subfunctions) - rust-lang/rust#154606 (misc test cleanups) - rust-lang/rust#154612 (Add a test for a now fixed ICE with `offset_of!()`)

rustbot assigned jieyouxu Mar 26, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Mar 26, 2026

rustbot assigned nnethercote and unassigned jieyouxu Mar 26, 2026

bjorn3 reviewed Mar 27, 2026

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compiler/rustc_data_structures/src/sync/parallel.rs Outdated Show resolved Hide resolved

Take first task group for further execution

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zetanumbers force-pushed the take-first-group branch from f0c55d6 to 576a727 Compare March 27, 2026 10:39

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Auto merge of #154419 - zetanumbers:take-first-group, r=<try>

7162208

Take first task group for further execution

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Rollup of 21 pull requests #154629

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Rollup of 12 pull requests #154637

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rust-bors bot merged commit 8427445 into rust-lang:main Mar 31, 2026
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rust-timer commented Mar 31, 2026

Overall result: ✅ improvements - no action needed

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