Test out parallel frontend via crater by zetanumbers · Pull Request #146237 · rust-lang/rust

zetanumbers · 2025-09-05T11:46:13Z

I would like to help push progress on parallel compiler. I think it will be necessary at some point to test compiler in its multi-threaded mode (we already "have" parallel compiler but it is single-threaded by default) via crater. It would allow us to diagnose new issues, measure impact on the ecosystem and estimate amount of remaining work.

I have modified compiler to use ~~"num_cpus" number of~~ 8 threads by default. But I am not sure if there's better value for the cloud environment, so please tell if there is one. I am also not aware of any other nuances of cloud computing, but @Mark-Simulacrum on zulip have confirmed it should be fine.

I believe build-and-test crater mode should provide us with more useful information than the other modes, but if necessary you can scale it down to just build only.

Related zulip thread: #t-compiler/parallel-rustc > Run crater with `-Zthreads=$NUM_CPUS` set

EDIT: Switched from num_cpus to 8 threads

rustbot · 2025-09-05T11:46:18Z

r? @petrochenkov

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lqd · 2025-09-05T12:02:01Z

@bors try

…try> Test out parallel frontend via crater

bjorn3 · 2025-09-05T13:03:06Z

@bors try spurious failure

rust-bors · 2025-09-05T13:03:09Z

Unknown argument "spurious". Run @bors2 help to see available commands.

bjorn3 · 2025-09-05T13:03:30Z

Never mind, that was PR CI, not the try job.

rust-bors · 2025-09-05T14:10:11Z

☀️ Try build successful (CI)
Build commit: d79d87d (d79d87d4c0f7f0d0785ad6e493cf8cf718ed89a6, parent: ad85bc524b1ad696e42061ad8338d382dffbdbe5)

petrochenkov · 2025-09-05T14:12:58Z

@craterbot build-and-test

craterbot · 2025-09-05T14:13:01Z

🚨 Error: failed to parse the command

🆘 If you have any trouble with Crater please ask in t-infra on Zulip
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

petrochenkov · 2025-09-05T14:16:56Z

@craterbot run mode=build-and-test

craterbot · 2025-09-05T14:17:24Z

👌 Experiment pr-146237 created and queued.
🤖 Automatically detected try build d79d87d
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2025-09-10T02:54:09Z

🚧 Experiment pr-146237 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2025-09-11T15:27:41Z

🎉 Experiment pr-146237 is completed!
📊 182 regressed and 154 fixed (694485 total)
📊 115635 spurious results on the retry-regessed-list.txt, consider a retry¹ if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-146237/retry-regressed-list.txt ↩

zetanumbers · 2025-09-18T16:25:10Z

OOM build failures increasing from 4.5% to 13.5% is concerning if we will have more that one thread enabled by default one day. I hypothesized in the zulip thread it comes from increased RAM consumption during parallel compilation. I guess if we set threads to num_cpus (which is our eventual goal) this problem would become less noticeable. So I have reverted to this behavior to see what happens now.

@craterbot run mode=build-only crates=https://crater-reports.s3.amazonaws.com/pr-146237/retry-regressed-list.txt

petrochenkov · 2025-09-23T12:38:32Z

@craterbot p=1

craterbot · 2025-09-23T12:38:36Z

📝 Configuration of the pr-146237-1 experiment changed.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2025-09-27T19:29:27Z

🚧 Experiment pr-146237-1 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2025-09-28T06:48:53Z

🎉 Experiment pr-146237-1 is completed!
📊 5 regressed and 3 fixed (115451 total)
📊 30677 spurious results on the retry-regessed-list.txt, consider a retry¹ if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-146237-1/retry-regressed-list.txt ↩

ywxt · 2025-09-28T07:05:19Z

⚠️ Try build based on commit 34fd685, but latest commit is 77f6c01. Did you forget to make a new try build?

Did we forget to try build or not?

SparrowLii · 2025-09-28T07:22:10Z

@bors try

…try> Test out parallel frontend via crater

rust-bors · 2025-09-28T09:31:28Z

☀️ Try build successful (CI)
Build commit: 4987e9d (4987e9d4227139400384ab59296ffee3b0fb1183, parent: c7f6aa2869acdbf014d094c6e427e554e160b6db)

SparrowLii · 2025-09-29T03:11:50Z

@craterbot run mode=build-only crates=https://crater-reports.s3.amazonaws.com/pr-146237/retry-regressed-list.txt

craterbot · 2025-09-29T03:12:01Z

👌 Experiment pr-146237-2 created and queued.
🤖 Automatically detected try build 4987e9d
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

SparrowLii · 2025-09-29T03:12:26Z

@craterbot p=1

craterbot · 2025-09-29T03:12:29Z

📝 Configuration of the pr-146237-2 experiment changed.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2025-09-29T15:20:17Z

🚧 Experiment pr-146237-2 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2025-09-30T02:09:17Z

🎉 Experiment pr-146237-2 is completed!
📊 3 regressed and 1 fixed (115430 total)
📊 32026 spurious results on the retry-regessed-list.txt, consider a retry¹ if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-146237-2/retry-regressed-list.txt ↩

SpriteOvO · 2026-02-13T11:34:33Z

Hi, ping from triage team. This PR has been inactive for a while. What's the status of this PR now? Thanks.

zetanumbers · 2026-02-15T09:42:06Z

Hi, ping from triage team. This PR has been inactive for a while. What's the status of this PR now? Thanks.

I plan to rerun crater to see if I am right about zero regressions after #149849 is merged and other small stuff is done. See #t-compiler/parallel-rustc > Steps to enable/stabilize multi-threaded rustc front-end @ 💬

rust-bors · 2026-03-28T02:15:28Z

☔ The latest upstream changes (presumably #154466) made this pull request unmergeable. Please resolve the merge conflicts.

Enable parallel frontend by default

77f6c01

rustbot assigned petrochenkov Sep 5, 2025

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 Sep 5, 2025

rust-bors bot added a commit that referenced this pull request Sep 5, 2025

Auto merge of #146237 - zetanumbers:parallel-frontend-by-default, r=<…

d79d87d

…try> Test out parallel frontend via crater

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petrochenkov added S-waiting-on-crater Status: Waiting on a crater run to be completed. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 5, 2025

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This was referenced Sep 15, 2025

ICE: parallel: internal error: entered unreachable code #142064

Open

Add the parallel front-end test suite #143953

Open

ywxt mentioned this pull request Sep 16, 2025

Unstabler query depth limit check on parallel frontend #146616

Open

This was referenced Sep 17, 2025

ICE: queries overflow the depth limit , ice when trying to get query map #142159

Closed

Irreproducible "index out of bounds" error in rustc_thread_pool #146677

Closed

zetanumbers force-pushed the parallel-frontend-by-default branch from 34fd685 to 77f6c01 Compare September 18, 2025 16:16

craterbot added S-waiting-on-crater Status: Waiting on a crater run to be completed. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 23, 2025

craterbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-crater Status: Waiting on a crater run to be completed. labels Sep 28, 2025

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rust-bors bot added a commit that referenced this pull request Sep 28, 2025

Auto merge of #146237 - zetanumbers:parallel-frontend-by-default, r=<…

4987e9d

…try> Test out parallel frontend via crater

craterbot added S-waiting-on-crater Status: Waiting on a crater run to be completed. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 29, 2025

craterbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-crater Status: Waiting on a crater run to be completed. labels Sep 30, 2025

petrochenkov added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 30, 2025

Dylan-DPC added S-blocked Status: Blocked on something else such as an RFC or other implementation work. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Feb 15, 2026

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