mono collector: Reduce # of locking while walking the graph

[osiewicz]

While profiling Zed's dev build I've noticed that while most of the time upstream_monomorphizations takes a lot of time in monomorpization_collector, in some cases (e.g. build of editor itself) the rest of monomorphization_collector_graph_walk dominates it. Most of the time is spent in collect_items_rec.

This PR aims to reduce the number of locks taking place; instead of locking output MonoItems once per children of current node, we do so once per current node. We also get to reuse locks for mentioned and used items. While this commit does not reduce Wall time of Zed's build, it does shave off CPU time (measured with cargo build -j1) from 48s to 47s. I've also tested it with parallel frontend against Zed and ripgrep and found no regressions.

[rustbot]

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[lqd]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 578ea26 with merge f3a2b0e...

[bors]

☀️ Try build successful - checks-actions
Build commit: f3a2b0e (f3a2b0ecccb63c2cbcca1daf45b27db4bd25cbba)

[rust-timer]

Finished benchmarking commit (f3a2b0e): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

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

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.2%, 0.2%]	1
Regressions ❌ (secondary)	0.7%	[0.7%, 0.7%]	1
Improvements ✅ (primary)	-2.9%	[-2.9%, -2.9%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.3%	[-2.9%, 0.2%]	2

Max RSS (memory usage)

Results (primary -0.5%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.7%	[0.7%, 0.7%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.6%	[-1.0%, -0.4%]	11
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.5%	[-1.0%, 0.7%]	12

Cycles

Results (primary -0.3%, secondary 6.7%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.5%	[0.4%, 0.5%]	4
Regressions ❌ (secondary)	6.7%	[6.7%, 6.7%]	1
Improvements ✅ (primary)	-1.2%	[-2.7%, -0.4%]	3
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.3%	[-2.7%, 0.5%]	7

Binary size

Results (primary -1.1%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.1%	[-1.1%, -1.1%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.1%	[-1.1%, -1.1%]	1

Bootstrap: 763.233s -> 764.766s (0.20%)
Artifact size: 365.38 MiB -> 365.24 MiB (-0.04%)

[wesleywiser]

Thanks @osiewicz! Just out of curiosity, do you see this as unblocking subsequent wins in collect_items_rec?

@bors r+

[bors]

📌 Commit 578ea26 has been approved by wesleywiser

It is now in the queue for this repository.

[bors]

⌛ Testing commit 578ea26 with merge 2ad5f86...

[bors]

☀️ Test successful - checks-actions
Approved by: wesleywiser
Pushing 2ad5f86 to master...

[github-actions]

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing 2d5ffc5 (parent) -> 2ad5f86 (this PR)

Test differences

Show 6 test diffs

6 doctest diffs were found. These are ignored, as they are noisy.

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 2ad5f8607d0e192b60b130e5cc416b477b351c18 --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

1. dist-aarch64-linux: 5573.8s -> 7611.6s (36.6%)
2. aarch64-apple: 4360.9s -> 3648.7s (-16.3%)
3. x86_64-apple-2: 5816.6s -> 4901.1s (-15.7%)
4. dist-apple-various: 6080.3s -> 6561.6s (7.9%)
5. x86_64-apple-1: 6823.9s -> 7309.2s (7.1%)
6. x86_64-msvc-2: 6911.5s -> 6618.1s (-4.2%)
7. x86_64-gnu-llvm-20-2: 5962.7s -> 5716.0s (-4.1%)
8. dist-armv7-linux: 5528.0s -> 5731.3s (3.7%)
9. i686-msvc-2: 6985.9s -> 7236.1s (3.6%)
10. x86_64-msvc-ext2: 5819.7s -> 5633.0s (-3.2%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

[rust-timer]

Finished benchmarking commit (2ad5f86): comparison URL.

Overall result: ❌ regressions - please read the text below

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

• If the regression was expected or you think it can be justified,
  please write a comment with sufficient written justification, and add
  @rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
• If you think that you know of a way to resolve the regression, try to create
  a new PR with a fix for the regression.
• If you do not understand the regression or you think that it is just noise,
  you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
  were already notified of this PR).

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	1.4%	[1.4%, 1.4%]	1
Regressions ❌ (secondary)	0.3%	[0.3%, 0.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.4%	[1.4%, 1.4%]	1

Max RSS (memory usage)

Results (primary 0.2%, secondary 2.0%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.9%	[0.4%, 2.4%]	18
Regressions ❌ (secondary)	2.0%	[2.0%, 2.0%]	1
Improvements ✅ (primary)	-1.0%	[-2.0%, -0.4%]	10
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[-2.0%, 2.4%]	28

Cycles

Results (primary 0.2%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.7%	[0.4%, 1.3%]	29
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.8%	[-3.7%, -0.5%]	6
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[-3.7%, 1.3%]	35

Binary size

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

Bootstrap: 769.328s -> 769.364s (0.00%)
Artifact size: 365.53 MiB -> 365.54 MiB (0.00%)

[Mark-Simulacrum]

Both changes look within noise thresholds (bimodality). Dropping regression.

[osiewicz]

@wesleywiser I am not quite sure to be fair; in Zed's case we are mostly bottlenecked on incremental query result deserialization, so (imho) it makes sense to make the overhead of these functions as low as possible. I did not dig beyond this PR yet though.