Fix performance regression with jemalloc #148925

madsmtm · 2025-11-13T22:04:52Z

Fixes the performance regression introduced in #146627.

This effectively reverts 65f0b7a (second commit in that PR), and adds a comment explaining why extern crate is needed here instead of use (we need to load tikv_jemalloc_sys from the sysroot for some reason).

r? Kobzol

madsmtm · 2025-11-13T22:05:10Z

I could reproduce the performance regression locally, let's see if rustc-perf agrees:

@bors try
@rust-timer queue

Fix performance regression with jemalloc

rust-bors · 2025-11-14T00:22:43Z

☀️ Try build successful (CI)
Build commit: f30bc23 (f30bc23f4189bac695183835d28078b38cf28d74, parent: 2286e5d224b3413484cf4f398a9f078487e7b49d)

rust-timer · 2025-11-14T01:42:24Z

Finished benchmarking commit (f30bc23): comparison URL.

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

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.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@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)	3.1%	[3.1%, 3.1%]	1
Regressions ❌ (secondary)	1.2%	[0.2%, 1.5%]	8
Improvements ✅ (primary)	-2.1%	[-3.4%, -1.1%]	20
Improvements ✅ (secondary)	-1.4%	[-2.7%, -0.2%]	25
All ❌✅ (primary)	-1.9%	[-3.4%, 3.1%]	21

Max RSS (memory usage)

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

Cycles

Results (primary -1.7%, secondary 0.1%)

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

	mean	range	count
Regressions ❌ (primary)	2.6%	[2.6%, 2.6%]	1
Regressions ❌ (secondary)	2.5%	[2.5%, 2.5%]	1
Improvements ✅ (primary)	-2.5%	[-3.2%, -2.1%]	6
Improvements ✅ (secondary)	-2.3%	[-2.3%, -2.3%]	1
All ❌✅ (primary)	-1.7%	[-3.2%, 2.6%]	7

Binary size

Results (primary 1.1%)

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

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

Bootstrap: 474.605s -> 473.65s (-0.20%)
Artifact size: 388.41 MiB -> 388.64 MiB (0.06%)

Using the new `override_allocator_on_supported_platforms` feature in `tikv-jemalloc-sys v0.6.1` we can avoid the manual statics.

madsmtm · 2025-11-14T01:54:04Z

@bors try
@rust-timer queue

Fix performance regression with jemalloc

rust-bors · 2025-11-14T04:11:24Z

☀️ Try build successful (CI)
Build commit: aef7be6 (aef7be6c425e18433bd4afc8a6ffcccfe9ffe796, parent: 7a72c5459dd58f81b0e1a0e5436d145485889375)

rust-timer · 2025-11-14T05:31:53Z

Finished benchmarking commit (aef7be6): comparison URL.

Overall result: no relevant changes - 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

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

Max RSS (memory usage)

Results (primary -0.6%)

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

	mean	range	count
Regressions ❌ (primary)	7.2%	[7.2%, 7.2%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-8.5%	[-8.5%, -8.5%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.6%	[-8.5%, 7.2%]	2

Cycles

Results (secondary 2.2%)

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

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.2%	[2.2%, 2.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 475.201s -> 474.025s (-0.25%)
Artifact size: 388.70 MiB -> 388.66 MiB (-0.01%)

Kobzol

Thanks! This is a great cleanup.

Feel free to r=me once you undraft the PR.

View changes since this review

Kobzol · 2025-11-14T08:13:42Z

src/librustdoc/lib.rs

+/// and https://github.com/rust-lang/rust/pull/146627 for why we need this.
+///
+/// NOTE: This is loaded from the sysroot that was built with the other `rustc` crates above, for
+/// some unknown reason this is more performant than building this with Cargo.


That might be the LTO config, although we should also pass LTO to rustdoc.

lqd · 2025-11-14T15:11:07Z

src/tools/miri/src/bin/miri.rs

+/// See docs in https://github.com/rust-lang/rust/blob/HEAD/compiler/rustc/src/main.rs
+/// and https://github.com/rust-lang/rust/pull/146627 for why we need this `use` statement.
+#[cfg(any(target_os = "linux", target_os = "macos"))]
+use tikv_jemalloc_sys as _;


would extern crate be needed here too instead of the use?

madsmtm added T-rustdoc Relevant to the rustdoc team, which will review and decide on the PR/issue. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Nov 13, 2025

rustbot assigned Kobzol Nov 13, 2025

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-clippy Relevant to the Clippy team. labels Nov 13, 2025