Set DontFork and Unmergeable on all mmap sites in turbo-persistence by sokra · Pull Request #90941 · vercel/next.js

sokra · 2026-03-05T18:25:46Z

Summary

Add madvise(MADV_DONTFORK) and madvise(MADV_UNMERGEABLE) to all mmap sites in turbo-persistence
Extract a shared advise_mmap_for_persistence helper into a dedicated mmap_helper module

Why

read_blob in db.rs already correctly sets DontFork and Unmergeable on its mmap, but three other mmap sites were missing these flags:

static_sorted_file.rs (open_internal) — SST file mmaps used for lookups and compaction
meta_file.rs (open_internal) — meta file mmaps storing AMQF filters and SST metadata
sst_inspect.rs (analyze_sst_file) — diagnostic tool mmap

MADV_DONTFORK prevents mmap regions from being copied into child processes on fork(), avoiding unnecessary memory duplication and potential SIGBUS issues if the parent unmaps before the child accesses the page. MADV_UNMERGEABLE opts the pages out of KSM (Kernel Same-page Merging), avoiding the overhead of scanning these pages for deduplication since they contain unique compressed data.

Both flags are Linux-only (#[cfg(target_os = "linux")]), matching the existing pattern in read_blob.

Implementation

Rather than duplicating #[cfg(target_os = "linux")] madvise calls at each mmap site, a shared helper advise_mmap_for_persistence() is extracted into src/mmap_helper.rs. The function applies both flags on Linux and is a no-op on other platforms. All four mmap sites (db.rs, static_sorted_file.rs, meta_file.rs, sst_inspect.rs) now call this single helper.

Test Plan

cargo check -p turbo-persistence passes
cargo fmt clean
No behavioral change on non-Linux platforms (flags are gated behind #[cfg(target_os = "linux")])

Previously only read_blob set these madvise flags. Apply them consistently to SST file, meta file, and sst_inspect mmaps as well.

turbopack/crates/turbo-persistence/src/bin/sst_inspect.rs

codspeed-hq · 2026-03-05T18:48:47Z

Merging this PR will not alter performance

✅ 17 untouched benchmarks
⏩ 3 skipped benchmarks¹

_{Comparing sokra/dont-fork (4178046) with canary (9d0fbf1)}

3 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

nextjs-bot · 2026-03-05T18:52:24Z

Stats from current PR

✅ No significant changes detected

📊 All Metrics

📖 Metrics Glossary

Dev Server Metrics:

Listen = TCP port starts accepting connections
First Request = HTTP server returns successful response
Cold = Fresh build (no cache)
Warm = With cached build artifacts

Build Metrics:

Fresh = Clean build (no .next directory)
Cached = With existing .next directory

Change Thresholds:

Time: Changes < 50ms AND < 10%, OR < 2% are insignificant
Size: Changes < 1KB AND < 1% are insignificant
All other changes are flagged to catch regressions

⚡ Dev Server

Metric	Canary	PR	Change	Trend
Cold (Listen)	455ms	456ms	✓	▇▇▁▁▁
Cold (Ready in log)	439ms	439ms	✓	▇▇▁▁▁
Cold (First Request)	1.268s	1.258s	✓	▇▇▃▁▃
Warm (Listen)	456ms	456ms	✓	▇▇▁▁▁
Warm (Ready in log)	444ms	443ms	✓	▆▆▁▁▁
Warm (First Request)	348ms	351ms	✓	▆▆▂▂▁

📦 Dev Server (Webpack) (Legacy)

📦 Dev Server (Webpack)

Metric	Canary	PR	Change	Trend
Cold (Listen)	456ms	455ms	✓	▁▁▁█▁
Cold (Ready in log)	436ms	436ms	✓	▂▅▆█▁
Cold (First Request)	1.945s	1.949s	✓	▁▄▄█▂
Warm (Listen)	457ms	456ms	✓	▁▁▁█▁
Warm (Ready in log)	436ms	436ms	✓	▂▄▅█▁
Warm (First Request)	1.948s	1.932s	✓	▁▄▄█▁

⚡ Production Builds

Metric	Canary	PR	Change	Trend
Fresh Build	3.937s	3.851s	✓	▆▆▁▃▁
Cached Build	3.889s	3.853s	✓	▆▆▁▃▁

📦 Production Builds (Webpack) (Legacy)

📦 Production Builds (Webpack)

Metric	Canary	PR	Change	Trend
Fresh Build	14.013s	13.900s	✓	▁▂▂█▁
Cached Build	14.087s	14.036s	✓	▁▁▂█▁
node_modules Size	476 MB	476 MB	✓	▁▁▁▁▁

📦 Bundle Sizes

Bundle Sizes

⚡ Turbopack

Client

Main Bundles: **401 kB** → **401 kB** ✅ -14 B

80 files with content-based hashes (individual files not comparable between builds)

Server

Middleware

	Canary	PR	Change
middleware-b..fest.js gzip	767 B	764 B	✓
Total	767 B	764 B	✅ -3 B

Build Details

Build Manifests

	Canary	PR	Change
_buildManifest.js gzip	450 B	452 B	✓
Total	450 B	452 B	⚠️ +2 B

📦 Webpack

Client

Main Bundles

	Canary	PR	Change
5528-HASH.js gzip	5.54 kB	N/A	-
6280-HASH.js gzip	59.4 kB	N/A	-
6335.HASH.js gzip	169 B	N/A	-
912-HASH.js gzip	4.59 kB	N/A	-
e8aec2e4-HASH.js gzip	62.6 kB	N/A	-
framework-HASH.js gzip	59.7 kB	59.7 kB	✓
main-app-HASH.js gzip	255 B	255 B	✓
main-HASH.js gzip	39.1 kB	39.1 kB	✓
webpack-HASH.js gzip	1.68 kB	1.68 kB	✓
262-HASH.js gzip	N/A	4.59 kB	-
2889.HASH.js gzip	N/A	169 B	-
5602-HASH.js gzip	N/A	5.55 kB	-
6948ada0-HASH.js gzip	N/A	62.6 kB	-
9544-HASH.js gzip	N/A	60.2 kB	-
Total	233 kB	234 kB	⚠️ +735 B

Polyfills

	Canary	PR	Change
polyfills-HASH.js gzip	39.4 kB	39.4 kB	✓
Total	39.4 kB	39.4 kB	✓

Pages

	Canary	PR	Change
_app-HASH.js gzip	194 B	194 B	✓
_error-HASH.js gzip	183 B	180 B	🟢 3 B (-2%)
css-HASH.js gzip	331 B	330 B	✓
dynamic-HASH.js gzip	1.81 kB	1.81 kB	✓
edge-ssr-HASH.js gzip	256 B	256 B	✓
head-HASH.js gzip	351 B	352 B	✓
hooks-HASH.js gzip	384 B	383 B	✓
image-HASH.js gzip	580 B	581 B	✓
index-HASH.js gzip	260 B	260 B	✓
link-HASH.js gzip	2.51 kB	2.51 kB	✓
routerDirect..HASH.js gzip	320 B	319 B	✓
script-HASH.js gzip	386 B	386 B	✓
withRouter-HASH.js gzip	315 B	315 B	✓
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Total	7.98 kB	7.98 kB	✅ -1 B

Server

Edge SSR

	Canary	PR	Change
edge-ssr.js gzip	125 kB	125 kB	✓
page.js gzip	254 kB	254 kB	✓
Total	378 kB	379 kB	⚠️ +499 B

Middleware

	Canary	PR	Change
middleware-b..fest.js gzip	618 B	614 B	✓
middleware-r..fest.js gzip	156 B	155 B	✓
middleware.js gzip	43.6 kB	43.5 kB	✓
edge-runtime..pack.js gzip	842 B	842 B	✓
Total	45.2 kB	45.1 kB	✅ -119 B

Build Details

Build Manifests

	Canary	PR	Change
_buildManifest.js gzip	715 B	718 B	✓
Total	715 B	718 B	⚠️ +3 B

Build Cache

	Canary	PR	Change
0.pack gzip	4.04 MB	4.04 MB	✓
index.pack gzip	102 kB	103 kB	✓
index.pack.old gzip	103 kB	102 kB	✓
Total	4.24 MB	4.25 MB	⚠️ +2.48 kB

🔄 Shared (bundler-independent)

Runtimes

	Canary	PR	Change
app-page-exp...dev.js gzip	321 kB	321 kB	✓
app-page-exp..prod.js gzip	170 kB	170 kB	✓
app-page-tur...dev.js gzip	321 kB	321 kB	✓
app-page-tur..prod.js gzip	170 kB	170 kB	✓
app-page-tur...dev.js gzip	318 kB	318 kB	✓
app-page-tur..prod.js gzip	168 kB	168 kB	✓
app-page.run...dev.js gzip	318 kB	318 kB	✓
app-page.run..prod.js gzip	168 kB	168 kB	✓
app-route-ex...dev.js gzip	70.3 kB	70.3 kB	✓
app-route-ex..prod.js gzip	48.6 kB	48.6 kB	✓
app-route-tu...dev.js gzip	70.4 kB	70.4 kB	✓
app-route-tu..prod.js gzip	48.7 kB	48.7 kB	✓
app-route-tu...dev.js gzip	69.9 kB	69.9 kB	✓
app-route-tu..prod.js gzip	48.4 kB	48.4 kB	✓
app-route.ru...dev.js gzip	69.9 kB	69.9 kB	✓
app-route.ru..prod.js gzip	48.4 kB	48.4 kB	✓
dist_client_...dev.js gzip	324 B	324 B	✓
dist_client_...dev.js gzip	326 B	326 B	✓
dist_client_...dev.js gzip	318 B	318 B	✓
dist_client_...dev.js gzip	317 B	317 B	✓
pages-api-tu...dev.js gzip	43.2 kB	43.2 kB	✓
pages-api-tu..prod.js gzip	32.9 kB	32.9 kB	✓
pages-api.ru...dev.js gzip	43.2 kB	43.2 kB	✓
pages-api.ru..prod.js gzip	32.9 kB	32.9 kB	✓
pages-turbo....dev.js gzip	52.6 kB	52.6 kB	✓
pages-turbo...prod.js gzip	38.5 kB	38.5 kB	✓
pages.runtim...dev.js gzip	52.6 kB	52.6 kB	✓
pages.runtim..prod.js gzip	38.5 kB	38.5 kB	✓
server.runti..prod.js gzip	61.8 kB	61.8 kB	✓
Total	2.83 MB	2.83 MB	✅ -5 B

📎 Tarball URL

https://vercel-packages.vercel.app/next/commits/41780464ee454e72f9293ace99215d9f56c13f19/next

bgw · 2026-03-05T19:25:35Z

We should apply all these flags in a single helper function?

Move duplicated DontFork + Unmergeable madvise calls into a shared helper function in a dedicated mmap_helper module. All mmap sites (db.rs, static_sorted_file.rs, meta_file.rs, sst_inspect.rs) now call this single function instead of inlining the cfg-gated advise calls.

…stence

nextjs-bot · 2026-03-05T20:18:05Z

Tests Passed

lukesandberg · 2026-03-06T05:45:15Z

Merge activity

Mar 6, 5:45 AM UTC: @lukesandberg merged this pull request with Graphite.

…90941) ## Summary - Add `madvise(MADV_DONTFORK)` and `madvise(MADV_UNMERGEABLE)` to all mmap sites in turbo-persistence - Extract a shared `advise_mmap_for_persistence` helper into a dedicated `mmap_helper` module ## Why `read_blob` in `db.rs` already correctly sets `DontFork` and `Unmergeable` on its mmap, but three other mmap sites were missing these flags: - **`static_sorted_file.rs`** (`open_internal`) — SST file mmaps used for lookups and compaction - **`meta_file.rs`** (`open_internal`) — meta file mmaps storing AMQF filters and SST metadata - **`sst_inspect.rs`** (`analyze_sst_file`) — diagnostic tool mmap `MADV_DONTFORK` prevents mmap regions from being copied into child processes on `fork()`, avoiding unnecessary memory duplication and potential SIGBUS issues if the parent unmaps before the child accesses the page. `MADV_UNMERGEABLE` opts the pages out of KSM (Kernel Same-page Merging), avoiding the overhead of scanning these pages for deduplication since they contain unique compressed data. Both flags are Linux-only (`#[cfg(target_os = "linux")]`), matching the existing pattern in `read_blob`. ## Implementation Rather than duplicating `#[cfg(target_os = "linux")]` madvise calls at each mmap site, a shared helper `advise_mmap_for_persistence()` is extracted into `src/mmap_helper.rs`. The function applies both flags on Linux and is a no-op on other platforms. All four mmap sites (`db.rs`, `static_sorted_file.rs`, `meta_file.rs`, `sst_inspect.rs`) now call this single helper. ## Test Plan - `cargo check -p turbo-persistence` passes - `cargo fmt` clean - No behavioral change on non-Linux platforms (flags are gated behind `#[cfg(target_os = "linux")]`)

Set DontFork and Unmergeable on all mmap sites in turbo-persistence

1595e3b

Previously only read_blob set these madvise flags. Apply them consistently to SST file, meta file, and sst_inspect mmaps as well.

nextjs-bot added created-by: Turbopack team PRs by the Turbopack team. Turbopack Related to Turbopack with Next.js. labels Mar 5, 2026

sokra requested review from bgw and lukesandberg March 5, 2026 18:27

lukesandberg reviewed Mar 5, 2026

View reviewed changes

turbopack/crates/turbo-persistence/src/bin/sst_inspect.rs Outdated Show resolved Hide resolved

lukesandberg approved these changes Mar 5, 2026

View reviewed changes

bgw approved these changes Mar 5, 2026

View reviewed changes

sokra added 2 commits March 5, 2026 20:38

Use imports instead of absolute crate paths for advise_mmap_for_persi…

4178046

…stence

lukesandberg merged commit 83ae8ad into canary Mar 6, 2026
288 of 291 checks passed

lukesandberg deleted the sokra/dont-fork branch March 6, 2026 05:45

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Set DontFork and Unmergeable on all mmap sites in turbo-persistence#90941

Set DontFork and Unmergeable on all mmap sites in turbo-persistence#90941
lukesandberg merged 3 commits intocanaryfrom
sokra/dont-fork

sokra commented Mar 5, 2026 •

edited

Loading

Uh oh!

Uh oh!

codspeed-hq bot commented Mar 5, 2026 •

edited

Loading

Uh oh!

nextjs-bot commented Mar 5, 2026 •

edited

Loading

⚡ Dev Server

📦 Dev Server (Webpack)

⚡ Production Builds

📦 Production Builds (Webpack)

Bundle Sizes

⚡ Turbopack

📦 Webpack

🔄 Shared (bundler-independent)

Uh oh!

bgw commented Mar 5, 2026

Uh oh!

nextjs-bot commented Mar 5, 2026 •

edited

Loading

Uh oh!

Uh oh!

lukesandberg commented Mar 6, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

4 participants

Conversation

sokra commented Mar 5, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Summary

Why

Implementation

Test Plan

Uh oh!

Uh oh!

codspeed-hq bot commented Mar 5, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Merging this PR will not alter performance

Footnotes

Uh oh!

nextjs-bot commented Mar 5, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Stats from current PR

✅ No significant changes detected

⚡ Dev Server

📦 Dev Server (Webpack)

⚡ Production Builds

📦 Production Builds (Webpack)

Bundle Sizes

⚡ Turbopack

📦 Webpack

🔄 Shared (bundler-independent)

Uh oh!

bgw commented Mar 5, 2026

Uh oh!

nextjs-bot commented Mar 5, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Tests Passed

Uh oh!

Uh oh!

lukesandberg commented Mar 6, 2026

Merge activity

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

4 participants

sokra commented Mar 5, 2026 •

edited

Loading

codspeed-hq bot commented Mar 5, 2026 •

edited

Loading

nextjs-bot commented Mar 5, 2026 •

edited

Loading

nextjs-bot commented Mar 5, 2026 •

edited

Loading