Optimize how we track data for persistence #89370

lukesandberg · 2026-02-02T07:18:02Z

What?

Optimizes the change tracking mechanism used for persistent cache snapshots in turbo-tasks-backend.

Why?

The previous implementation used a FxDashMap<TaskId, ModifiedState> to track modified tasks, where ModifiedState was an enum with Modified and Snapshot(Option<Box<TaskStorage>>) variants. This had several inefficiencies:

Memory overhead: The ModifiedState enum was 16 bytes to account for the rare 'snapshot race' case
Datastructure overhead: the DashMap for the modified map was highly sharded leading to large amounts of memory overhead. Most acquisitions were extremely short lived and so this isn't necessary.
Separate task cache log: New task types were tracked in a separate TaskCacheLog structure (Sharded<ChunkedVec<...>>), requiring additional synchronization and a separate persistence path.

How?

Restructured change tracking into two separate data structures:

modified: Sharded<Vec<TaskId>> - A sharded append-only list for tracking modified task IDs. Since modifications are guarded by a transition in the any_modified flag on TaskStorage, each task is only added once, avoiding duplicates without additional synchronization.
snapshots: FxDashMap<TaskId, Option<Box<TaskStorage>>> - A small, rarely-used map for the uncommon case where a task is modified during an active snapshot operation. Uses only 16 shards since this is rare (only during dev mode idle-callback persistence races).

Unified task cache persistence:

Removed the separate TaskCacheLog and ChunkedVec infrastructure
New tasks now include their task_type directly in the SnapshotItem struct
Task cache entries are written inline with task data during persistence, simplifying the write path

Other optimizations:

SmallVec::into_boxed_slice() now called directly instead of into_vec().into_boxed_slice() (avoids intermediate allocation)
Removed swap_retain utility (no longer needed)
Simplified snapshot iteration using scope_and_block with chunked shards, this reduces the number of 'scratch buffers' allocated to 'one per chunk' instead of 'one per original shard'
Fixed a very subtle race in end_snapshot where multiple racing save_snapshot calls could corrupt state.

Results on vercel-site

I ran a sequence of builds on vercel-site comparing canary and this branch

Cold Cache Builds

Metric	Optimization (`87071fe`)	Canary (`479877d`)	Difference
Duration (avg)	82.98s	82.56s	+0.51%
Duration (min)	82.44s	80.39s
Duration (max)	83.20s	87.24s
Duration (std dev)	0.27s	2.22s
Peak Memory (avg)	21.92 GB	23.07 GB	-4.98%
Peak Memory (min)	21.83 GB	22.64 GB
Peak Memory (max)	22.06 GB	23.78 GB
Sample Size	n=6	n=6

Warm Cache Builds

Metric	Optimization (`87071fe`)	Canary (`479877d`)	Difference
Duration (avg)	13.71s	14.75s	-7.02%
Duration (min)	10.65s	11.41s
Duration (max)	17.77s	19.84s
Peak Memory (avg)	11.10 GB	11.39 GB	-2.49%
Peak Memory (min)	10.76 GB	10.68 GB
Peak Memory (max)	11.48 GB	12.06 GB
Sample Size	n=12	n=12

The latency deltas are in the noise but there are substantial memory wins from this change

lukesandberg · 2026-02-02T07:18:28Z

Remove task snapshots from persistence #89412
Optimize how we track data for persistence #89370 👈 (View in Graphite)
canary

This stack of pull requests is managed by Graphite. Learn more about stacking.

turbopack/crates/turbo-tasks-backend/src/backend/storage.rs

codspeed-hq · 2026-02-02T07:38:29Z

CodSpeed Performance Report

Merging this PR will improve performance by 3.76%

_{Comparing optimized_change_tracking (4dada51) with canary (7437fed)}

Summary

⚡ 1 improved benchmark
✅ 16 untouched benchmarks
⏩ 3 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	`BASE`	`HEAD`	Efficiency
⚡	Simulation	`jsonwebtoken.js[tracing]`	280.6 ms	270.4 ms	+3.76%

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 · 2026-02-02T07:40:44Z

Stats from current PR

🟢 1 improvement

Metric	Canary	PR	Change	Trend
Webpack Build Time	16.159s	14.536s	🟢 1.623s (-10%)	▁▁▃▁▅

📊 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)	506ms	505ms	✓	▁▁▁▁▁
Cold (Ready in log)	461ms	460ms	✓	▂▂▂▂▁
Cold (First Request)	829ms	834ms	✓	▅▅▅▄▁
Warm (Listen)	458ms	456ms	✓	▁▁▁▁▁
Warm (Ready in log)	459ms	456ms	✓	▁▁▁▁▁
Warm (First Request)	363ms	364ms	✓	▁▁▂▂▁

📦 Dev Server (Webpack) (Legacy)

📦 Dev Server (Webpack)

Metric	Canary	PR	Change	Trend
Cold (Listen)	507ms	506ms	✓	▁█▅▅█
Cold (Ready in log)	475ms	467ms	✓	▇▆█▇█
Cold (First Request)	1.963s	1.930s	✓	▄▃▅▄▆
Warm (Listen)	458ms	458ms	✓	▅▅▅▅█
Warm (Ready in log)	451ms	454ms	✓	▇▆▆▆█
Warm (First Request)	1.907s	1.917s	✓	▅▄▆▄▇

⚡ Production Builds

Metric	Canary	PR	Change	Trend
Fresh Build	4.194s	4.144s	✓	▁▁▁▁▃
Cached Build	4.241s	4.201s	✓	▁▁▁▁▃

📦 Production Builds (Webpack) (Legacy)

📦 Production Builds (Webpack)

Metric	Canary	PR	Change	Trend
Fresh Build	16.159s	14.536s	🟢 1.623s (-10%)	▁▁▃▁▅
Cached Build	14.925s	14.785s	✓	▁▁▃▁▅
node_modules Size	464 MB	464 MB	✓	▁▁▁▁▁

📦 Bundle Sizes

Bundle Sizes

⚡ Turbopack

Client

Main Bundles: **434 kB** → **435 kB** ⚠️ +131 B

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

Server

Middleware

	Canary	PR	Change
middleware-b..fest.js gzip	763 B	764 B	✓
Total	763 B	764 B	⚠️ +1 B

Build Details

Build Manifests

	Canary	PR	Change
_buildManifest.js gzip	449 B	451 B	✓
Total	449 B	451 B	⚠️ +2 B

📦 Webpack

Client

Main Bundles

	Canary	PR	Change
5528-HASH.js gzip	5.47 kB	N/A	-
6280-HASH.js gzip	54.5 kB	N/A	-
6335.HASH.js gzip	169 B	N/A	-
912-HASH.js gzip	4.53 kB	N/A	-
e8aec2e4-HASH.js gzip	62.5 kB	N/A	-
framework-HASH.js gzip	59.7 kB	59.7 kB	✓
main-app-HASH.js gzip	255 B	254 B	✓
main-HASH.js gzip	39 kB	39.1 kB	✓
webpack-HASH.js gzip	1.68 kB	1.68 kB	✓
262-HASH.js gzip	N/A	4.52 kB	-
2889.HASH.js gzip	N/A	169 B	-
5602-HASH.js gzip	N/A	5.48 kB	-
6948ada0-HASH.js gzip	N/A	62.5 kB	-
9544-HASH.js gzip	N/A	55.2 kB	-
Total	228 kB	229 kB	⚠️ +796 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	2.49 kB	2.49 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	7.97 kB	7.97 kB	✅ -1 B

Server

Edge SSR

	Canary	PR	Change
edge-ssr.js gzip	126 kB	126 kB	✓
page.js gzip	249 kB	249 kB	✓
Total	375 kB	375 kB	⚠️ +403 B

Middleware

	Canary	PR	Change
middleware-b..fest.js gzip	616 B	614 B	✓
middleware-r..fest.js gzip	156 B	155 B	✓
middleware.js gzip	33.1 kB	33.1 kB	✓
edge-runtime..pack.js gzip	842 B	842 B	✓
Total	34.7 kB	34.8 kB	⚠️ +56 B

Build Details

Build Manifests

	Canary	PR	Change
_buildManifest.js gzip	732 B	736 B	✓
Total	732 B	736 B	⚠️ +4 B

Build Cache

	Canary	PR	Change
0.pack gzip	3.79 MB	3.81 MB	🔴 +15.3 kB (+0%)
index.pack gzip	104 kB	102 kB	🟢 2.03 kB (-2%)
index.pack.old gzip	103 kB	102 kB	✓
Total	4 MB	4.01 MB	⚠️ +12.7 kB

🔄 Shared (bundler-independent)

Runtimes

	Canary	PR	Change
app-page-exp...dev.js gzip	311 kB	311 kB	✓
app-page-exp..prod.js gzip	166 kB	166 kB	✓
app-page-tur...dev.js gzip	311 kB	311 kB	✓
app-page-tur..prod.js gzip	166 kB	166 kB	✓
app-page-tur...dev.js gzip	308 kB	308 kB	✓
app-page-tur..prod.js gzip	164 kB	164 kB	✓
app-page.run...dev.js gzip	308 kB	308 kB	✓
app-page.run..prod.js gzip	164 kB	164 kB	✓
app-route-ex...dev.js gzip	70.4 kB	70.5 kB	✓
app-route-ex..prod.js gzip	48.9 kB	49 kB	✓
app-route-tu...dev.js gzip	70.4 kB	70.5 kB	✓
app-route-tu..prod.js gzip	49 kB	49 kB	✓
app-route-tu...dev.js gzip	70 kB	70.1 kB	✓
app-route-tu..prod.js gzip	48.7 kB	48.8 kB	✓
app-route.ru...dev.js gzip	70 kB	70.1 kB	✓
app-route.ru..prod.js gzip	48.7 kB	48.7 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	43.1 kB	43.2 kB	✓
pages-api-tu..prod.js gzip	32.9 kB	32.9 kB	✓
pages-api.ru...dev.js gzip	43.1 kB	43.2 kB	✓
pages-api.ru..prod.js gzip	32.8 kB	32.9 kB	✓
pages-turbo....dev.js gzip	52.4 kB	52.5 kB	✓
pages-turbo...prod.js gzip	39.4 kB	39.4 kB	✓
pages.runtim...dev.js gzip	52.4 kB	52.5 kB	✓
pages.runtim..prod.js gzip	39.3 kB	39.4 kB	✓
server.runti..prod.js gzip	62.6 kB	62.6 kB	✓
Total	2.77 MB	2.78 MB	⚠️ +1.01 kB

📝 Changed Files (25 files)

Files with changes:

app-page-exp..ntime.dev.js
app-page-exp..time.prod.js
app-page-tur..ntime.dev.js
app-page-tur..time.prod.js
app-page-tur..ntime.dev.js
app-page-tur..time.prod.js
app-page.runtime.dev.js
app-page.runtime.prod.js
app-route-ex..ntime.dev.js
app-route-ex..time.prod.js
app-route-tu..ntime.dev.js
app-route-tu..time.prod.js
app-route-tu..ntime.dev.js
app-route-tu..time.prod.js
app-route.runtime.dev.js
app-route.ru..time.prod.js
pages-api-tu..ntime.dev.js
pages-api-tu..time.prod.js
pages-api.runtime.dev.js
pages-api.ru..time.prod.js
... and 5 more

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nextjs-bot · 2026-02-03T05:38:16Z

Failing test suites

Commit: 4dada51 | About building and testing Next.js

pnpm test-dev test/e2e/app-dir-export/test/dynamicapiroute.test.ts (job)

app dir - with output export - dynamic api route > should work in prod with dynamicApiRoute 'force-static' > should work (DD)

Expand output

● app dir - with output export - dynamic api route › should work in prod with dynamicApiRoute 'force-static' › should work

expect(received).toContain(expected) // indexOf

Expected substring: "Another"
Received string:    "Home"

  306 |
  307 |       await retry(async () =>
> 308 |         expect(await browser.elementByCss('h1').text()).toContain('Another')
      |                                                         ^
  309 |       )
  310 |       expect(await browser.elementByCss(a(1)).text()).toBe(
  311 |         'Visit the home page'

  at toContain (e2e/app-dir-export/test/utils.ts:308:57)
  at retry (lib/next-test-utils.ts:831:14)
  at Object.<anonymous> (e2e/app-dir-export/test/utils.ts:307:7)

sokra · 2026-02-03T07:36:33Z

turbopack/crates/turbo-tasks-backend/src/backend/mod.rs

        let process = |task_id: TaskId,
-                       (meta, data): (Option<TaskStorage>, Option<TaskStorage>),
+                       (meta, data, task_type): (
+            Option<TaskStorage>,
+            Option<TaskStorage>,
+            Option<Arc<CachedTaskType>>,
+        ),
                       buffer: &mut TurboBincodeBuffer| {


Funny formatting..

Maybe move it into a type X = to avoid that

sokra · 2026-02-03T07:38:10Z

turbopack/crates/turbo-tasks-backend/src/backend/mod.rs

-                            let meta = match meta {
-                                Some(Ok(meta)) => {
-                                    #[cfg(feature = "print_cache_item_size")]
-                                    task_cache_stats
-                                        .lock()
-                                        .entry(self.debug_get_task_name(task_id))
-                                        .or_default()
-                                        .add_meta(&meta);
-                                    Some(meta)
-                                }
-                                None => None,
-                                Some(Err(err)) => {
-                                    println!(
-                                        "Serializing task {} failed (meta): {:?}",
-                                        self.debug_get_task_description(task_id),
-                                        err
-                                    );
-                                    None
-                                }
-                            };
-                            let data = match data {
-                                Some(Ok(data)) => {
-                                    #[cfg(feature = "print_cache_item_size")]
-                                    task_cache_stats
-                                        .lock()
-                                        .entry(self.debug_get_task_name(task_id))
-                                        .or_default()
-                                        .add_data(&data);
-                                    Some(data)
-                                }
-                                None => None,
-                                Some(Err(err)) => {
-                                    println!(
-                                        "Serializing task {} failed (data): {:?}",
-                                        self.debug_get_task_description(task_id),
-                                        err
-                                    );
-                                    None
-                                }
-                            };


Could you keep the print_cache_item_size logic?

sokra · 2026-02-03T07:54:09Z

turbopack/crates/turbo-tasks-backend/src/backend/operation/aggregation_update.rs

            && !task.has_persistent_task_type()
        {
            let _ = task.set_persistent_task_type(task_type);
+            task.set_new_persistent_task(true);


I was a bit worried that this will add a race condition, because the task can be persisted before we set the new_persistent_task flag.
I think it's fine, as the task id can't be referenced by other tasks before that.
So in the very rare case of this happening we only end up with a dangling unused task.

sokra · 2026-02-03T07:58:14Z

turbopack/crates/turbo-tasks-backend/src/backend/storage.rs

-                process_snapshot,
-                scratch_buffer: TurboBincodeBuffer::with_capacity(SCRATCH_BUFFER_SIZE),
+        // Ideally these shards would be perfectly aligned with the dashmap so we could
+        // monolithically lock shards instead of axquiring a lock for each item.  But doing this


Optimize how we track data for persistence

eb9af29

nextjs-bot added created-by: Turbopack team PRs by the Turbopack team. Turbopack Related to Turbopack with Next.js. labels Feb 2, 2026

vercel bot reviewed Feb 2, 2026

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turbopack/crates/turbo-tasks-backend/src/backend/storage.rs Outdated Show resolved Hide resolved

lukesandberg added 2 commits February 1, 2026 23:44

fix clippy issue

91e1a12

fix some documentation and parallise end_snapshot

87071fe

lukesandberg mentioned this pull request Feb 2, 2026

Remove task snapshots from persistence #89412

Draft

lukesandberg added 3 commits February 2, 2026 12:55

simplify callbacks

b3d4105

fix a subtle issue with state management

8339c65

fix a subtle race condition

4dada51

lukesandberg marked this pull request as ready for review February 3, 2026 05:23

lukesandberg requested review from bgw and sokra February 3, 2026 05:23

sokra reviewed Feb 3, 2026

View reviewed changes

Optimize how we track data for persistence #89370

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CodSpeed Performance Report

Merging this PR will improve performance by 3.76%

Summary

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⚡ Turbopack

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