QuickJS engine: threshold-based VM-memory snapshotting (WORKFLOW_SNAPSHOT_THRESHOLD) - #3053
Merged
Merged
Conversation
🦋 Changeset detectedLatest commit: f1b1e10 The changes in this PR will be included in the next version bump. This PR includes changesets to release 16 packages
Not sure what this means? Click here to learn what changesets are. Click here if you're a maintainer who wants to add another changeset to this PR |
Contributor
Contributor
🧪 E2E Test Results✅ All tests passed E2E Test SummarySummary
Details by Category✅ ▲ Vercel Production
✅ 💻 Local Development
✅ 📦 Local Production
✅ 🐘 Local Postgres
✅ 🪟 Windows
✅ 📋 Other
✅ vercel-multi-region
|
TooTallNate
force-pushed
the
quickjs-vm-snapshots
branch
from
July 31, 2026 01:07
1e0b6e6 to
f43ee63
Compare
TooTallNate
force-pushed
the
quickjs-vm-threshold-snapshots
branch
from
July 31, 2026 01:09
64f40c3 to
f1b1e10
Compare
TooTallNate
force-pushed
the
quickjs-vm-snapshots
branch
from
July 31, 2026 02:15
f43ee63 to
f64932e
Compare
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
1 participant
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
Summary
PR 4 of the QuickJS VM roadmap: threshold-based VM-memory snapshotting — the middle ground that motivated reviving this effort (see #1298 / #1300 discussion). Instead of snapshotting at every suspension (the original branch's model, which cost ~25% on e2e wall clock), snapshots are taken only once
WORKFLOW_SNAPSHOT_THRESHOLDevents have been processed since the last one:How it works
WORKFLOW_SNAPSHOT_THRESHOLDenv var (default0= disabled) or per-runexecutionContext.snapshotThreshold, stamped atstart()for run affinity likeWORKFLOW_VM.session.snapshot()) → compress (zstd/gzip via the shared serialization pipeline; QuickJS heaps compress ~4×, measured 16.5 MB → 3.9 MB) → encrypt with the run's key when configured →world.snapshots.savewith the events cursor at the VM's feed frontier.world.snapshots.load→ decrypt → decompress →QuickJS.restoreover the cached WASM module, re-register host callbacks, fetch events from the snapshot's cursor and feed only the delta. Runs seamlessly through PR 2's inline continuation loop.Determinism model (restore + partial replay)
The threshold model's new mechanism vs. the original branch: a resumption may restore a snapshot older than the log head (suspensions since the snapshot weren't persisted) and must deterministically re-derive everything in between:
eventsCursor: the heap already consumed pre-snapshot draws, so re-seeding from the base would replay the first-N draws and collide with recorded correlationIds. The cursor is identical for every resume from the same snapshot (concurrent resumes still collide ids for the world's dedup) and advances only when a newer snapshot is taken.Validation
WORKFLOW_SNAPSHOT_THRESHOLD=1(maximum churn: snapshot on every qualifying suspension), wall clock within ~10% of the node baselinerestored: trueresumptions, save/restore/delete lifecycle, and threshold gating (threshold=100 short run ⇒ zero snapshots, pure replay)quickjs-snapshotmatrix leg (nextjs-turbopack, threshold=1) across local dev/prod/postgres e2e jobsNotes / follow-ups
quickjs-wasibuild that produced them. Per the deployment contract (runs continue on the version they started on — free on Vercel), this is a non-issue in production; environments without skew protection are covered by the restore-failure fallback to full replay.WORKFLOW_SNAPSHOT_THRESHOLDsection added to v5 Runtime Tuning.