[Kamirus]

Supersedes #889 by @timohanke, whose branch on an organization-owned fork doesn't accept maintainer pushes. The first commit is #889 unchanged, so the squash keeps @timohanke as co-author. Fixes #888.

Problem

mops bench instruction counts depended on the order of the cells. The bench canister was compiled with --force-gc, so the incremental GC started a new cycle at the end of every message, and on a large heap that cycle ran on into the next cell. While a cycle is in progress, the GC's write and allocation barriers are on, and the cell pays for them: identical cells differed by 20–40%. #888 has the compiler details.

Dropping --force-gc isn't enough on its own. A cell that leaves enough garbage still starts a cycle, and the next cell runs mid-cycle. In the fixture below, that cell costs 16,100,150 instructions instead of 12,500,150.

Fix

• The incremental GC is no longer compiled with --force-gc. The legacy GCs of moc 1.x keep it, because they finish a collection within the message.
• Before each cell, mops bench calls a new collectorInstructions() method on the bench canister until two calls in a row return the same value. rts_collector_instructions is the GC cost at the end of the previous message, not a running total: about 330 instructions while the GC is paused, and hundreds of millions per increment while a cycle runs. After 1000 messages without a pause, mops bench fails.

Before / after

cli/tests/bench-gc-barrier, moc 2.0.0: identical fill cells, each after a garbage cell, on a 1M-object live heap.

Before (3.5.0):

|    |       fill |     garbage |
| :- | ---------: | ----------: |
| a  | 17_300_150 |  75_001_653 |
| b  | 16_100_150 | 158_552_560 |
| c  | 12_500_150 |  76_051_963 |
| d  | 16_100_150 | 157_503_563 |

After:

|    |       fill |    garbage |
| :- | ---------: | ---------: |
| a  | 12_500_150 | 75_003_204 |
| b  | 12_500_150 | 75_001_630 |
| c  | 12_500_150 | 75_001_681 |
| d  | 12_500_150 | 75_001_428 |

On a real benchmark, research-ag/sort, 23 of 36 cells were inflated by 23–40% on 3.5.0. With this fix, it gives the same counts in both cell orders.

Migration

Baselines saved with 3.5.0 can show lower instruction counts for cells that used to start mid-cycle. To compare against fair numbers, re-save them with mops bench --save. Saved and published results now record forceGc: false for the incremental GC.

What is unchanged

• The Heap table still shows exactly what a cell allocated, and the GC still never runs inside the measured window.
• --gc copying|compacting|generational on moc 1.x still compiles with --force-gc and skips the wait.

Follow-up worth flagging

If two GC increments in a row cost exactly the same number of instructions, the wait would end mid-cycle. That's unlikely, and the Motoko runtime exposes no GC phase to check instead.

🤖 Generated with Claude Code

[github-actions]

Cursor AI review

👍 APPROVE — looks safe to merge

Category	Assessment	Details
Summary	✅	mops bench stops compiling the incremental GC with --force-gc and waits until rts_collector_instructions is stable before each cell, so identical cells no longer pay mid-cycle write/allocation barriers.
Code Quality	✅	Reuses isLegacyGc, cliError, and the existing bench wrapper; waitForGcPause is a small local helper with no speculative API.
Consistency	✅	Changelog ## Next, docs/docs/cli/4-dev/02-mops-bench.md, and cli/declarations/bench/* all match. No new flag, so --help and sibling commands stay consistent; --force-gc remains commented out in cli/cli.ts.
Security	✅	collectorInstructions is a local PocketIC harness method, not the registry. Traced publish.ts bench("", { silent: true }) → uploadBenchmarks; forceGC is existing metadata. Frontend PackageBenchmarks.svelte does not display forceGC. No auth/integrity path changed.
Tests	✅	No snapshot hunks. New cli/tests/bench.test.ts property test plus bench-gc-barrier fixture asserts four identical fill cells and forceGc: false. Existing EOP (bench/, moc 1.16.1) and allocation (bench-allocation) tests still cover compile/run and heap isolation.
Maintainability	✅	Comments in bench.ts and bench-canister.mo state why --force-gc is skipped and what equal collector readings mean. Fail-loud after 1000 messages via cliError (throws; bench() finally still stops PocketIC).

Verdict

Decision: APPROVE
Risk: Low
Reason: Contained, tested bugfix in the local bench harness — not registry, install, auth, or release. Saved and published instruction counts will drop versus 3.5.0 mid-cycle cells, which the changelog and docs already describe.

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Generated for commit 55dfced

[automation-sa-sre]

Automated approval: the AI review verdict for c35ee91 is APPROVE. See the "Cursor AI review" comment for details.

[automation-sa-sre]

Automated approval: the AI review verdict for 55dfced is APPROVE. See the "Cursor AI review" comment for details.