Aleksey Shakhmatov
504179acd1
PLAN D7's six items, with the numbers and the command that reproduces each in
tests/e2e/results/m0-gates.txt. Unit tests green in both optimize modes, the
whole e2e matrix green, the spec scorecard byte-identical at 131/161/195, and
the large smoke run at the scale D7.3 asked for:
21.47 GB collection (1,310,720 x 16 KiB)
data file 21.75 GB (+1.3% over the documents)
log after the load 2.5 MB (checkpoints reclaim it)
kill -9 then reopen 0.5 s (0.5 s at 4 GB too -- flat)
RSS after reopen 237 MB (1.1% of the data)
count after restart 1,310,720 last document byte-intact
acked writes after kill 200/200
That is the milestone's claim, measured: an open costs the working set rather
than the size of the database. Before M0 the same measurement was 523 MB
resident for a 512 MB database, because recovering each document's `_id` meant
reading every document at open.
Two gates need reading rather than a tick, and m0-gates.txt says so where a
reader would otherwise take a tick for granted.
The churn gate settles at 1.65x live data (delete-heavy) to 2.47x
(update-heavy), flat, above the ~1.3x amendment A2 hoped for. Rebuild-only
reclamation cannot reach that: it needs a whole second copy of the live data
before the first can be freed. The gate existed to decide whether doc-level
free lists are needed after M0, and that is the answer.
Benchmark parity holds for every read and latency row inside the run-to-run
spread, and bulk insert regresses 24% (732 -> 555 MB/s), reproducibly across
three runs. Risk 1 as written: document bytes now reach the disk uncompressed
on top of the LZ4 log. createIndex improves 62% from the same change.
Three measurement bugs fixed while running the gates, because each would have
put a false number in the README:
- `compare-run.sh` measured "db on disk" as `du` of the log alone against
`du` of mongod's whole dbpath. It reported 20 MB for a 1 GB collection --
the documents had moved to <db>.data. Honest figure, measured: 914 MB of
allocated blocks against mongod's compressed 85 MB.
- `big.js` counted "compaction events" as "the log shrank", which is a
*checkpoint* now. It claimed 12 compaction rewrites during a pure insert
load, which has no garbage to compact.
- `big.js` labelled peak RSS "in-memory engine: docs live in RAM" and its
summary said the collection was held "fully in RAM". Both were true of the
engine this milestone replaced.
README: the storage section described an all-in-RAM engine; the comparison
table mixed one old run's body with three new rows; and `findOne({_id})` was
documented as a full scan for integer ids, which the ordered `_id_` index made
false (2 ms against 55 s for a scan of the same 21.5 GB collection). The table
is now best-of-three for both servers, with the measured variance stated, since
two runs of the same binary moved the sub-10 ms rows by 27-51%.
157 lines
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157 lines
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Plaintext
# M0 gate results — mmap + WAL storage foundation (PLAN D7)
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#
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# Machine: Apple Silicon, macOS 25.5.0, 16 KiB system pages, APFS.
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# Server: MultiforaDB at the commit named below, ReleaseFast.
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# Driver: mongodb@7.5.0 (pinned in tests/e2e/package-lock.json).
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# mongod: 8.3.7, for the parity rows.
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#
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# Read this alongside PLAN.md §5. Every number here is reproducible with the
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# command printed under it; where a gate was not met, the number is recorded as
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# measured and the reason is stated rather than the workload being tuned until
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# it passed.
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[D7.1] unit tests, ReleaseFast and ReleaseSafe
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zig build test -Doptimize=ReleaseFast 122/122 pass
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zig build test -Doptimize=ReleaseSafe 122/122 pass
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zig build fuzz fuzz_split, spill, spill2, stress clean
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Both modes matter: `protect_stable` (the mprotect belt over the published
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image) is comptime-off in ReleaseFast, and `std.posix.mprotect` not existing
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in Zig 0.16 was a ReleaseSafe-only compile error.
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[D7.2] end-to-end suites — all green
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reproduce: zig-out/bin/multiforadb --port 27020 --db /tmp/mfdb-e2e.log \
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--ttl-sweep-secs 1 & then node tests/e2e/<suite>
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e2e.js CRUD, operators, aggregate, errors 35/35
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e2e3.js secondary indexes: unique/sparse/compound 16/16
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e2e4.js TTL indexes: expiry and rejected specs 17/17
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e2e5.js miscellaneous command surface 3/3
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e2e2.js concurrent, 4 writers + 4 readers 2/2
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e2e2.js crash-a / crash-b (kill -9, then verify) 1/1 then 3/3
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e2e6.js compaction and log growth (own server) 72/72
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[D7.3] large-collection smoke — big.js
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reproduce: node tests/e2e/big.js --size 20g --doc-size 16k
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20g run 4g run
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documents 1,310,720 262,144 (x 16 KiB)
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live data 21.47 GB 4.29 GB
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data file 21.75 GB 4.35 GB (+1.3%, +1.4%)
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log file, min -> final 16 B -> 2.5 MB 16 B -> 7.7 MB
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bulk insert 411.0 MB/s 560.3 MB/s
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peak server RSS during load 18,492 MB 3,318 MB
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kill -9 then reopen 0.5 s 0.5 s
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RSS after reopen 237 MB (1.1%) 130 MB (3.0%)
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count after restart 1,310,720 OK 262,144 OK
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last doc intact after restart payload 16254B OK
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kill -9 after 200 acked writes 200/200 200/200
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This is the milestone's central claim, measured: an open costs the working
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set, not the size of the database, and it no longer replays the whole log.
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Reopen is flat at 0.5 s from 4 GB to 21.5 GB, and RSS after reopen falls as a
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*share* of the data as the database grows — which is what "RSS = working set"
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has to mean to be worth anything.
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Before M0 the same shape of run reported 523 MB resident after reopen for a
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512 MB database, because recovering each document's `_id` meant reading every
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document at open. That scan is what the mmap foundation deleted.
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Two other things the 20g run shows, honestly. Peak RSS during the *load* is
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18.5 GB: writing 21 GB touches 21 GB of pages and the kernel keeps them until
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it needs the memory, so a bulk load is not where the mmap win shows up. And a
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cold full scan of 21 GB costs ~55 s — countDocuments, the range scan and the
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unindexed updateMany all land there, ~390 MB/s off the disk — which is correct
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and is the reason M1's cursors and index-only paths matter.
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[D7.4] churn gate — doc/slab garbage (PLAN amendment A2 retargets D6.2 here)
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40,000 x 16 KiB documents (655 MB live), one secondary index, steady state
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measured as data-file size / live data after the ratio stopped moving.
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before the fixes after
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delete half and refill, 6 rounds 4.10x, climbing 1.65x, flat
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random $set over 5x the collection 3.58x, climbing 2.47x, flat
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"Climbing" is the whole finding: nothing was being reclaimed at all. Three
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bugs, all fixed in `db/pager: reclaim what churn abandons` — a compaction
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trigger that had read `log.data_bytes` since before the log was truncated at
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every checkpoint, a numeric stable mark that made recycled pages look
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published, and a first-fit free list that let one-page requests dismantle
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the extents. A fourth fix (a rebuild publishes twice, so the space it frees
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is reusable immediately) took the interleaved case from 3.58x to 2.47x.
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The gate hoped for ~1.3x and this is above it, structurally: a rebuild needs
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a whole second copy of the live data before the first can be freed, and
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two-generation retention holds the old copy through two more publishes. The
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gate's purpose was to decide whether doc-level free lists are needed after
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M0. They are; that is an M1 item. What M0 owes is a bound, and there is one.
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[D7.5] benchmark parity vs the pre-mmap engine
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reproduce: bash tests/e2e/bench-run.sh 1g 16k "1 4 8 16 32"
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baseline: tests/e2e/results/phase8.txt (c8d547f, all-in-RAM engine)
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M0: three runs, tests/e2e/results/bench-2026080{3-223947,3-225128,3-225322}.txt
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Read the range, not a single delta. Two consecutive runs of the *same* binary
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moved the sub-10 ms rows by 27-51% on this machine, so any one comparison
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reads whatever the noise did that minute. Best-of-three against the baseline:
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row pre-mmap M0 min..max best
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insertOne (sequential) x200 0.20 ms 0.20..0.22 ms +0%
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bulk insert throughput 732.4 MB/s 463.4..555.5 MB/s -24%
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createIndex({k: 1}) 74.4 ms 27.9..28.9 ms -62%
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countDocuments({}) 3.1 ms 2.6..3.6 ms -16%
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findOne({_id: <ObjectId>}) 0.59 ms 0.60..0.80 ms +2%
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findOne({k: 500}) (indexed) 0.56 ms 0.54..0.73 ms -4%
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find({p: {$gte,$lt}}).count() (scan) 13.0 ms 11.2..14.5 ms -14%
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find({}).sort({_id:-1}).limit(20) 2.2 ms 1.5..1.9 ms -32%
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find({}, {proj}).limit(1000) 3.5 ms 3.6..3.8 ms +3%
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aggregate $group by k 8.3 ms 7.4..11.2 ms -11%
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updateOne({_id}) x50 0.14 ms 0.15..0.18 ms +7%
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updateMany({k: 7}, {$inc}) 1.9 ms 1.9..2.0 ms +0%
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deleteOne({_id}) + insertOne 0.58 ms 0.63..0.69 ms +9%
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node client RSS 158 MB 150..163 MB -5%
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concurrent durable insertOne, docs/s:
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clients 1 8,435 7,709..8,373 -1%
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clients 4 22,480 20,122..20,880 -7%
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clients 8 25,920 26,210..26,349 +2%
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clients 16 31,079 27,794..29,594 -5%
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clients 32 34,041 32,235..32,817 -4%
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One reproducible regression: bulk insert, -24%, steady across all three runs.
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This is PLAN risk 1 exactly as written -- document bytes now reach the disk
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uncompressed in the data file on top of the LZ4 log, and that writeback
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bandwidth is new. Every other row is inside the run-to-run spread. The gate
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as stated was "no phase8 row regressed"; it is met for the read and latency
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rows and not for bulk load, which is the trade the milestone makes and which
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risk 1 anticipated. Untried mitigations, in the order worth trying:
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MADV_HUGEPAGE / a larger growth chunk, and not writing doc bytes twice
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(log and slab) for a bulk path that could log an extent reference instead.
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createIndex at -62% is the other reproducible number, and it comes from the
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same change: a bulk build now packs pages in the mapping instead of growing
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an ArrayList.
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[D7.6] MongoDB spec-test scorecard
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reproduce: bash tests/spec/fetch.sh && node tests/spec/run.js --scorecard
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total 131 pass 161 fail 195 skip 175 files 0 errored
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Byte-identical to the scorecard recorded before the storage rewrite. That is
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the intended result and it is worth stating plainly: M0 replaced the document
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store, the node arena and the overflow slab, added copy-on-write, a watermark
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and a checkpoint, and moved every index leaf's payload -- and changed no
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observable CRUD or aggregate semantics.
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# WHAT THE GATES FOUND
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# Five bugs, none of which any unit test or e2e suite had reached:
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# 1. The compaction trigger had been dead since commit 14 (log truncation
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# zeroes the counter it gated on). Nothing reclaimed doc-slab garbage.
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# 2. `page_mut_cow` asked `p >= stable_pages`, which is false for a recycled
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# page, so every write to one copied and freed it again.
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# 3. First fit let copy-on-write's one-page requests shave the extents the
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# doc slab needs, so the free list drained every generation and the file
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# grew anyway.
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# 4. A rebuild's freed space took two unrelated checkpoints to become
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# reusable, so the next rebuild grew the file instead of reusing it.
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# 5. `reserve_pages`' promise was a single counter on the pager. Two upserts
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# on different collections release it independently, so the first to
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# finish revoked the second's promise mid-write -- aborting the server at
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# four concurrent clients, on the first concurrent benchmark since the
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# data file landed. PLAN risk 3, whose mitigation was never built.
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# Four of the five are invisible without a workload that runs long enough to
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# reach a steady state, or wide enough to have two writers. That is the
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# argument for keeping both the churn gate and the concurrent benchmark in the
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# gate list rather than treating them as optional.
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#
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# ONE COMPATIBILITY GAP FOUND, NOT FIXED (out of M0's scope, storage-only)
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# `update.apply` (src/update.zig:18) rejects any update document whose first
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# key is not `$`, so replacement-style writes -- replaceOne, findOneAndReplace,
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# bulkWrite's replaceOne -- fail with "bad update". Found while building the
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# churn workload. It is a CRUD feature, not storage, and it is already part of
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# what the 161 spec failures cover.
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