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%.
Collection's slab of malloc'd 8 MiB segments becomes extents in the data file,
and a document's offset becomes an absolute file offset. That makes `doc_bytes`
base + off instead of a binary search over segment starts, and it removes the
hazard the segment list carried: the mapping's base never moves, so a slice into
it cannot be invalidated by growth. phase8 records a dangling-slab-pointer bug
of exactly the shape this deletes.
`slab_reserve` now runs before the log append and `slab_append` after it, and is
infallible. It had no reservation before because appending to an ArrayList could
only fail on OOM; a file-backed slab can also fail on growth, and failing after
the record is durable would report an error for a write the next open produces
anyway.
The data file is still recreated empty on every open and the log still replays in
full, so nothing durable depends on it yet and open/close semantics are
byte-for-byte what they were. The watermark that turns it into a checkpoint comes
next.
--
One bug, and it is worth reading because the milestone keeps producing this
shape. Collection holds a `*Pager`, and `Engine.open` builds an Engine on the
*stack* and returns it by value -- so every pointer taken during replay dangled
the moment it was moved. It surfaced as SIGBUS, then as a corrupt hashmap in the
*second* engine of an unrelated test: nothing resembling its cause. The pager is
heap-allocated now, as `Index` already was, for the same reason.
--
Measured on one harness, 512 MB / 16 KB docs, before and after:
bulk insert throughput 742.6 MB/s -> 746.7 MB/s
createIndex({k: 1}) 26.8 ms -> 16.2 ms
countDocuments({}) 2.1 ms -> 1.1 ms
findOne({k: 500}) indexed 0.75 ms -> 0.53 ms
find({p: range}).count() 6.6 ms -> 4.1 ms
aggregate $group by k 5.8 ms -> 3.7 ms
insertOne (sequential) 0.20 ms -> 0.20 ms
Every row equal or faster. The regression this commit was scheduled early to
catch -- document bytes now reaching disk uncompressed on top of the LZ4 log --
did not appear at this size; bulk insert is flat and the reads gain from one
contiguous mapping instead of separately allocated segments.
What did *not* improve, stated plainly because it is the milestone's headline
claim: RSS is unchanged, 552 MB against 563 MB for 512 MB of data. It cannot
improve yet. Every open still replays the whole log and rewrites the whole slab,
so every page is touched and resident regardless of where it lives. "RSS =
working set" only becomes measurable once an open loads a checkpoint instead of
rebuilding, and the honest test for it is the multi-GB reopen in big.js, not this
microbenchmark.
Prose and benchmark tables use MultiforaDB; the binary, the CLI usage
line, the log-message prefix and the default database file use
multiforadb.
Two consequences worth noting:
- build.zig.zon's fingerprint is derived from the package name, so it
had to change with it (Zig refuses to build otherwise). A consumer
pinning this package by fingerprint needs updating.
- the default --db path is now multiforadb.log, and getCmdLineOpts
reports it as dbpath. An existing mongo-lite.log has to be passed
explicitly with --db.
The e2e harness abbreviated the old name as ML_; that is now MFDB_,
including the documented ML_BIN override (MFDB_BIN) and the scratch
file names. MD_ (mongod) is untouched.
compare-run.sh spawned the server by absolute path under a
sandbox/mongo-lite directory that no longer exists; that block already
runs from tests/e2e, so it uses a relative path now.
The archived reports under tests/e2e/results/ keep the old name: they
record what the old binary measured.
Two streams of work land together: they are interleaved in storage.zig
and db.zig and only build as a unit.
Already in the working tree before this session:
- ReleaseFast as the default zig build (Debug was 10-200x slower)
- group commit: one fsync per write command instead of per document
- plan_id returned a pointer to a stack temporary; ReleaseFast read
garbage and silently broke findOne({_id: ObjectId})
- perf suite: big.js, compare.js, compare-run.sh, e2e6.js
Phase 1 performance work:
Record integrity hash CRC32 -> XxHash3. std.hash.Crc32 is the
table-driven byte-at-a-time Crc32IsoHdlc, measured at 408 MB/s against
XxHash3's 31 GB/s: 38us versus 0.5us on a 16 KiB document, which was
about two thirds of the entire bulk-insert cost. The record header
grows from u32 crc to u64 hash (header_len 20 -> 24), a breaking
format change. Bulk insert 260 -> 700 MB/s, reopen 1.1 -> 0.5s.
Compaction fsynced once per live document, because Log.open leaves
defer_sync false and compact never set it. It now issues one sync for
the whole rewrite, before the rename that publishes it.
Compaction triggers on the share of the log that is garbage
(live_docs/dead_docs, maintained at evict_doc, the single point where
a document dies) rather than on bytes appended. A fixed byte count is
wrong in both directions: a 1 GB bulk load holds no garbage at all yet
would compact ~64 times under the 16 MiB default, rewriting 1 GB each
time, while a small collection rewritten in place accumulates garbage
indefinitely without ever reaching the count. Pure inserts now never
compact, and the file stays near 1.25x the live data. Bulk load at the
default threshold: 41.6 -> 702.7 MB/s.
remove() never called maybe_compact, so a delete-heavy workload grew
the log without bound.
e2e6's compaction check required the file to bloat past 30 MiB before
being reclaimed, which encoded the old policy and failed on strictly
better behaviour (ends at 15.8 MB against ~12 MB live, was ~30 MB). It
now asserts the file ends near the live size and peaked well above it,
which does not depend on when the trigger fires. Sampling interval 50
-> 10ms: the operations now finish inside the old window.
Verified: 70 unit tests under both ReleaseFast and ReleaseSafe, e2e
29/16/17/3/2 checks, the crash-a/kill -9/crash-b pair, and e2e6 72/72
across three consecutive runs.
