databasev2 4 part A, task 5.
Controlled before/after — same machine, same workload (wmix 4000 32),
same build except db.c and vm.c, two runs each interleaved:
- per-statement barrier: 2213 / 2177 ops/sec, p50 7183 / 7251us
- group commit: 6216 / 6525 ops/sec, p50 3458 / 3444us
- ~2.9x throughput, ~2.1x lower p50
The full campaign confirms it a second way: s1 takes the inline path and
commits per statement BY DESIGN, so within one build the shard configs are
batching-off vs batching-on — 1467 -> 5117 ops/sec, mean batch 1.0 -> 5.43,
peak 1 -> 57. 3.5x, agreeing with the 2.9x above.
Recorded honestly:
- the BEFORE p99 is at the histogram ceiling (hist_add clamps at 20000us
and both runs pinned there), so the true figure is >=20ms and unknown.
The improvement is AT LEAST 2.3x; the old p99 was off the instrument
- durable.sN.mixwrite went 480 -> 492 ops/sec, i.e. UNCHANGED. That was
the spec's original payoff metric and correcting it was part of the
brainstorm: mix performs 20 writes at C=4, mean batch 1.01. A workload
that never has two writes in flight cannot be helped by batching them
- seed is likewise unchanged: a serial writer has nothing to batch with
- so the payoff is real but CONDITIONAL — it appears where concurrent
durable writes fan into the owner shard, and nowhere else
Two traps recorded in perf-targets §6:
- do not benchmark durability on /tmp: it is tmpfs here, where fdatasync
is free. The same run reported 195000 ops/sec at p50 1us there against
2200 at p50 7200us on ext4 — no barrier to amortise, so the measurement
measures nothing. db-bench keeps its stores under bench/ for this reason
- the record count is not the update count: 7755 records for 4000 updates,
because hist_dump and the done-marker are themselves durable inserts
- FIXED a regression I introduced in T4: master's committed baseline is
FULL mode (N=20000, crash_reps=3, msg_n=200000) and I had overwritten it
with quick-mode values. Regenerated from a full campaign; the full run
now passes 106 checks 0 failures against it
- gate still bites: sN wmix ops_sec -70% -> FAIL on exactly that metric
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
shoney.arickathil
6183a67dfc