`slab_reserve` and `reserve_overflow` abandoned the rest of their extent
whenever a checkpoint froze the page the tail pointed into, and took a fresh
8 MiB one. The comment called the waste "bounded by one extent per collection
per checkpoint", which is true per checkpoint and says nothing about the sum:
nothing reclaims it except a rebuild, and a rebuild only runs when there is
garbage. A pure-insert workload produces none.
Measured, 40 collections of inserts with incompressible payloads so the log
actually reaches the checkpoint threshold:
live data file log
29 MB 340 MB 29 MB
38 MB 542 MB 5 MB <- checkpoint
67 MB 681 MB 33 MB
76 MB 1076 MB 9 MB <- checkpoint
115 MB 1357 MB 14 MB <- checkpoint
11.8x the live data and climbing by ~335 MB per checkpoint (40 x 8 MiB), which
would exhaust the 64 GB address-space reservation after roughly 6 GB of real
data -- and after ~1.2 GB with 200 collections. `DatabaseTooLarge` on a
database that is nowhere near too large.
The fix is what the plan called for and never got: round the cursor up to the
next *system* page and keep the extent. Only the page holding the live tail is
in the published image; the rest of the extent holds nothing referenced by the
image or by an index, so `Pager.mark_appendable` hands it back for appending
(and unprotects it, since it may sit below the stable mark where
`protect_image` made it read-only). System pages rather than 4 KiB ones because
writeback tears at the granularity the kernel manages: a 4 KiB store dirties a
whole 16 KiB page on Apple Silicon, and tearing there would take out the
published bytes sharing it.
Same 40 collections after: 340 MB -> 352 MB across three checkpoints, the ratio
falling monotonically toward the 8 MiB-per-collection floor. 64,000 documents
across 8 collections verified byte-for-byte and after a kill -9. The churn gate
is unchanged at 1.65x, big.js at 4 GB unchanged (4.32 GB file, reopen 0.5 s,
RSS after reopen 130 MB).
Two mutations, verified red: dropping the resume branch (a fresh extent per
checkpoint), and rounding to `page_size` instead of `map_align` (the resumed
append then shares a system page with the published image).
Aleksey Shakhmatov
1814020df9