Give a compacted file the zeros of its whole cycle (085)

Commits grew the tree store's tail 256 KiB at a time, each growth a sync
that also committed the ext4 journal for the new size and blocks. A file
installed by compaction now carries written zeros up to where the next
compaction will come (4 x live + slack, plus a commit's margin), capped at
the size of the file it replaces, so a store never takes more space than
before and shrinks when its live data does. The cycle's commits then only
overwrite blocks the file has.

Store commits: 317.5 -> 385.5 a second, ext4 journal commits 109 -> 45 for
4,000 commits; durable app posts +5.3%, their p99 the same within this SD
card's noise. The new cycles test keeps sizes still between compactions
once the live data is whole, and shows the file shrinking with it.

1 files changed, 7 insertions, 3 deletions

diff --git a/research/experiments/013-parallel-syncs/variants.py b/research/experiments/013-parallel-syncs/variants.py
index 2c48ab6..ee49d2a 100644
--- a/research/experiments/013-parallel-syncs/variants.py
+++ b/research/experiments/013-parallel-syncs/variants.py
@@ -1,9 +1,10 @@
 #!/usr/bin/env python3
 """Where the tree store's extra sync work goes (docs/research/084-parallel-syncs.md).
 
-Builds pure-serve with three stores from store/store-tree.c: as it is; with no
-compaction; and with no compaction and a 64 MiB tail written at creation (no
-tail growth during a run). Then durable score posts (64 connections), the
+Builds pure-serve with stores from store/store-tree.c: as it is; with no
+compaction; with no compaction and a 64 MiB tail written at creation (no tail
+growth during a run); and as before 085, compacted files getting only 256 KiB
+of zeros and commits growing the tail as they need it. Then durable score posts (64 connections), the
 builds in rotated order per round, each on a fresh store after 64 warm-up
 posts. Prints one JSON line per run and a summary.
 """
@@ -25,6 +26,9 @@ PATCHES = {
     'no compaction': [('enum { COMPACT_SLACK = 1048576 };', 'enum { COMPACT_SLACK = 1 << 30 };')],
     'no compaction, no growth': [('enum { COMPACT_SLACK = 1048576 };', 'enum { COMPACT_SLACK = 1 << 30 };'),
                                  ('TAIL_STEP = 262144', 'TAIL_STEP = 67108864')],
+    # Before 085: a compacted file got 256 KiB of zeros, and commits grew the tail as they needed it.
+    'tail grown as commits need it': [('zero_fill(fd, head->end, cycle_end(head, replaced));',
+                                       'zero_fill(fd, head->end, tail_end(head->end));')],
 }