probe: ROOT CAUSE — multi-BRANCH anomaly is ext4 writeback throttle (#146)#151
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Round 5 of #146. Two complementary findings: 1. perf with explicit hybrid sampling (-e cpu_core/cycles/P -e cpu_atom/cycles/P) — fixes the Alder Lake E-core blindness from round 4. Got 13 FC samples in a 20s window with 4 slow BRANCHes, 46% of which are in ext4 kernel paths: io_schedule ← wbt_wait ← submit_bio ← ext4_bio_write_folio ← ext4_do_writepages down_write ← ext4_da_map_blocks ← ext4_da_write_begin ← vfs_write crc32c_x86_3way ← ext4_block_bitmap_csum_set ← ext4_mb_mark_diskspace_used ← ext4_mb_new_blocks = ext4 delayed allocation + writeback throttle (wbt) + multi-block allocator + block-bitmap checksumming. 2. tmpfs control — second daemon with --snapshot-root /dev/shm/... Ran 10 consecutive diff BRANCHes: BRANCH 1 (cold): pause_ms=728 BRANCH 2-10: pause_ms 111-259, flat No slow regime. Anomaly is 100% filesystem-layer. Synthesis of all 5 probe rounds: Pass 1 strace -c → user-space CPU (wrong) Pass 2 bpftrace → 94% off-CPU + futex (side obs) Pass 3 futex args → passive waiters (confirmed) Pass 4 perf -a -g → kernel sleep dominates (right direction, blind) Pass 5 perf hybrid + tmpfs → ext4 wbt + mballoc (ROOT CAUSE) Fix path is now clear and small: 1. fallocate memory.bin to full size before FC writes (~30 LOC Rust) 2. O_DIRECT in FC's snapshot writer (upstream FC PR) 3. io_uring (Phase 2 original case, now justified) 4. memfd-backed memory.bin (v0.4) Recommended first step: fallocate. If pause stays flat → ship as forkd v0.3.4 + close #146. Ships: - bench/pause-window/probe-perf-hybrid.sh - PROBE-multi-branch-anomaly.md "ROOT CAUSE FOUND" section - probe-offcpu.sh referenced (already shipped in #150) Refs #146. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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…balloc (#152) 5 rounds of probe (#128, #140, #143, #150, #151) traced the multi-BRANCH pause_ms anomaly to ext4 delayed allocation + writeback throttle + multi-block allocator + block-bitmap CRC compounding per BRANCH. tmpfs control confirmed: anomaly is 100% in the fs layer. Fix: posix_fallocate the destination memory.bin to source full size right after we create snap_dir, before either the diff-mode background copy OR FC's snapshot/create write. ext4 reserves the extents up-front; subsequent writes don't run mballoc or update block bitmap. Best-effort — on tmpfs / unsupported FS the syscall returns ENOSYS, we log at WARN and continue (no behavior change). On ext4 (the actual problem case), this should flatten pause_ms across the BRANCH chain. Adds libc 0.2 as a cfg(unix) dep for posix_fallocate. Refs #146. Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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Summary
Round 5 of #146. Two findings together identify the root cause.
(a) Hybrid CPU sampling finds ext4 in FC's stacks
Re-ran perf with explicit hybrid event sampling (`-e cpu_core/cycles/P -e cpu_atom/cycles/P`) — fixes the Alder Lake E-core blindness from round 4 (#150). 13 FC samples in a 20 s window with 4 slow BRANCHes:
ext4 stacks include:
= ext4 delayed allocation + writeback throttle (`wbt_wait`) + multi-block allocator + block-bitmap checksumming.
(b) tmpfs control: anomaly vanishes
Second daemon, `--snapshot-root /dev/shm/forkd-snapshots`, same 10-BRANCH sweep:
Pause stays in the 110-260 ms band for all 10 BRANCHes on tmpfs. No slow regime. 100 % of the anomaly is in the fs layer.
Five-round synthesis
Fix path
`#118`'s original Phase 2/3 scope can finally be re-evaluated against real data. Options in order of leverage:
Recommended first step: fallocate. If pause stays flat → ship as forkd v0.3.4 patch + close #146.
Files
Closes investigation phase of #146.
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