docs(sync): record real-repo commit bench and TreeBuilder handoff
The real jcalixte/notes clone (570 MB pack, 1179 files) proved both index strategies are O(N_tree) and unshippable: index.write re-hashes the tree (611 s), and the index-free read_tree is 77 s cold and OOMs the mmap cache (zlib crash). Record the run in the tradeoff curve, revise the verdict to an O(depth) TreeBuilder walk, and add a handoff note with the design, firmware call sites, and bench steps for the next session.
This commit is contained in:
Julien Calixte
@@ -12,3 +12,4 @@
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| [`git-sync-images-and-repo-size.md`](git-sync-images-and-repo-size.md) | Why we don't shrink the notes repo — its 153 MB of media is remanso's image CDN, so rewriting history to slim the on-device clone breaks the web app. |
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| [`boot-time-budget.md`](boot-time-budget.md) | Where the ~4.3 s to cursor goes — and why the ≤ 3 s v1.0 target is hard: one ~1.9 s full refresh is unavoidable at cold boot, so the splash is nearly free. |
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| [`sync-latency.md`](sync-latency.md) | Where the ~16 s cold `:sync` goes — Wi-Fi + SNTP + one TLS push; why optimistic-retry cut a whole handshake, and why the rest is close to the protocol floor. |
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| [`sync-commit-handoff.md`](sync-commit-handoff.md) | **Handoff:** the on-device commit must become an O(depth) TreeBuilder walk — `add_all`/`index.write` re-hash the whole 570 MB-pack tree (611 s / OOM) and the real repo has never synced. Design + firmware plumbing + bench steps for the next session. |
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138
docs/notes/sync-commit-handoff.md
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138
docs/notes/sync-commit-handoff.md
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@@ -0,0 +1,138 @@
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# Handoff — the on-device commit must become an O(depth) TreeBuilder walk
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> **Start here if you're picking up `:sync` performance.** Written 2026-07-12 after
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> benching `git_bench` on a full clone of the real `jcalixte/notes` repo. Full
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> analysis + numbers: [`../tradeoff-curves/sync-commit-staging.md`](../tradeoff-curves/sync-commit-staging.md).
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> Latency memory: `sync-timing`. Why the repo can't shrink:
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> [`git-sync-images-and-repo-size.md`](git-sync-images-and-repo-size.md).
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## TL;DR
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The current `firmware::git_sync::stage_and_commit` (`add_all` → `index.write` →
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`write_tree`) **cannot commit the real repo** — it is O(N_tree) and the real repo is
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1179 files / 158 dirs / 570 MB pack / 150 MB of images. Measured on device:
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- `index.write()` re-hashes the whole working tree → **up to 611 s** (10-min freeze).
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- The index-free alternative (`Index::new` + `read_tree(HEAD)` + `write_tree_to`)
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still reads the whole tree cold → **77 s**, and drove the `esp_map.c` mmap cache
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to 7.4 MB, starving zlib so `repo.blob()` crashed (`zlib (5)`, 508 KB heap left).
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**The real repo has almost certainly never completed a `:sync` on device** — only
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the toy `typoena-test` (`notes.md`) has. The repo cannot be shrunk (the 150 MB of
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images serve another app — see the images note). So the fix is a **new commit
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mechanism**, not a tuning knob.
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## The fix — O(depth) TreeBuilder walk
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Rebuild only the edited file's ancestor subtree chain onto HEAD's tree. Never
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materialise the 1179-entry index; never `index.write()`; never `read_tree` the whole
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tree. Cost is O(depth × dirty_files), flat in repo size, tiny heap, and it carries
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every unchanged entry (all 260 images, the other 1176 files) forward untouched —
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which means **the device doesn't even need the images in its working tree.**
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Sketch (git2 0.20 API — all present: `Repository::treebuilder(Option<&Tree>)`,
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`TreeBuilder::{insert,remove,write}`, `Tree::{get_name,get_path}`,
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`TreeEntry::{to_object,filemode}`):
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```rust
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/// Return a new tree OID = `base` with `path` set to `new` (Some(blob_oid) to
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/// add/replace, None to delete). Reads ~depth subtree objects, writes ~depth trees.
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fn splice(repo: &Repository, base: &Tree, path: &[&str], new: Option<Oid>) -> Result<Oid> {
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let (head, rest) = path.split_first().unwrap();
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if rest.is_empty() {
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// leaf level: patch this tree directly
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let mut tb = repo.treebuilder(Some(base))?;
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match new {
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Some(oid) => tb.insert(head, oid, 0o100644)?, // FileMode::Blob
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None => { tb.remove(head).ok(); } // delete
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};
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return Ok(tb.write()?);
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}
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// descend: find (or synthesize) the subtree, recurse, re-insert its new OID
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let sub = base.get_name(head)
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.and_then(|e| e.to_object(repo).ok())
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.and_then(|o| o.peel_to_tree().ok());
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let empty; let sub_ref = match &sub { Some(t) => t, None => { empty = repo.treebuilder(None)?; /* ... */ unreachable!() } };
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let new_sub = splice(repo, sub_ref, rest, new)?;
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let mut tb = repo.treebuilder(Some(base))?;
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// if new_sub is the empty tree and this was a delete, remove the dir entry instead
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tb.insert(head, new_sub, 0o040000)?; // FileMode::Tree
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Ok(tb.write()?)
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}
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```
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Then, for the dirty set, fold each change through `splice` (thread the running root
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tree), and `commit(Some("HEAD"), …, &final_tree, &[&parent])`. Edge cases to handle:
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new files where an intermediate dir doesn't exist yet (build subtree from `None`),
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deletes that empty a directory (remove the dir entry rather than insert an empty
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tree), and path splitting on `/` (paths are repo-relative POSIX).
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**Bench it first** (per the established discipline): add a `treebuilder splice→tree`
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op to `firmware/src/bin/git_bench.rs` alongside the existing ones and confirm it's
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sub-second cold on the real repo, with heap staying healthy (no OOM). Only then wire
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it into the firmware.
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## Firmware plumbing (after the bench validates)
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1. **`firmware/src/git_sync.rs`**
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- Rewrite `stage_and_commit` (currently ~L271–332) to the `splice`-walk above.
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Drop `add_all`/`update_all`/`index.write`/`index.write_tree`. Keep the
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`commit split —` timing log, the `tree unchanged → nothing to publish` check,
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and the signature/message code.
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- `PublishRequest` (~L79) is currently an **empty struct** — it must carry the
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dirty set: `{ changed: Vec<(String /*repo-rel path*/, ...)>, deleted: Vec<String> }`.
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The commit needs the blob content or a way to read it; simplest is to pass the
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paths and let the git thread `repo.blob_path(abs)` / read `/sd/repo/<path>`.
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- `reconcile_onto_origin` (~L377/L394) uses `repo.reset(Mixed)` — with an
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index-free commit there's no index to reset; switch to `ResetType::Soft` (move
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HEAD only) or drop the reset and just re-`splice` onto the new origin tip.
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- The macOS-cruft filter (`skip_macos_cruft`) is no longer needed — the walk only
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ever touches paths the editor explicitly hands it, so `._*`/`.DS_Store` can't
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sneak in. (Keep a note; don't silently lose the Spike-14 lesson.)
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2. **Dirty-set source — `firmware/src/persistence.rs` + `main.rs`**
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- Writes funnel through `Storage::save_path` (~L296) and deletes through
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`Storage::delete_path` (~L332), both `&self`. Accumulate a dirty/deleted set
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(needs `RefCell` interior mutability, or move the set up to `main.rs`).
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- `Effect::Publish` handler in `main.rs` (~L222) builds the `PublishRequest` from
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that set and clears it on a successful `Pushed`/`UpToDate` outcome.
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3. **`esp_map.c` cache fix (same pass) — `firmware/components/libgit2/esp_map.c`**
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- Bug: cached buffers are freed only lazily in `p_mmap`'s `evict_for`, so a
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window libgit2 has `p_munmap`'d stays resident until the *next* map — defeating
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`MWINDOW_MAPPED_LIMIT` and starving non-mmap `git__malloc` (zlib).
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- Fix: on `p_munmap` when refcount hits 0, evict down to a low-water mark (or free
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outright); lower `ESP_MAP_CACHE_CAP` (4 MB → ~1.5–2 MB) and/or `SLOTS`.
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- ⚠️ Editing this `.c` needs the fingerprint dance or the change won't rebuild —
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see the `esp-idf-component-rebuild` memory:
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`rm -rf firmware/target/xtensa-esp32s3-espidf/release/.fingerprint/esp-idf-sys-*`
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then rebuild.
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## How to bench / flash
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`git_bench.rs` runs git ops on the 96 KB `GIT_STACK` thread (the main task stack
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overflows on these ops — that's why the real service has a dedicated thread). It's
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Rust-only, so a plain rebuild picks it up (no fingerprint dance unless you also
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touched `esp_map.c`).
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```
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just flash-gitbench
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# = . ~/export-esp.sh && LIBGIT2_SRC=<repo>/firmware/components/libgit2/vendor \
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# LIBGIT2_NO_VENDOR=1 PKG_CONFIG_ALLOW_CROSS=1 \
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# PKG_CONFIG_LIBDIR=<repo>/firmware/pkgconfig \
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# cargo run --release --bin git_bench --features git
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```
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Bench on the **real repo** clone (`/sd/repo` = full `jcalixte/notes`), not the toy —
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the toy pack understates everything by ~2 orders of magnitude.
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## What's proven vs open
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**Proven (2026-07-12, real repo):** `odb.write` 142 ms (mmap cache holds);
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`index.write` 611 s (whole-tree re-hash via `truncate_racily_clean`, index.c:822 /
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index.h:117); index-free `read_tree` 77 s cold; mmap cache OOM at 7.4 MB → zlib
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crash. `Repository::open` 88 ms, odb-open ~6 s cold (maps 1.7 MB `.idx`).
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**Open:** the TreeBuilder walk is **designed but not yet benched or built.** Confirm
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its cold-real-repo latency and heap before wiring. Push (network half, ~6.5 s) is a
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separate floor, untouched here.
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@@ -1,12 +1,17 @@
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# Commit-staging cost vs working-tree size
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> **Decision (pending measurement):** keep the file-agnostic `add_all(["*"])`
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> staging, or switch to explicit-path staging (`add_path` over the editor's dirty
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> set)? The fork is worth taking **only if the FAT working-tree walk dominates the
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> ~4 s commit** — which the split-timer added to
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> [`../../firmware/src/git_sync.rs`](../../firmware/src/git_sync.rs)
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> (`stage_and_commit`, the `commit split —` log line) resolves. This note records
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> the cost model and the rule the measurement decides.
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> **Decision (RESOLVED 2026-07-12, real-repo bench):** neither `add_all(["*"])`
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> nor an index-free `read_tree`+`write_tree` is viable on the real
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> `jcalixte/notes` clone — **both are O(N_tree)** and blow up on the 570 MB pack /
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> 1179-file tree (611 s hash one end, 77 s tree-read + OOM the other; see
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> [Real-repo run](#real-repo-run-2026-07-12-jcalixtenotes-570 mb-pack--the-index-is-the-wrong-primitive)
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> below). The commit must be rebuilt with an **O(depth) TreeBuilder walk** —
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> patch only the edited file's ancestor subtree chain onto HEAD's tree, never
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> materialise all 1179 entries. Shrinking the repo is **not** an option
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> ([`../notes/git-sync-images-and-repo-size.md`](../notes/git-sync-images-and-repo-size.md):
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> the images are load-bearing for another app), which is exactly why the O(depth)
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> mechanism is the only lever left. This note records the cost model and the full
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> measurement trail that got here.
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>
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> Tradeoff-curves index: [`README.md`](README.md). Docs index:
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> [`../README.md`](../README.md). Where the whole sync goes:
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@@ -124,9 +129,97 @@ write amplification / better card / SPI-clock" framing is refuted: **the SD card
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not the bottleneck.** fsync is still confirmed off; the extra ~600 ms/op is CPU or
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repeated `.git` I/O *inside* libgit2 (candidates: ODB refresh scanning
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`objects/`, the treebuilder's per-entry `git_odb_exists`, ref-lock + reflog writes,
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config/attributes re-reads). `git_bench` (`firmware/src/bin/git_bench.rs`) times
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`odb.write` / `index.write` / `write_tree` in isolation to localize it — **run
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pending.**
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config/attributes re-reads). `git_bench` (`firmware/src/bin/git_bench.rs`) localizes
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it — see below.
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### It's the pack, read through an un-caching emulated mmap (`git_bench`, 2026-07-12)
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`git_bench` times the git2 primitives in isolation on `/sd/repo` (git ops on the
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same 96 KB thread the real service uses — the main-task stack overflows on
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`index.write`, which is itself the reason the service has a dedicated thread):
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| git2 op | p50 | note |
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| --- | ---: | --- |
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| `Repository::open` | 100 ms | one-time |
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| `odb.write(blob)` (unique) | **45 ms** | writes a fresh object; touches no existing object |
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| `repo.index()` open | ~0 ms | cached |
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| `index.write()` | 376 ms | index + `index.lock` rename + tree-cache |
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| `write_tree` [unchanged] | ~0 ms | tree exists → freshen-skips the write |
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| **`write_tree` [changed]** | **1136 ms** | writes ONE 45 ms object |
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| **`commit(None)` orphan obj** | **563 ms** | writes ONE 45 ms object, no ref/reflog |
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Writing a fresh object is 45 ms; the ops that wrap one are 8–25×. The cause, from
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the vendored source: `git_odb_write` calls `git_odb__freshen` (odb.c:1011), which
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on a not-found object runs **`git_odb_refresh`** (re-reads the pack dir + reloads
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pack indexes), and existence checks (`freshen(tree)` in `commit.c:169`, base-object
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lookups in `write_tree`) hit the **pack**. Pack access goes through our
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`p_mmap` (`esp_map.c`), which **`malloc`s and `read()`s the mapped range from the
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card on every call — no cache** — with a 32 MB window on this 32-bit target. So
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each write re-reads pack bytes from SD; `odb.write` of a fresh blob is 45 ms only
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because it touches no packed object.
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**This scales with pack size.** The toy repo's pack is tiny; the real
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`jcalixte/notes` clone has a **570 MB pack**, and provisioning rsyncs a full clone
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onto the card — so a real-repo commit has **never been benchmarked** and, on this
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mechanism, will be far worse than the ~3.3 s toy number. That is the single biggest
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open risk in sync.
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### Real-repo run (2026-07-12, `jcalixte/notes`, 570 MB pack) — the index is the wrong primitive
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`git_bench` was finally run against a full clone of the real repo (1179 files, 158
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dirs, 570 MB pack). It settles the design: **any index-based commit is O(N_tree)
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and does not fit this device.** Two independent walls:
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| op | result | reading |
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| --- | ---: | --- |
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| `Repository::open` | 88 ms | fine |
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| odb open (implicit) | ~6 s cold | maps the 1.7 MB pack `.idx` once (16 miss / 1790 KB) |
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| `odb.write(blob)` | **142 ms** p50 | the mmap cache win **holds** (was 862 ms uncached) ✅ |
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| `repo.index()` load (1179 entries) | 514 ms max | the on-disk index we were trying to avoid |
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| `index.write()` | **min 360 ms / p50 12.8 s / max 611 s** | ⚠️ hangs — see root cause |
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| **seed `read_tree(HEAD)` (cold, 1×)** | **~77 s** | ⚠️ reads all ~158 tree objects, 22.7 MB of pack windows |
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| `Index::new + read_tree` (warm) | 447 ms p50 | windows still mapped → pure CPU |
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| **index-free `stage→tree`** | **💥 crash** | `zlib (5)`: `deflateInit` failed, **508 KB heap left** |
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**Wall 1 — `index.write()` hashes the whole working tree (up to 611 s).**
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`git_index_write` unconditionally calls `truncate_racily_clean` (index.c:822),
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which runs `git_diff_index_to_workdir` over **every** entry flagged "racy" and
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re-hashes its file. On a fresh FAT clone the mtime granularity is 2 s and
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`index.stamp.mtime <= entry.mtime` for ~all 1179 entries (index.h:117), so the
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whole tree looks racy → it re-hashes ~170 MB (mostly the 150 MB of images) over
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10 MHz SPI. The 611 s → 12.8 s → 360 ms decay across three iterations is the
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signature: each write bumps the index mtime, shrinking the racy set. **Implication:
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the shipping `stage_and_commit` calls `index.write()`, so `:sync` on the real repo
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effectively bricks on the first commit** — the user sees a 10-minute freeze,
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resets, the index mtime never advances, and it re-hashes forever. The real repo has
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almost certainly never completed a sync on device (only the toy `typoena-test` has).
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**Wall 2 — the index-free path is still O(N_tree), and the mmap cache OOMs.**
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Skipping `index.write()` entirely (fresh `Index::new()`, stamp = 0, so
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`truncate_racily_clean` can never fire) removes Wall 1. But to seed the in-memory
|
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index, `read_tree(HEAD)` materialises all 1179 entries and reads every tree object
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from the 570 MB pack — **77 s cold** (447 ms only once the windows are resident).
|
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`write_tree_to` is O(changed), but you pay O(N_tree) to build the cache it needs, so
|
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the index-free path only trades a 611 s hash for a 77 s tree-read. Worse, that
|
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`read_tree` drove the `esp_map.c` cache to **7.4 MB resident** — past its own 4 MB
|
||||
soft cap — which left 508 KB of heap and made `repo.blob()`'s zlib `deflateInit`
|
||||
fail. **The one write we cannot skip crashed.** Root cause of the OOM: our cache
|
||||
holds pack windows *after* libgit2 `p_munmap`s them (refcount 0, freed only lazily
|
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on the next `p_mmap`), which **defeats `GIT_OPT_SET_MWINDOW_MAPPED_LIMIT`** —
|
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libgit2 thinks it released the memory; we didn't.
|
||||
|
||||
**Conclusion — use an O(depth) TreeBuilder walk.** "Replace K files in a 1179-file
|
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tree" should touch `O(depth × K)` objects, not `O(N_tree)`. Walk HEAD's tree down
|
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the edited path (`tree.get_name`/`get_path` → read ~depth subtree objects), then
|
||||
rebuild bottom-up with `repo.treebuilder(Some(&subtree))` → `insert`/`remove` →
|
||||
`write()`, and `commit` the new root. That never materialises the 1179 entries,
|
||||
never re-hashes anything, never visits the 150 MB of images, reads only a handful
|
||||
of tree windows (so the cache stays small and zlib keeps its heap), and — crucially
|
||||
— **carries the image entries forward untouched from HEAD's tree, so the device
|
||||
does not even need the images in its working tree.** The `esp_map.c` cache still
|
||||
needs an evict-on-`munmap` fix (drop the cap, free past a low-water mark) so it can
|
||||
never again starve a downstream `git__malloc`, but with the TreeBuilder walk the
|
||||
pressure it was under largely disappears.
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||||
|
||||
### The walk is ~1.4 s even at N ≈ 2
|
||||
|
||||
@@ -139,27 +232,42 @@ For orientation: `publish(commit+push)` was 9846 ms cold, so the **network half
|
||||
~6.5 s** — still the biggest single block of a warm sync (10.1 s total), a separate
|
||||
floor ([`../notes/sync-latency.md`](../notes/sync-latency.md)).
|
||||
|
||||
## The verdict (provisional — pending `git_bench`)
|
||||
## The verdict
|
||||
|
||||
Two things are now settled and one is open:
|
||||
The real-repo run (above) overturned the earlier ranking. Both index strategies
|
||||
are O(N_tree) and fail on the 570 MB-pack clone, and the repo cannot be shrunk. The
|
||||
work, ranked:
|
||||
|
||||
- **Settled: the card is fast.** The SD-clock and better-card levers are off the
|
||||
table — they target I/O that costs ~86 ms, not the ~700 ms we see. Do not spend
|
||||
the PCB's 20 MHz budget expecting a commit-latency win here.
|
||||
- **Settled: explicit-path staging is still worth doing** — but on *design +
|
||||
big-repo* grounds, not toy-repo latency (its measured payoff there is ~0.7 s). It
|
||||
**caps the O(N) walk on the 1179-file target**, **never visits the ~260 images**
|
||||
(150 MB it would otherwise scan), lets us **drop the macOS-cruft filter**, and
|
||||
aligns the git layer with what the editor changed.
|
||||
- **Open: the ~600 ms/op libgit2 overhead** is now the largest single mystery in
|
||||
the commit and likely the highest-value fix — if it's ODB refresh or reflog
|
||||
writes, it may be a cheap config/flag change that speeds up *every* commit
|
||||
regardless of repo size or staging. `git_bench` decides. **Localize it before
|
||||
committing effort to (a).**
|
||||
1. **Rewrite the commit as an O(depth) TreeBuilder walk (the fix — build this).**
|
||||
Rebuild only the edited path's ancestor subtree chain onto HEAD's tree; never
|
||||
materialise the 1179-entry index, never `index.write()`, never `read_tree` the
|
||||
whole tree. This is the ONLY mechanism that fits: O(depth × dirty) reads/writes,
|
||||
flat in repo size, small heap, images carried forward untouched. Replaces
|
||||
`stage_and_commit`'s `add_all`/`update_all`/`index.write`/`write_tree`. Needs the
|
||||
editor's dirty set (+ deleted set) plumbed to the git service — the editor
|
||||
already knows both. **Bench the walk on the real repo before wiring it in.**
|
||||
2. **Fix the `esp_map.c` cache so it can't OOM (do it alongside #1).** It grew to
|
||||
7.4 MB resident past its 4 MB cap and starved zlib. Evict on `p_munmap` (not
|
||||
only lazily on the next `p_mmap`) down to a low-water mark, and lower the cap, so
|
||||
a released window's memory is actually returned to `git__malloc`. The cache's
|
||||
`odb.write` win (862 → 142 ms) is real and worth keeping — this is a
|
||||
memory-discipline fix, not a removal.
|
||||
3. **Retired: `add_all`/explicit-path *index* staging.** Explicit-path `add_path`
|
||||
still goes through the index and `index.write` → `truncate_racily_clean`, so it
|
||||
hits Wall 1 just the same. The TreeBuilder walk supersedes it entirely; the
|
||||
"explicit-path staging" idea survives only as "the editor's dirty set feeds the
|
||||
walk."
|
||||
4. **Retired: SD clock / better card.** The card does a full object write in
|
||||
~86 ms; raw I/O is not the bottleneck. Do not spend the PCB's 20 MHz budget
|
||||
expecting a commit-latency win.
|
||||
5. **Kept: the mmap cache + mwindow tuning** (`GIT_OPT_SET_MWINDOW_*`, 256 KB
|
||||
window / 4 MB mapped limit). It fixed `odb.write` and the push read path; #2 just
|
||||
makes it well-behaved under memory pressure.
|
||||
|
||||
**Recommendation:** run `git_bench` to pin the libgit2 overhead; then implement
|
||||
explicit-path staging for the design + big-repo reasons; the SD/card levers are
|
||||
retired.
|
||||
**Recommendation:** build #1 (O(depth) TreeBuilder walk) with #2 (cache
|
||||
evict-on-munmap) in the same pass. See
|
||||
[`../notes/sync-commit-handoff.md`](../notes/sync-commit-handoff.md) for the
|
||||
concrete next-session plan (bench design, firmware plumbing, exact call sites).
|
||||
|
||||
## Adjacent lever: should the images be on the card at all?
|
||||
|
||||
|
||||
Reference in New Issue
Block a user