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refactor(sync): remove the never-hit mmap window cache

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Four instrumented real-repo bench runs scored 0 cache hits: libgit2's
mwindow layer reuses its open windows, so only genuinely new
(offset, len) ranges ever reach p_mmap — there is no repetition to
cache. The 7.4 MB OOM the v2 discipline fixed was caused by the cache
itself holding buffers past p_munmap; plain free-at-munmap is honest
with MWINDOW_MAPPED_LIMIT by construction. Run 5 confirmed removal is
I/O-neutral on device (byte-identical read pattern, warm splice 1953
vs 1949 ms) and even 15 reads better — v2's low-water eviction was
fighting mwindow. Stats counters kept to spot any future workload
that does repeat ranges.
This commit is contained in:
Julien Calixte
2026-07-13 00:20:02 +02:00
parent ce3204a350
commit 1a5e1736f5
2 changed files with 35 additions and 167 deletions
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13
firmware/src/bin/git_bench.rs
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@@ -293,22 +293,25 @@ fn find_edit_path(repo: &Repository, root: &Tree) -> Result<Vec<String>> {
}
unsafe extern "C" {
/// Counters from the p_mmap cache in `components/libgit2/esp_map.c`.
/// Counters from the p_mmap emulation in `components/libgit2/esp_map.c`.
/// Post cache-removal: `hits` is always 0, `misses` counts every mapping,
/// `cached_kb` reports the LIVE mapped bytes (the mwindow working set).
fn esp_map_stats(hits: *mut u32, misses: *mut u32, read_kb: *mut u32, cached_kb: *mut u32);
}
/// Log the p_mmap cache counters — hits vs misses (SD reads avoided), total KB
/// read from the card, and KB currently resident. If the pack-read hypothesis is
/// right, hits climb and `KB read` stops growing across the write ops.
/// Log the p_mmap counters — mappings performed, total KB read from the card,
/// and KB currently live-mapped (should track mwindow's open windows and stay
/// well under MWINDOW_MAPPED_LIMIT now that munmap frees immediately).
fn log_map_stats(label: &str) {
let (mut hits, mut misses, mut read_kb, mut cached_kb) = (0u32, 0u32, 0u32, 0u32);
unsafe { esp_map_stats(&mut hits, &mut misses, &mut read_kb, &mut cached_kb) };
let _ = hits; // always 0 since the cache removal; slot kept for ABI stability
// Free heap spans PSRAM here; a drop toward 0 during write_tree/commit on the
// real repo would point at mwindow/idx allocation pressure (or thrash) as the
// cause of an apparent hang, not CPU.
let free_kb = unsafe { esp_idf_svc::sys::esp_get_free_heap_size() } / 1024;
log::info!(
"mmap cache @ {label:<11} {hits} hit / {misses} miss, {read_kb} KB read, {cached_kb} KB resident, {free_kb} KB heap free"
"mmap @ {label:<11} {misses} maps, {read_kb} KB read, {cached_kb} KB live, {free_kb} KB heap free"
);
}