vdm/core/docs/buffer-sizing.md

# CORE → PROTO — `bufferBytes` range + the RSS-budget reconciliation

`contracts/` froze `bufferBytes` at **4 KiB – 8 MiB** in four places; `docs/04` §4 line 70
says **64 KiB – 64 MiB, default 4 MiB**. The floor and ceiling are settled below (64 KiB /
16 MiB — my earlier reasoning holds). The default and `max_total_buffer_bytes` are
**not** — my first pass counted one buffer per download; it is one per *segment*
("Per-segment ring buffer", line 68; "Write buffer per connection", line 70), which
changes the whole RSS picture. This version fixes that.

PROTO to land schema + `docs/04` §4 + §8 + an ADR together.

## Recommendation

| Knob | Value | |
|---|---|---|
| `bufferBytes` minimum | **65536** (64 KiB) | frozen 4 KiB is below one libcurl callback — see §Floor |
| `bufferBytes` maximum | **16777216** (16 MiB) | see §Ceiling; the `256/64` unreachability argument is *stronger* per-segment |
| `bufferBytes` default | **1048576** (1 MiB) | see §Default+RSS; 2 MiB and 4 MiB both bust line 125 |
| `maxActiveSegments` (**new**) | **32** | global cap on concurrently-transferring segments — the actual bound on "20 active downloads" |
| `maxTotalBufferBytes` | **134217728** (128 MiB), down from 256 | backstop for the buffer knob; RSS-safe given the 1 MiB default + the segment cap |

## Floor — 64 KiB, not 4 KiB

The buffer's first job is to coalesce libcurl write-callback deliveries into one `pwrite`.
Over HTTP/2 a single write callback is routinely 16–64 KiB, up to ~256 KiB. A 4 KiB ring
buffer is **smaller than one callback**: the "one `pwrite` per fill" design degrades to a
syscall per curl chunk — the exact thing the buffer exists to prevent. 64 KiB (≈4 typical
chunks) is the smallest floor that buys anything. 4 KiB is a page size that wandered into
a throughput knob.

## Ceiling — 16 MiB, not 64 MiB

Two things the buffer buys past coalescing:

1. **Large sequential writes.** On NVMe, throughput vs. write size is flat by ~1–4 MiB.
   4→8 MiB gains a little on syscall overhead at multi-Gbit; past 8 MiB there is no
   throughput left to get, only `fdatasync` latency and page-cache pressure (a 40 GB ISO
   must not evict the user's working set — `docs/04` §4).
2. **Absorbing a disk stall without stalling the socket** — the only reason to exceed
   8 MiB. Fast link + bursty storage (HDD, SMR, USB, network mount): 1 Gbit ≈ 125 MB/s,
   so 16 MiB/segment ≈ 130 ms of write-stall cover, ~0.5 s across 4 segments — enough to
   ride out a seek storm. Beyond 16 MiB the marginal cover isn't worth the footprint.

**8 MiB captures all throughput; 8–16 MiB is stall-absorption headroom for the
fast-pipe/slow-disk case; >16 MiB is waste.**

### `maxTotalBufferBytes / segments` unreachability — worse per-segment

`effective = clamp(requested, 64 KiB, floor(maxTotalBufferBytes / live_segment_count))`.

With a **64 MiB** ceiling and the old 256 MiB cap, 64 MiB is unreachable once total live
segments exceed 4 (256/64). Under correct per-segment accounting "4 total segments" is
*half of one default download* (8 segments) — so the frozen-doc ceiling is unreachable in
essentially every real configuration. 16 MiB with a 128 MiB cap is reachable up to 8 live
segments (128/16) — i.e. exactly the single-download slow-disk case the ceiling exists
for — and clamps predictably beyond that, where per-segment buffering has stopped
mattering because each segment holds a small fraction of the link.

## Default + RSS — the part that didn't land

`docs/04` line 125: **"≤ 60 MB RSS with 20 active downloads at default buffers."**

Per-segment accounting: 20 downloads × the default 8 segments = **160 buffers**, not 20.

| default | 160 buffers | after a 256 MiB cap | vs 60 MB RSS |
|---|---|---|---|
| 4 MiB | 640 MiB | 256 MiB | ~4× over |
| 2 MiB | 320 MiB | 256 MiB | ~4× over |
| 1 MiB | 160 MiB | 160 MiB (cap not binding) | still ~3× over |
| any | — | **cap is the binding constraint at 20 tasks, not the default** | — |

So the per-segment default is *not* what decides the 20-task case — the global cap is,
whatever the default. **256 MiB and 60 MB RSS cannot both hold.** Hitting 60 MB across
160 segments is ~48 MiB of buffers total (≈300 KiB each), leaving ~12 MB for 160 curl
handles + TLS + the daemon — which is not achievable; 160 live TLS connections alone are
~10–15 MB.

### The real fix: cap concurrent segments, not just total buffer bytes

No download manager runs 160 simultaneous connections for 20 downloads. `docs/01` §2
already implies this ("1 curl-multi transfer thread per ~8 active segments (capped)");
`docs/04` never states the cap. Add **`maxActiveSegments` (default 32)** — a global
ceiling on segments actually transferring at once. 20 "active" downloads then means ~32
live connections with the rest of each download's segments queued, not 160.

RSS with `maxActiveSegments = 32`, default `bufferBytes = 1 MiB`:

- buffers: 32 × 1 MiB = **32 MiB** (the 128 MiB cap isn't even engaged at the default)
- 32 curl/HTTP2/TLS connections: ~2–3 MiB
- daemon base (RPC loop, event batching, SQLite cache + WAL, ~8 thread stacks resident,
  task table): ~8–12 MiB
- **total ≈ 45–50 MiB RSS — line 125 holds**, at the default, with margin.

`docs/04` §8 keeps the 60 MB number but must state it now depends on
`maxActiveSegments = 32` and default buffers. A power user who overrides every download to
16 MiB gets clamped by `maxTotalBufferBytes` to `128 / 32 = 4 MiB` per live segment →
128 MiB of buffers, ~140 MiB RSS — deliberately, and outside "at default buffers", so
line 125 is unaffected.

### If 60 MB is the wrong target

It's defensible to raise it instead. 160 (or even 32) live TLS connections have an
irreducible cost, and IDM itself uses more. If PROTO/docs prefer, change line 125 to
**"≤ 120 MB RSS with 20 active downloads at default buffers"** and keep
`maxActiveSegments` higher (64). CORE's recommendation is the 60 MB + cap-at-32 route
because "lean daemon" is in the brief and 45–50 MiB is comfortably achievable — but it
must be an explicit decision in the ADR, not the number that quietly loses.

## Clamp behaviour CORE will implement

- On task start and on any change to `live_segment_count` (new task, task finishing, a
  steal), recompute `effective` for every live segment by the formula above.
- Never below the 64 KiB floor. If `maxTotalBufferBytes / live_segment_count` < 64 KiB
  (needs >2048 live segments at a 128 MiB cap — not reachable under
  `maxActiveSegments = 32`), CORE admits fewer concurrent segments rather than shipping a
  sub-floor buffer.
- `effective` and `requested` both reported upward so the GUI shows "16 MiB (using
  4 MiB)" — request **B2a**.

## Net contract delta for PROTO

1. **`bufferBytes` bounds in FOUR schema files** (not three): `DownloadSpec`,
   `TaskDetail`, `download.update` patch, **and `Settings.schema.json`
   `connection.bufferBytes`** — all currently `4096`–`8388608`. Change every one to
   `minimum: 65536`, `maximum: 16777216`, `default: 1048576`.
2. **`maxTotalBufferBytes` is absent from the contract entirely.** The clamp CORE
   implements has no wire representation, so the Options dialog can neither show nor set
   it. Add `Settings.schema.json connection.maxTotalBufferBytes` (default `134217728`) —
   fold into the **B2a** follow-up alongside `effectiveBufferBytes`.
3. **`maxActiveSegments` is new.** Add `Settings.schema.json connection.maxActiveSegments`
   (default `32`). CORE enforces it; DAEMON's scheduler needs to know it to decide what to
   start.
4. **`docs/04` §4 line ~70:** "Default 1 MiB, range 64 KiB – 16 MiB. Reduced to fit
   `maxTotalBufferBytes` (128 MiB) across all live segments; effective value reported
   back."
5. **`docs/04` §8 line 125:** keep "≤ 60 MB RSS / 20 downloads" but add "given
   `maxActiveSegments = 32` and default buffers" — or raise to 120 MB (see above). ADR
   records which and why.
6. **ADR:** throughput-plateau reasoning for the ceiling; the per-segment RSS arithmetic;
   the `maxActiveSegments` addition as the mechanism that makes line 125 hold; 8 MiB
   considered and rejected for the ceiling in favour of 16 MiB stall absorption.