111 lines
6.3 KiB
Markdown
111 lines
6.3 KiB
Markdown
# US-044 — Tracker as model-file source; nodes download only their shard
|
||
|
||
Status: in progress
|
||
Priority: High (blocks multi-machine big-model serving; pairs with US-042)
|
||
Stage: Designed (grill remaining decisions before build)
|
||
|
||
## Context
|
||
|
||
Common deployment: the tracker and the first node share a machine that already
|
||
holds the model files (e.g. `/run/media/popov/DATA/llm/safetensor`). When a
|
||
second node joins with no model selected, the tracker assigns it the uncovered
|
||
layer range — and today that node then downloads the **entire snapshot from
|
||
HuggingFace**, even for a 20-layer shard of a 160 GB model.
|
||
|
||
What exists already (build on it, don't duplicate):
|
||
|
||
- Nodes serve their shard dir as a tar at `GET /v1/shards/download` with
|
||
checksum verification; `download_shard` tries assignment-provided `peers`
|
||
before HF (`downloader.py`). But it only matches **identical layer ranges**,
|
||
and the HF fallback runs `snapshot_download` of the whole repo.
|
||
- The torch path (`--model-id`) bypasses `download_shard` entirely:
|
||
`TorchModelShard` → `from_pretrained` downloads **and loads into RAM** the
|
||
full model, then executes only the assigned layers. Sharding currently saves
|
||
compute, not memory or bandwidth.
|
||
|
||
## Design
|
||
|
||
1. **Tracker `--models-dir PATH`** (env `MESHNET_MODELS_DIR`). When set, the
|
||
tracker indexes HF-layout snapshots under it and advertises itself as a
|
||
model-file source in `/v1/nodes/assign` responses.
|
||
2. **Layer-aware file selection.** For safetensors models, read
|
||
`model.safetensors.index.json` and map the assigned layer range → the
|
||
subset of weight files containing those layers, plus the always-needed
|
||
files (config, tokenizer, index, embeddings/head files for head/tail
|
||
shards). Serve exactly that subset (tar stream, per-file checksums).
|
||
GGUF (US-042): single file or naive byte-range — phase 2.
|
||
3. **Node download order**: exact-shard peer (existing) → tracker/peer file
|
||
subset (new) → HF `snapshot_download` with `allow_patterns` for the same
|
||
subset (new — stop downloading the whole repo even from HF) → full snapshot
|
||
(last resort).
|
||
- The `allow_patterns` subset must not depend on the tracker having a local
|
||
snapshot: when the tracker has no `--models-dir` match for a repo (or
|
||
hasn't cached it yet — the common case for a fresh public tracker),
|
||
`model_sources` comes back empty and `download_shard` falls straight to
|
||
`_download_huggingface_subset(..., allow_patterns=None)`, i.e. the full
|
||
repo. Reported 2026-07-06: a CPU node assigned layers 0–2 of
|
||
`unsloth/Qwen3.6-35B-A3B` (42 safetensor shards) sat downloading the
|
||
entire model unauthenticated because of this. Fix: fetch
|
||
`model.safetensors.index.json` + `config.json` directly from HF (a few
|
||
KB) and compute the same layer-scoped file subset client-side, so the
|
||
HF-fallback path is filtered even with an empty `model_sources`.
|
||
4. **Partial LOAD (the hard half).** Downloading a subset is wasted unless the
|
||
node stops instantiating the full model: build the model skeleton on the
|
||
`meta` device, materialize only assigned layers (+embeddings/norm/head as
|
||
role requires) from the local files, leave the rest on meta. Without this,
|
||
an 80 GB machine can never hold a shard of a 160 GB model regardless of
|
||
how the bytes arrive. This is the acceptance bar for the issue.
|
||
|
||
## Open questions (grill before building)
|
||
|
||
- Trust: joining nodes fetch weights from the tracker/peers — checksum against
|
||
what root of trust? (HF etag/sha vs tracker-signed manifest.)
|
||
- Disk layout: partial snapshots must not corrupt the HF cache dir; probably
|
||
a meshnet-owned layout keyed by repo+revision.
|
||
- Serving cost: a 100 GB tar stream per joining node on the tracker box —
|
||
rate-limit/queue? LAN-only heuristic?
|
||
|
||
## Acceptance criteria
|
||
|
||
- [x] Tracker can be started with `--models-dir PATH` / `MESHNET_MODELS_DIR`
|
||
and advertises a local model-file source in assignment responses when it has
|
||
a matching HF snapshot.
|
||
- [x] Tracker serves a tar stream containing only the safetensors files selected
|
||
for the assigned layer range plus config/tokenizer/index metadata.
|
||
- [x] Node downloader keeps exact-shard peers first, then races tracker model
|
||
sources against a HuggingFace `snapshot_download(..., allow_patterns=...)`
|
||
subset download, using the first successful source.
|
||
- [x] When no tracker model source is available at all, the HuggingFace
|
||
fallback still computes `allow_patterns` from the repo's own
|
||
`model.safetensors.index.json` (fetched directly, not via the tracker) —
|
||
it never silently downloads the full model just because the tracker has
|
||
nothing cached.
|
||
- [x] Real PyTorch model startup can use tracker `full_url` sources to fetch
|
||
the full local snapshot over LAN before `from_pretrained`, so local-network
|
||
testing no longer has to pull from HuggingFace first.
|
||
- [ ] Two-machine test: machine A (tracker + node, holds full snapshot) serves
|
||
layers 0–k; machine B joins with no model and receives **only** the files
|
||
for its assigned range from A — nothing fetched from HF
|
||
- [x] Machine B's resident memory scales with its shard size, not model size
|
||
- [x] Checksums verified end-to-end; corrupted transfer falls back cleanly
|
||
- [x] Single-node/full-model flows unchanged
|
||
- [x] `python -m pytest` passes from repo root
|
||
|
||
## Implementation notes
|
||
|
||
- 2026-07-06: Added the tracker/node download path. For immediate Qwen3.6-35B
|
||
LAN testing, real PyTorch nodes fetch the full snapshot from the tracker via
|
||
`full_url`; HuggingFace remains fallback-only, and when it is used the node
|
||
computes `allow_patterns` from the repo's remote SafeTensors index so it
|
||
stays layer-filtered even without tracker-cached files. Remaining hard half
|
||
is partial model materialization: the backend can prefer a downloaded
|
||
local model directory, but Transformers still needs a `meta`-device load
|
||
path that materializes only assigned layers.
|
||
- 2026-07-13: Partial LOAD implemented. `_load_partial_model_from_snapshot` builds
|
||
on `meta` via `init_empty_weights`, materializes only layer-scoped checkpoint
|
||
tensors, and finalizes device placement without copying unmaterialized meta
|
||
weights (`_finalize_active_shard_modules_on_device`). Tests cover memory
|
||
scaling (`test_partial_snapshot_resident_weight_numel_scales_with_shard`)
|
||
and real-torch meta-vs-materialized counts. Remaining: live two-machine LAN
|
||
verification.
|