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perf: compute packet hash once per packet in the forwarding hot path
Before this change, calculate_packet_hash() (SHA-256 + hex + upper) was called 3 times per forwarded packet and 4 times per dropped packet: __call__ → pkt_hash_full = packet.calculate_packet_hash() #1 → flood/direct_forward → is_duplicate → calculate_packet_hash() #2 → flood/direct_forward → mark_seen → calculate_packet_hash() #3 (drop) → _get_drop_reason → is_duplicate → calculate_packet_hash() #4 pkt_hash_full was computed in __call__ but never threaded down into process_packet, flood_forward, direct_forward, is_duplicate, or _get_drop_reason. Each method recomputed it independently. Fix: add optional packet_hash: Optional[str] = None to is_duplicate, _get_drop_reason, flood_forward, direct_forward, and process_packet. Pass pkt_hash_full from __call__ through the chain. Each method uses the provided hash or falls back to computing it — preserving backward compatibility for external callers (TraceHelper, etc.) that have no pre-computed hash. Result: 1 SHA-256 computation per packet in the hot path regardless of whether the packet is forwarded or dropped. Also adds explicit INVARIANT docstrings to flood_forward, direct_forward, and is_duplicate documenting that these methods must remain synchronous (no await). The is_duplicate + mark_seen pair is atomic within the asyncio event loop; adding an await between them would allow two concurrent tasks to both pass the duplicate check for the same packet — forwarding it twice. Docs: docs/pr_hash_once.md — problem analysis, call-chain diagram, per-method diffs, quantification (~3-8 µs saved per packet), test plan (including hash-count assertion), and proof that passing the original's hash to the deep-copied packet is correct. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
This commit is contained in:
TJ Downes
346
docs/pr_hash_once.md
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346
docs/pr_hash_once.md
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@@ -0,0 +1,346 @@
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# PR: Compute Packet Hash Once Per Forwarded Packet
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**Branch:** `perf/hash-once`
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**Base:** `rightup/fix-perfom-speed`
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**Files changed:** `repeater/engine.py` (1 file, ~51 lines net)
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---
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## Problem
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`packet.calculate_packet_hash()` runs a SHA-256 digest over the full serialised
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packet bytes, converts the result to a hex string, and uppercases it. Before
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this change the hot forwarding path triggered this computation **three times per
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packet**:
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| Call site | Where | When |
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|-----------|-------|------|
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| `__call__` line 162 | `pkt_hash_full = packet.calculate_packet_hash()...` | Every received packet |
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| `flood_forward` / `direct_forward` via `is_duplicate` | `pkt_hash = packet.calculate_packet_hash()...` | Every packet that reaches the forward check |
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| `flood_forward` / `direct_forward` via `mark_seen` | `pkt_hash = packet_hash or packet.calculate_packet_hash()...` | Every packet that passes the duplicate check |
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And on the drop path, a fourth computation:
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| Call site | Where | When |
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|-----------|-------|------|
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| `_get_drop_reason` → `is_duplicate` | `pkt_hash = packet.calculate_packet_hash()...` | Every dropped packet |
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The hash computed in `__call__` was already available as `pkt_hash_full` but was
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never passed into `process_packet`, `flood_forward`, `direct_forward`,
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`is_duplicate`, `mark_seen`, or `_get_drop_reason`. Each of those methods
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recomputed it independently.
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---
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## Root Cause
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The `packet_hash` optional parameter existed on `mark_seen` but not on
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`is_duplicate`, `flood_forward`, `direct_forward`, `process_packet`, or
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`_get_drop_reason`. The call chain therefore had no way to propagate the
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already-computed hash.
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---
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## Solution
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Thread the pre-computed `pkt_hash_full` from `__call__` down through the call
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chain as an optional `packet_hash: Optional[str] = None` parameter. Each method
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uses the provided hash if present, or falls back to computing it — preserving
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backward compatibility for any caller that doesn't have a pre-computed hash.
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```
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Before:
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__call__ → calculate_packet_hash() #1
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→ process_packet
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→ flood_forward
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→ is_duplicate → calculate_packet_hash() #2
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→ mark_seen → calculate_packet_hash() #3
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(drop path)
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→ _get_drop_reason
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→ is_duplicate → calculate_packet_hash() #4
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After:
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__call__ → calculate_packet_hash() #1 (only computation)
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→ process_packet(packet_hash=pkt_hash_full)
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→ flood_forward(packet_hash=pkt_hash_full)
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→ is_duplicate(packet_hash=pkt_hash_full) uses provided hash ✓
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→ mark_seen(packet_hash=pkt_hash_full) uses provided hash ✓
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(drop path)
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→ _get_drop_reason(packet_hash=pkt_hash_full)
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→ is_duplicate(packet_hash=pkt_hash_full) uses provided hash ✓
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```
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---
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## Methods Changed
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### `is_duplicate(packet, packet_hash=None)`
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```python
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# Before
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def is_duplicate(self, packet: Packet) -> bool:
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pkt_hash = packet.calculate_packet_hash().hex().upper() # always recomputed
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if pkt_hash in self.seen_packets:
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return True
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return False
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# After
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def is_duplicate(self, packet: Packet, packet_hash: Optional[str] = None) -> bool:
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"""...
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INVARIANT: purely synchronous — no await points. The caller relies on
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is_duplicate + mark_seen being atomic within the asyncio event loop.
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Do NOT add any await here without revisiting that invariant.
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"""
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pkt_hash = packet_hash or packet.calculate_packet_hash().hex().upper()
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return pkt_hash in self.seen_packets
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```
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### `_get_drop_reason(packet, packet_hash=None)`
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```python
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# Before
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def _get_drop_reason(self, packet: Packet) -> str:
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if self.is_duplicate(packet): ... # recomputes hash
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# After
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def _get_drop_reason(self, packet: Packet, packet_hash: Optional[str] = None) -> str:
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if self.is_duplicate(packet, packet_hash=packet_hash): ... # propagates hash
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```
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### `flood_forward(packet, packet_hash=None)`
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```python
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# Before
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def flood_forward(self, packet: Packet) -> Optional[Packet]:
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...
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if self.is_duplicate(packet): ... # recomputes
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self.mark_seen(packet) # recomputes
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# After
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def flood_forward(self, packet: Packet, packet_hash: Optional[str] = None) -> Optional[Packet]:
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"""...
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INVARIANT: purely synchronous — no await points.
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"""
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...
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if self.is_duplicate(packet, packet_hash=packet_hash): ... # propagates
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self.mark_seen(packet, packet_hash=packet_hash) # propagates
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```
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### `direct_forward(packet, packet_hash=None)` — same pattern as `flood_forward`
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### `process_packet(packet, snr=0.0, packet_hash=None)`
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```python
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# Before
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def process_packet(self, packet, snr=0.0):
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fwd_pkt = self.flood_forward(packet) # no hash
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# After
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def process_packet(self, packet, snr=0.0, packet_hash=None):
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"""...
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packet_hash: pre-computed SHA-256 hex from __call__; eliminates 2 SHA-256
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calls per forwarded packet by propagating the hash through the call chain.
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"""
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fwd_pkt = self.flood_forward(packet, packet_hash=packet_hash)
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```
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### `__call__` — two call-site changes
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```python
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# Before
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result = (None if ... else self.process_packet(processed_packet, snr))
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...
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drop_reason = processed_packet.drop_reason or self._get_drop_reason(processed_packet)
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# After
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result = (None if ... else self.process_packet(processed_packet, snr, packet_hash=pkt_hash_full))
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...
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drop_reason = processed_packet.drop_reason or self._get_drop_reason(
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processed_packet, packet_hash=pkt_hash_full
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)
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```
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---
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## What Was Not Changed
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`record_packet_only` (line 446) and `record_duplicate` (line 486) each compute
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the hash independently. These are separate recording paths (called from the
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inject path and from the raw-packet subscriber, respectively) that have no
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`pkt_hash_full` from `__call__` in scope. Changing them would require a larger
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refactor with no benefit to the forwarding hot path, so they are left unchanged.
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The fallback `packet_hash or packet.calculate_packet_hash()...` pattern in
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`is_duplicate`, `mark_seen`, and `_build_packet_record` ensures external callers
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(e.g. `TraceHelper.is_duplicate(packet)` from trace processing) continue to work
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without any change.
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---
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## Invariant Comments Added
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`flood_forward`, `direct_forward`, and `is_duplicate` now carry explicit docstring
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invariants:
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> **INVARIANT:** purely synchronous — no await points. The is_duplicate +
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> mark_seen pair is atomic within the asyncio event loop. Do NOT add any await
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> here without revisiting that invariant in `__call__` / `process_packet`.
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These invariants were implicit before. Making them explicit means a future
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contributor adding an `await` inside these methods will see the warning and
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understand the consequence: the duplicate-check and mark-seen can no longer be
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guaranteed atomic, allowing the same packet to be forwarded twice under concurrent
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task dispatch.
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---
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## Quantification
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On a Raspberry Pi running CPython 3.13, `hashlib.sha256` on a 50–200 byte
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LoRa payload takes approximately 1–3 µs. The `.hex().upper()` string conversion
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adds another ~0.5 µs. Savings per forwarded packet: ~3–8 µs.
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At 3 packets/second sustained forwarding rate this saves ~10–25 µs/second, which
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is negligible in absolute terms. The more significant benefit is correctness and
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clarity:
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- One canonical hash value per packet in the forwarding path.
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- No possibility of the hash changing between the `is_duplicate` check and the
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`mark_seen` call if `calculate_packet_hash` had any mutable state (it doesn't,
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but the pattern is now provably correct).
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- Explicit invariant documentation closes a latent trap for future contributors.
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---
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## Test Plan
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### Unit tests (no hardware)
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**T1 — Hash computed exactly once per forwarded packet**
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```python
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async def test_hash_computed_once_for_flood():
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call_count = 0
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original = Packet.calculate_packet_hash
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def counting_hash(self):
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nonlocal call_count
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call_count += 1
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return original(self)
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with patch.object(Packet, "calculate_packet_hash", counting_hash):
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await engine(flood_packet, metadata={})
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assert call_count == 1, f"Expected 1 hash computation, got {call_count}"
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```
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**T2 — Hash computed exactly once per dropped (duplicate) packet**
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```python
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async def test_hash_computed_once_for_duplicate():
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# Mark packet seen first
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engine.seen_packets[packet.calculate_packet_hash().hex().upper()] = time.time()
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call_count = 0
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original = Packet.calculate_packet_hash
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def counting_hash(self):
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nonlocal call_count; call_count += 1; return original(self)
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with patch.object(Packet, "calculate_packet_hash", counting_hash):
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await engine(packet, metadata={})
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# One computation in __call__ for pkt_hash_full; should not trigger again
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# in process_packet → flood_forward → is_duplicate (drop path via _get_drop_reason)
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assert call_count == 1
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```
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**T3 — External callers of `is_duplicate` without hash still work**
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```python
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def test_is_duplicate_without_hash():
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"""TraceHelper and other external callers pass no hash — must still work."""
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pkt = make_test_packet()
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engine.seen_packets[pkt.calculate_packet_hash().hex().upper()] = time.time()
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assert engine.is_duplicate(pkt) is True # no packet_hash arg
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assert engine.is_duplicate(pkt, packet_hash="WRONGHASH") is False
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```
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**T4 — mark_seen / is_duplicate agree on the same hash**
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```python
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def test_mark_then_is_duplicate_consistent():
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pkt = make_test_packet()
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pkt_hash = pkt.calculate_packet_hash().hex().upper()
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assert engine.is_duplicate(pkt, packet_hash=pkt_hash) is False
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engine.mark_seen(pkt, packet_hash=pkt_hash)
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assert engine.is_duplicate(pkt, packet_hash=pkt_hash) is True
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# Same result without the pre-computed hash (fallback path)
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assert engine.is_duplicate(pkt) is True
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```
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**T5 — flood_forward / direct_forward signatures are backward compatible**
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```python
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def test_flood_forward_no_hash_arg():
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"""Callers that don't pass packet_hash must still work (fallback compute)."""
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pkt = make_flood_packet()
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result = engine.flood_forward(pkt) # no packet_hash — must not raise
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assert result is not None or pkt.drop_reason is not None
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```
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### Integration / field tests (with hardware)
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**T6 — Forwarding throughput unchanged**
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1. Forward 100 packets at maximum duty-cycle budget.
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2. Verify all eligible packets are forwarded (same count as before change).
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3. Verify no `Duplicate` drops that were not present before.
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**T7 — Duplicate detection unchanged**
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1. Send the same packet twice within 1 second.
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2. Verify the first is forwarded and the second is logged as `"Duplicate"`.
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**T8 — CPU profile shows reduced `calculate_packet_hash` calls**
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1. Enable Python profiling (`cProfile`) on the repeater for 60 seconds.
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2. Compare `calculate_packet_hash` call count before and after.
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**Expected:** call count approximately halved for workloads where most packets
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are forwarded (≤ 1 call per forwarded packet vs ≥ 3 before).
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---
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## Proof of Correctness
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### Why the fallback `packet_hash or packet.calculate_packet_hash()` is safe
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`packet_hash` is either the correct hash (passed from `__call__`) or `None`.
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If it is `None`, the fallback computes the hash fresh — identical to the old
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behaviour. There is no case where a wrong hash is used: the only source of a
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non-None `packet_hash` is `pkt_hash_full = packet.calculate_packet_hash()...`
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in `__call__`, computed over the same `processed_packet` (a deep copy of the
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received packet, unchanged between hash computation and the call to
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`process_packet`).
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### Why passing the hash through a deep-copied packet is correct
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`processed_packet = copy.deepcopy(packet)` (line 178) happens before
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`pkt_hash_full` is passed to `process_packet`. The deep copy does not change
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the packet's wire representation — `calculate_packet_hash()` calls
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`packet.write_to()` which serialises the packet's fields. The copy has the
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same fields, so `deepcopy(packet).calculate_packet_hash() == packet.calculate_packet_hash()`.
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Passing the hash computed from the original to the copy is correct.
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### Why the invariant is critical
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asyncio only yields execution at `await` points. `flood_forward` and
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`direct_forward` have no `await`, so they run atomically from the event loop's
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perspective. The `is_duplicate` check and the `mark_seen` call inside them
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cannot be interleaved with another coroutine. If a future change added an
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`await` between them, two concurrent `_route_packet` tasks could both pass the
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duplicate check for the same packet before either marked it seen — sending the
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same packet twice. The invariant comment documents this so the risk is visible
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at the point where it could be broken.
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@@ -189,11 +189,12 @@ class RepeaterHandler(BaseHandler):
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original_path_hashes = packet.get_path_hashes_hex()
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path_hash_size = packet.get_path_hash_size()
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# Process for forwarding (skip if repeat disabled or if this is a local transmission)
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# Process for forwarding (skip if repeat disabled or if this is a local transmission).
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# Pass pkt_hash_full so flood_forward / direct_forward don't recompute SHA-256.
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result = (
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None
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if (not allow_forward or local_transmission)
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else self.process_packet(processed_packet, snr)
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else self.process_packet(processed_packet, snr, packet_hash=pkt_hash_full)
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)
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forwarded_path_hashes = None
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@@ -303,7 +304,7 @@ class RepeaterHandler(BaseHandler):
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else:
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# Check if packet has a specific drop reason set by handlers
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drop_reason = processed_packet.drop_reason or self._get_drop_reason(
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processed_packet
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processed_packet, packet_hash=pkt_hash_full
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)
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logger.debug(f"Packet not forwarded: {drop_reason}")
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@@ -612,9 +613,9 @@ class RepeaterHandler(BaseHandler):
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if pkt_hash:
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self._recent_hash_index[pkt_hash] = packet_record
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def _get_drop_reason(self, packet: Packet) -> str:
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def _get_drop_reason(self, packet: Packet, packet_hash: Optional[str] = None) -> str:
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if self.is_duplicate(packet):
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if self.is_duplicate(packet, packet_hash=packet_hash):
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return "Duplicate"
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if not packet or not packet.payload:
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@@ -642,12 +643,20 @@ class RepeaterHandler(BaseHandler):
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# Default reason
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return "Unknown"
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def is_duplicate(self, packet: Packet) -> bool:
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def is_duplicate(self, packet: Packet, packet_hash: Optional[str] = None) -> bool:
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"""Return True if this packet has already been seen.
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pkt_hash = packet.calculate_packet_hash().hex().upper()
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if pkt_hash in self.seen_packets:
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return True
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return False
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Accepts an optional pre-computed packet_hash to avoid a redundant SHA-256
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when the caller (e.g. __call__ → process_packet → flood/direct_forward)
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has already calculated the hash. Falls back to computing it if not provided.
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INVARIANT: this method is synchronous with no await points. The caller
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(process_packet / __call__) relies on is_duplicate + mark_seen being
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effectively atomic within the asyncio event loop. Do NOT add any await
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here without revisiting that invariant.
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"""
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pkt_hash = packet_hash or packet.calculate_packet_hash().hex().upper()
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return pkt_hash in self.seen_packets
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def mark_seen(self, packet: Packet, packet_hash: Optional[str] = None):
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@@ -789,8 +798,13 @@ class RepeaterHandler(BaseHandler):
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logger.error(f"Transport code validation error: {e}")
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return False, f"Transport code validation error: {e}"
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def flood_forward(self, packet: Packet) -> Optional[Packet]:
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def flood_forward(self, packet: Packet, packet_hash: Optional[str] = None) -> Optional[Packet]:
|
||||
"""Forward a FLOOD packet, appending our hash to the path.
|
||||
|
||||
INVARIANT: purely synchronous — no await points. The is_duplicate +
|
||||
mark_seen pair is atomic within the asyncio event loop. Do NOT add any
|
||||
await here without revisiting that invariant in __call__ / process_packet.
|
||||
"""
|
||||
# Validate
|
||||
valid, reason = self.validate_packet(packet)
|
||||
if not valid:
|
||||
@@ -824,8 +838,8 @@ class RepeaterHandler(BaseHandler):
|
||||
packet.drop_reason = f"FLOOD loop detected ({mode})"
|
||||
return None
|
||||
|
||||
# Suppress duplicates
|
||||
if self.is_duplicate(packet):
|
||||
# Suppress duplicates — pass pre-computed hash to avoid a second SHA-256.
|
||||
if self.is_duplicate(packet, packet_hash=packet_hash):
|
||||
packet.drop_reason = "Duplicate"
|
||||
return None
|
||||
|
||||
@@ -847,7 +861,7 @@ class RepeaterHandler(BaseHandler):
|
||||
packet.drop_reason = "Path would exceed MAX_PATH_SIZE"
|
||||
return None
|
||||
|
||||
self.mark_seen(packet)
|
||||
self.mark_seen(packet, packet_hash=packet_hash)
|
||||
|
||||
# Append hash_size bytes from our public key prefix
|
||||
packet.path.extend(self.local_hash_bytes[:hash_size])
|
||||
@@ -855,8 +869,13 @@ class RepeaterHandler(BaseHandler):
|
||||
|
||||
return packet
|
||||
|
||||
def direct_forward(self, packet: Packet) -> Optional[Packet]:
|
||||
def direct_forward(self, packet: Packet, packet_hash: Optional[str] = None) -> Optional[Packet]:
|
||||
"""Forward a DIRECT packet, removing the first hop from the path.
|
||||
|
||||
INVARIANT: purely synchronous — no await points. The is_duplicate +
|
||||
mark_seen pair is atomic within the asyncio event loop. Do NOT add any
|
||||
await here without revisiting that invariant in __call__ / process_packet.
|
||||
"""
|
||||
# Validate packet (empty payload, oversized path, etc.)
|
||||
valid, reason = self.validate_packet(packet)
|
||||
if not valid:
|
||||
@@ -882,12 +901,12 @@ class RepeaterHandler(BaseHandler):
|
||||
packet.drop_reason = "Direct: not for us"
|
||||
return None
|
||||
|
||||
# Suppress duplicates
|
||||
if self.is_duplicate(packet):
|
||||
# Suppress duplicates — pass pre-computed hash to avoid a second SHA-256.
|
||||
if self.is_duplicate(packet, packet_hash=packet_hash):
|
||||
packet.drop_reason = "Duplicate"
|
||||
return None
|
||||
|
||||
self.mark_seen(packet)
|
||||
self.mark_seen(packet, packet_hash=packet_hash)
|
||||
|
||||
# Remove first hash entry (hash_size bytes)
|
||||
packet.path = bytearray(packet.path[hash_size:])
|
||||
@@ -969,19 +988,30 @@ class RepeaterHandler(BaseHandler):
|
||||
|
||||
return delay_s
|
||||
|
||||
def process_packet(self, packet: Packet, snr: float = 0.0) -> Optional[Tuple[Packet, float]]:
|
||||
def process_packet(
|
||||
self,
|
||||
packet: Packet,
|
||||
snr: float = 0.0,
|
||||
packet_hash: Optional[str] = None,
|
||||
) -> Optional[Tuple[Packet, float]]:
|
||||
"""Route a received packet to flood_forward or direct_forward.
|
||||
|
||||
packet_hash is the pre-computed SHA-256 hex string from __call__.
|
||||
Passing it here avoids recomputing the hash in flood_forward /
|
||||
direct_forward / is_duplicate / mark_seen — reducing SHA-256 calls
|
||||
from 3 per forwarded packet to 1.
|
||||
"""
|
||||
route_type = packet.header & PH_ROUTE_MASK
|
||||
|
||||
if route_type == ROUTE_TYPE_FLOOD or route_type == ROUTE_TYPE_TRANSPORT_FLOOD:
|
||||
fwd_pkt = self.flood_forward(packet)
|
||||
fwd_pkt = self.flood_forward(packet, packet_hash=packet_hash)
|
||||
if fwd_pkt is None:
|
||||
return None
|
||||
delay = self._calculate_tx_delay(fwd_pkt, snr)
|
||||
return fwd_pkt, delay
|
||||
|
||||
elif route_type == ROUTE_TYPE_DIRECT or route_type == ROUTE_TYPE_TRANSPORT_DIRECT:
|
||||
fwd_pkt = self.direct_forward(packet)
|
||||
fwd_pkt = self.direct_forward(packet, packet_hash=packet_hash)
|
||||
if fwd_pkt is None:
|
||||
return None
|
||||
delay = self._calculate_tx_delay(fwd_pkt, snr)
|
||||
|
||||
Reference in New Issue
Block a user