Joseph Doherty
4e76b44391
Adds `write_message::encode_to_bytes_mut` (and the timestamped variant) returning a freshly-allocated `BytesMut`. Allocation count is identical to `encode` (2 allocs/op for fixed-width scalars); the benefit is downstream — consumers can `BytesMut::split_to` / `freeze` and forward the body bytes to a wire-level sink without an intermediate copy. The body builders (`encode_boolean` / `encode_fixed` / `encode_variable` / `encode_array`) were refactored to fill a pre-sized `&mut [u8]` rather than each allocating their own `Vec<u8>`. The dispatcher computes the body size up front via small `*_body_size` helpers and resizes the destination buffer (Vec or BytesMut) once. This is also the prerequisite refactor for F52.3. Bench delta in `design/M6-bench-baseline.md` § F52.1; existing `encode` row unchanged at 2 allocs/op. All 265 round-trip tests unchanged. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
96 lines
4.5 KiB
Markdown
96 lines
4.5 KiB
Markdown
# M6 — `mxaccess-codec` allocation-count baseline
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Source: `cargo bench -p mxaccess-codec` (commit recording this file).
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Harness: `crates/mxaccess-codec/benches/alloc_count.rs` — a thin
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`GlobalAlloc` wrapper that increments two atomics on every `alloc` /
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`dealloc` call, then runs each scenario for 10k iterations after a
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1k-iteration warm-up.
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## Target (per `70-risks-and-open-questions.md` R12)
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> Aim for < 5 allocations per write at steady state.
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The bench gates on this: any `write_message::encode` scenario at
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≥ 5 allocs/op causes the binary to exit with code 1.
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## Baseline (release profile, Windows x64)
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| scenario | iters | allocs/op | bytes/op | deallocs/op |
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|------------------------------------------------|--------:|----------:|---------:|------------:|
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| `write_message::encode` (Int32) | 10,000 | 2.00 | 44 | 2.00 |
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| `write_message::encode` (Float32) | 10,000 | 2.00 | 44 | 2.00 |
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| `write_message::encode` (Float64) | 10,000 | 2.00 | 52 | 2.00 |
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| `write_message::encode` (Boolean) | 10,000 | 1.00 | 37 | 1.00 |
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| `write_message::encode` (String, 5 chars) | 10,000 | 4.00 | 92 | 4.00 |
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| `write_message::encode_to_bytes_mut` (Int32) | 10,000 | 2.00 | 44 | 2.00 |
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| `MxReferenceHandle::from_names` | 10,000 | 2.00 | 22 | 2.00 |
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| `NmxSubscriptionMessage::parse_inner` | 10,000 | 1.00 | 72 | 1.00 |
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| (DataUpdate, Int32) | | | | |
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## Read
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R12's < 5 allocs/write target is **already met** across the proven matrix:
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- Scalar writes (Bool, Int32, Float32, Float64) sit at 1–2 allocs/op.
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The two allocs come from (1) the encoder's `Vec<u8>` output buffer
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and (2) an internal scratch buffer in the value-encode path.
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- String writes hit 4 allocs/op (output buffer, UTF-16LE conversion
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buffer, the inner-length wrapper, and one more downstream).
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- `MxReferenceHandle::from_names` allocates twice (one per
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`compute_name_signature` call — UTF-16LE buffer for each name).
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- `NmxSubscriptionMessage::parse_inner` allocates once for the
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`records: Vec<NmxSubscriptionRecord>` collection.
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## Implications for F39
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F39 (zero-copy pass) was scoped as the work to *hit* the R12 target.
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With the target already met, F39's scope tightens to:
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- Move the encoder's output buffer to `bytes::BytesMut` so consumers
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can split it without copying. Doesn't reduce alloc count but
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improves downstream zero-copy on the wire-write path.
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- Cache the per-handle UTF-16LE name conversion (the two
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`compute_name_signature` allocs per `from_names`) inside
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`MxReferenceHandle` if the same name is registered repeatedly.
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- Pool the per-frame scratch buffer at the session level so the
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per-write count drops from 2 → 1 on hot paths.
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These are nice-to-have optimisations rather than R12 blockers.
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## F52 deltas
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F52 split the three F39 sub-tasks into their own commits. Each
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optimisation lands with a before/after row in this section.
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### F52.1 — `BytesMut` output buffer (encoder)
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Adds `write_message::encode_to_bytes_mut` (and the timestamped
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variant) returning a freshly-allocated `BytesMut`. Allocation count
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is **identical** to the existing `encode` path — the benefit is
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downstream: consumers can `BytesMut::split_to` / `freeze` and forward
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the body bytes to a wire-level sink without an intermediate copy.
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| scenario | before (allocs/op) | after (allocs/op) |
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|----------------------------------------------|-------------------:|------------------:|
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| `write_message::encode` (Int32) | 2.00 | 2.00 |
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| `write_message::encode_to_bytes_mut` (Int32) | — | 2.00 |
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Internally this required refactoring the body builders
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(`encode_boolean` / `encode_fixed` / `encode_variable` / `encode_array`)
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to fill a pre-sized `&mut [u8]` rather than each allocating their own
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`Vec<u8>`. The dispatcher computes the body size up front via small
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`*_body_size` helpers and resizes the destination buffer (Vec or
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BytesMut) once. This is also the prerequisite refactor for F52.3.
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## Reproducing
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```powershell
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cd rust
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cargo bench -p mxaccess-codec
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```
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Numbers are deterministic per release-profile build on a given host.
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Numeric drift across hosts is expected (the warm-up + black_box hints
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keep iteration counts stable, not the underlying allocator's
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small-alloc fast-path heuristics).
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