perf: reduce partial response allocations - #510
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Mohamed Mansour (mohamedmansour) merged 2 commits intoSep 2, 2026
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Mohamed Mansour (mohamedmansour)
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mcritzjam
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🟡 Changes recommended
The commerce path still deep-clones state, benchmark setup needs safer batching, and the specification remains internally inconsistent.
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Pull request overview
Optimizes Rust partial navigation by projecting owned JSON state directly while retaining streaming JSON handling at host boundaries.
Changes:
- Adds owned-state
render_partialand serialized-staterender_partial_json. - Migrates Rust and host-binding call sites.
- Adds tests, benchmarks, and documentation updates.
File summaries
| File | Description |
|---|---|
examples/app/commerce/server/src/frontend.rs |
Uses owned-state partial rendering. |
docs/guide/integrations/rust.md |
Documents both Rust rendering paths. |
docs/guide/concepts/routing.md |
Updates partial-navigation API guidance. |
DESIGN.md |
Specifies owned and serialized-state contracts. |
crates/webui/tests/example_apps_hydration.rs |
Migrates integration coverage. |
crates/webui/src/server.rs |
Removes state serialization and parameter cloning. |
crates/webui/README.md |
Updates API examples. |
crates/webui/benches/server_request_bench.rs |
Adds sparse-state benchmark coverage. |
crates/webui/benches/README.md |
Documents the benchmark. |
crates/webui-wasm/src/handler.rs |
Retains serialized WASM boundary. |
crates/webui-python/src/lib.rs |
Retains serialized Python boundary. |
crates/webui-node/src/lib.rs |
Retains serialized Node boundary. |
crates/webui-handler/src/route_handler.rs |
Implements owned-state projection and tests. |
crates/webui-handler/benches/handler_bench.rs |
Repairs FAST v2 payload. |
crates/webui-handler/benches/bootstrap_state_bench.rs |
Migrates raw-state benchmarks. |
crates/webui-ffi/src/lib.rs |
Retains serialized FFI boundary. |
crates/webui-cli/src/commands/serve.rs |
Moves parsed CLI state directly. |
BENCHMARKS.md |
Describes sparse projection coverage. |
Review details
- Files reviewed: 18/18 changed files
- Comments generated: 4
- Review effort level: Balanced
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Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Akrosh Gandhi (akroshg)
approved these changes
Sep 2, 2026
Mohamed Mansour (mohamedmansour)
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mohamedmansour-optimize-rust-memory
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September 2, 2026 17:47
Mohamed Mansour (mohamedmansour)
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Sep 3, 2026
## Why Server memory is not cheap, and the handler's render path still paid a steady stream of avoidable heap allocations on every request. Following up on #510, this pass targets the remaining per-request allocation sources in `webui-handler` without giving up any throughput. The largest one was hiding in the fragment-graph walks. Every walk (reachability, inventory, chain, nested params) queued a `QueuedFragment { id: String, route_base: String }` per graph **edge** and keyed its visited set on `(String, String)`, so traversing a component graph cost roughly four `String` allocations per edge. These are not startup-only paths: `collect_reachable_component_order_for_request` fires at the `body_end` boundary on every full render that has a hydration plugin attached, which is the standard configuration. ## What changed **Allocation-free graph traversal.** `QueuedFragment` now borrows the protocol's fragment name (`&'protocol str`) and stores its route base as a `u32` index into a small interning arena, making the struct `Copy`. Route nesting is shallow, so the arena's linear dedup scan beats hashing and stops allocating after each level is seen once. The visited key becomes `(&'protocol str, Option<u32>)`, where `None` collapses every base into one bucket for route-insensitive walks, matching the previous empty-string sentinel. This required threading a `'protocol` lifetime through `collect_inventoryable_components_from_stack`, `walk_route_children`, `collect_route_boundary_components`, `collect_inventory_and_chain`, and `walk_children_for_inventory_and_chain`. **Stack-rendered JSON scalars.** Interpolating a non-string signal called `Value::to_string()` and then ran the result through `encode_safe`. Numbers, booleans, and null can never contain HTML-significant bytes, so they now render through a fixed-capacity `ScalarBuffer` and skip escaping entirely. Applied to `write_signal_value`, dynamic attribute emission, and template attribute assembly. **Residual per-route clones.** `route_base` is saved with `mem::replace` instead of a deep `Cow` clone (inside `is_matched`, `best_route` is always `Some`, so the slot is unconditionally overwritten). `process_component` and `emit_css_module` take `&str`, so generated route hosts no longer build a throwaway `WebUIFragmentComponent` just to pass a name. Nested route params merge entry-wise instead of cloning an entire `HashMap` only to move its entries out. ## Performance Measured with `cargo xtask bench streaming-resource` (exact counts from a custom `GlobalAlloc`), release build, 2000 iterations per row. | Metric | Before | After | Change | |---|---:|---:|---:| | string allocs/run | 156 | 141 | -9.6% | | streaming allocs/run | 170 | 155 | -8.8% | | streaming POOLED allocs/run | 163 | 148 | -9.2% | | streaming bytes/run | 43.7 KiB | 43.6 KiB | -0.2% | | POOLED wall us/run @1000 (median, n=5) | 13.92 | 13.65 | -1.9%, within noise | | Output size | 24152 B | 24152 B | unchanged | Output bytes are byte-identical at all three scales, which is the correctness proof. Wall time improved alongside the allocation count, so nothing was traded away for the memory win. > **Correction.** An earlier revision of this description listed `process RSS @1000 | 14.12 MiB | 12.83 MiB | -9.1%` as a headline metric. **That row was sampling noise and has been removed.** It was a single sample per build, and the benchmark's `process RSS` column turns out to be multimodal. Re-running the *identical* binary 8 times gives 12.66 / 12.73 / 12.94 / 13.20 / 13.70 / 13.97 / 13.98 / 14.02 MiB - a 1.36 MiB spread, which is wider than the 1.29 MiB "improvement" originally claimed. The column is dominated by where the allocator places the ~6.8 MiB scale-1000 fixture state, which is built before any render runs, so it is largely blind to the render path regardless. `allocs/run` and `bytes/run` are exact counts from a custom `GlobalAlloc` and are the numbers to judge this PR on. Tracked in #516. > **Second correction, same cause.** The wall-time row originally read `15.46 -> 13.74 us, -11.1%`, also a single sample per build. A proper A/B - baseline built from `78b7617f` in a separate worktree, the two binaries run alternately five times each - gives medians of 13.92 vs 13.65 us (POOLED/1000) and 13.28 vs 13.05 us (string/1000), about **-1.9% and -1.7%, with the two ranges overlapping**. The original `15.46` baseline was a cold first-run outlier. The honest statement is that this PR shows **no measurable wall-time regression and possibly a small improvement that n=5 cannot separate from noise** - not the double-digit speedup first claimed. > > The same A/B re-confirms the allocation counts are exact and deterministic: **156 and 163 in all five baseline runs, 141 and 148 in all five patched runs, zero variance.** That is the result this PR rests on. ## Notes for reviewers **These numbers understate the real-world gain.** The `streaming-resource` fixture renders the contact-book dashboard, whose output is ~24 KB whether the state carries 10, 100, or 1000 contacts, because the scaled `contacts` array is not rendered at `/`. That makes it a good zero-copy proof (state tree size does not affect render allocations) but it exercises a shallow route graph. Savings here scale with route depth and component-graph size, so apps with deeper route trees benefit proportionally more. **`ScalarBuffer` overflow is reported, not truncated.** `write_str` returns `fmt::Error` rather than silently clipping, so `format_plain_json_scalar` declines and the caller falls back to the allocating path. The 48-byte capacity is comfortably above the 24-byte worst case for an f64 (`serde_json` is built without `arbitrary_precision`), so the fallback is unreachable in practice while staying correct if that ever changes. Added `scalar_buffer_tests` asserting byte-identical output versus `Value::to_string()` across `i64::MIN`, `i64::MAX`, `u64::MAX`, `f64` extremes, and `MIN_POSITIVE`, plus coverage that strings/arrays/objects correctly decline so nothing escapes unescaped. **Deliberately left out of scope.** `context.route_children` is still a `Vec<WebUiFragmentRoute>` deep-cloned from the protocol per matched route, and `rendered_components` / the `local_vars` and `component_attrs` keys are still `String` despite always originating from `&'protocol str`. All three want the same fix, but it requires threading a lifetime through `SessionCore` and `ContinuationVm`, which are currently lifetime-free because they hold this data across streaming suspension. That is a real payoff but a much riskier refactor, so it belongs in its own PR rather than bundled here. ## Validation - `cargo xtask check` (all phases pass) - `cargo xtask bench streaming-resource` - 489 handler tests pass, including the 3 new scalar-rendering tests No public API, protocol schema, FFI surface, or user-facing behavior changed, so `DESIGN.md` and `docs/` need no updates. No new dependencies. --------- Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: 2edb7e98-5706-46bc-b191-51620cd290e5
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Why
Rust partial-navigation requests already own a parsed
serde_json::Value, but the server path serialized the complete state tree before the handler parsed it again and discarded unselected keys. Large sparsely projected states therefore paid substantial temporary memory and CPU costs per request.What changed
Protocol::render_partial(Value, ...)the ownership-taking Rust path so selected values move directly into the response.Protocol::render_partial_json(&str, ...)for Node, FFI, WASM, and Python boundaries, where the streaming JSON visitor avoids materializing a full duplicate state tree.$webuiexclusion for both full and keyed owned-state projection.Performance
The complete 12-target Rust benchmark suite completed successfully.
Validation
cargo xtask checkcargo xtask bench all