applyBatch serialization benchmark

August 28, 2026 · View on GitHub

Every React commit turns into one applyBatch(json) call. This benchmark measures what that costs on both sides of the FFI boundary, on the real ChatApp queue rather than a synthetic one.

Headline: the wire codec is the smallest lever. Swapping JSON for MessagePack buys 1.24x. Decoding straight into typed ops and sharing styles by content bought 4.2x on parse-and-apply, 7.9x fewer allocations, and 5.3x on tree memory, with JSON unchanged and no change to apply_styles.

That work has landed. What follows is how it was measured, what was rejected, and what is left.

What shipped

Measured end to end on the real path, apply_batch_to_tree, before and after:

	before	after
parse and apply	127.1 ms	30.1 ms
heap churn	900.5 MB	104.0 MB
allocations	1 476 196	186 090
retained tree	224.5 MB	42.6 MB
bytes per element	3116 B	592 B

Four changes, in the order they landed:

BatchOp::SetStyle stopped holding a StyleDesc inline. A Vec<BatchOp> is as wide as its widest variant, so 221 764 ops reserved 312 MB up front. Measured on its own: −205 MB churn
The serde_json::Value tree is gone. The batch deserializes straight from its bytes into typed ops through a SeqAccess visitor, and strings borrow out of the input
RetainedTree::elements uses FxHashMap. It is probed once per ancestor hop in mark_changed and twice per child per frame in build_virtual_list
Styles are hash-consed by raw payload and shared as Arc<StyleDesc>, swept after each batch by Arc::strong_count

Rejected, with reasons, in what was not done.

Running it

Two halves. The JS half writes the fixture the Rust half reads, so both measure the same bytes.

cd examples
TURNS=10000 SAFE_MDX=1 bun run bench:serde     # writes tmp/batch-fixture.json

cd ../packages/native
cargo run --release --example bench_serde

examples/bench-serialization.ts installs a stub NativeRenderer that owns applyBatch, so wrapWithBatching hands it the exact tuples production sends. Nothing about the fixture is invented.

ChatApp ► reconciler ► wrapWithBatching ► CaptureRenderer.applyBatch(json)
                                                 │
                                    ┌────────────┴────────────┐
                                    ▼                         ▼
                            JS codec bench          tmp/batch-fixture.json
                                                              │
                                                              ▼
                                                packages/native/examples/bench_serde.rs

The Rust half installs a counting GlobalAlloc, so "live heap" is the real resident cost of the tree, not a size_of estimate. It also asserts that the compact tree and the current tree hold the same parents, the same child order and the same styled elements before reporting any saving.

The fixture

10 000 turns with safe-mdx enabled. 221 764 ops, 72 010 elements, 13.06 MB of JSON.

op	count	JSON bytes	share
`setStyle`	59 320	6.37 MB	49.6%
`createElement`	72 010	2.10 MB	16.4%
`appendChild`	72 009	1.92 MB	15.0%
`setCustomProp`	6 142	1.70 MB	13.2%
`setText`	12 255	0.74 MB	5.8%

Two facts decide everything below.

Styles are half the payload and there are only 90 of them. 59 320 setStyle ops carry 90 distinct style objects. The same ~1.2 KB object is serialized, shipped and parsed hundreds of times.

261 unique strings account for 564 987 occurrences. 5.45 MB of string bytes collapse to 0.01 MB once deduplicated.

JS side: encode

codec	encode	decode	wire bytes	vs JSON	resident while alive
`JSON.stringify`	23.96 ms	18.96 ms	13.05 MB	1.00x	26.1 MB
JSON ► utf8 Buffer	25.20 ms	21.41 ms	13.05 MB	1.00x	13.0 MB
msgpackr	18.83 ms	25.01 ms	10.29 MB	0.79x	10.3 MB
msgpackr records	16.85 ms	11.15 ms	6.96 MB	0.53x	7.0 MB
cbor-x	18.05 ms	36.38 ms	10.30 MB	0.79x	10.3 MB

JSON.stringify costs two bytes of JS heap per wire byte. JSC keeps a string as latin1 only when every code unit fits in a byte, and the chat payload contains box-drawing characters. Encoding into a Buffer costs one byte per wire byte, and that byte lives outside the JS heap where the GC never has to walk it.

That is the one unambiguous JS-side memory win, and it does not require changing the codec.

JS side: protocol

Interning is not a codec feature. It changes what JS sends.

Protocol A replaces a repeated style object with ["setStyleRef", id, 7] plus one ["defineStyle", 7, {…}] the first time that style is seen. Protocol B adds a string table for every value that repeats.

variant	JSON bytes	vs today	msgpackr records	vs today
today	13.05 MB	1.00x	6.96 MB	0.53x
A — interned styles	8.10 MB	0.62x	6.05 MB	0.46x
B — A + string table	6.78 MB	0.52x	4.32 MB	0.33x

Protocol A alone also halves encode time, from 23.96 ms to 15.55 ms, because JSON.stringify no longer walks 59 320 style objects.

Rust side: decode

This is where the money is. Median of 5 runs, counting allocator.

decode path	median	vs today	heap churn	allocations
today — `Vec<Value>` + clone + `from_value`	63.7 ms	1.00x	206.8 MB	1 755 376
`Vec<Value>`, no clone	56.4 ms	1.13x	150.9 MB	1 294 094
typed JSON ► `StyleDesc`	71.1 ms	0.89x	374.5 MB	140 702
typed JSON ► `CompactStyle`	26.0 ms	2.45x	36.0 MB	100 358
typed msgpack ► `CompactStyle`	19.6 ms	3.24x	42.3 MB	100 356
typed JSON ► `Box<StyleDesc>`, interned	13.4 ms	4.74x	32.2 MB	41 330
typed JSON ► `CompactStyle`, interned	14.8 ms	4.30x	32.1 MB	41 115
typed msgpack ► `CompactStyle`, interned	10.8 ms	5.91x	38.4 MB	41 113

Once styles are interned, CompactStyle stops paying for itself. Only 90 of 221 764 ops parse a style, so the 1392-byte struct is built 90 times and its size no longer matters. Box<StyleDesc> keeps every named field, keeps apply_styles untouched, and is marginally faster. Intern first; only consider a compact style if a profile still points at style parsing.

Today's path allocates the same data four times

renderer.rs takes json: String, so napi copies and UTF-8 validates the whole payload before parsing starts. Then:

Rust String
   │
   ▼ serde_json::from_str            a String per key AND per value
Vec<serde_json::Value>
   │
   ▼ batch_payload -> value.clone()  a deep clone of the whole style map
serde_json::Value
   │
   ▼ serde_json::from_value          a String per StyleDesc field
StyleDesc

1.76 million allocations for 221 764 ops. Removing the value.clone() in batch_payload alone is worth 1.15x and 460 000 allocations, costs nothing, and needs no new dependency.

A fat enum variant poisons the whole vector

The most useful negative result: going typed without fixing the style makes things worse. Op<StyleDesc> is 1408 bytes, so a Vec<Op<StyleDesc>> of 221 764 ops reserves 312 MB regardless of how few ops carry a style. That path churns 374.5 MB and is slower than the serde_json::Value tree it replaces.

Op<CompactStyle> is 104 bytes. Same code, 10x less churn.

The data-oriented redesign

Everything in this section is a phase two. The recommended order above gets 5.4x on memory without any of it. Read this when 577 bytes per element is still too much.

`StyleDesc` is 1392 bytes for six properties

Around 80 Option fields, all inline, all allocated whether set or not. A real element sets six. Three observations shrink it:

GPUI consumes Pixels(f32). Storing f64 buys nothing
a colour is 4 bytes once parsed, not a 24-byte String plus a heap block
display, position, alignItems and friends are keyword enums, not text

So a style becomes a sorted list of 8-byte properties:

#[repr(C)]
struct StyleProp {
    key: u16,    // interned property name
    kind: u8,    // f32 | colour | keyword | bool | nested | percent | auto
    _pad: u8,
    bits: u32,   // f32 bits, or rgba8, or an id
}

struct CompactStyle { props: Box<[StyleProp]> }   // 16 B + 8 B per property

Parsing colours at ingest is a second win: paint stops calling parse_color on every frame.

`RetainedElement` costs 3102 bytes resident

Option<StyleDesc> is inline, so every element pays the full style size even when it has none. Add a String element type, a HashSet<String> of event names and a HashMap<String, Value> of custom props, and one node costs more than a kilobyte before it holds any content. The HashMap<u64, RetainedElement> that holds them rehashes 1.4 KB values as it grows.

The replacement is a fixed 44-byte record in a dense Vec, indexed by the id JS already allocates as a counter. Children are a sibling list, so no node owns a Vec. Everything variable-width moves to a side table that repeated values share.

#[repr(C)]
struct CompactElement {
    parent: u32, first_child: u32, last_child: u32, next_sibling: u32,
    style: u32,           // index into a deduplicated style table
    content: u32, test_id: u32, custom_props: u32,
    subtree_revision: u32,
    events: u32,          // one bit per event type, was HashSet<String>
    kind: u16,            // div | text | input | …, was String
    custom_len: u16,
}

Result

tree	elements	live heap	per element	effort
`RetainedTree` (today)	72 010	223.4 MB	3102 B	—
+ `Box<StyleDesc>`	72 010	124.6 MB	1729 B	one field
+ `Arc<StyleDesc>`, deduplicated	72 010	41.6 MB	577 B	one field plus an intern table
`CompactTree`	72 010	9.2 MB	127 B	rewrite every reader

size_of::<RetainedElement>() was 1624 B and is 248 B with the style behind a pointer. The HashMap holds the value inline, so that one field decides the size of the whole table. The benchmark's own replica prints 240 B because it leaves out search_revision.

5.4x of the available 24.4x costs one field change. Everything reading Option<&StyleDesc> keeps working through Deref, so apply_styles and every custom element stay untouched. The remaining 4.5x needs the dense-Vec rewrite.

Extrapolated to the goal of virtualizing in Rust instead of React:

elements	today	`Arc` deduplicated	`CompactTree`
100 000	310 MB	58 MB	12.7 MB
1 000 000	3.1 GB	577 MB	127 MB

What codecs can and cannot do

No mainstream JS↔Rust codec deduplicates repeated string values.

option	what it deduplicates	usable here
msgpackr `useRecords`	object keys per shape	yes, but keys are not the cost
msgpackr `bundleStrings`	nothing; groups strings for one JS `toString()`	no, Rust cannot read the extension
msgpackr `structuredClone`	repeated object references	no, React makes a fresh style object per element
Arrow dictionary arrays	string values, properly	columnar; a mutation stream is not a table
FlatBuffers `create_shared_string`	string values, properly	needs a schema and codegen for every op

msgpackr with useRecords: true is also not plain MessagePack. rmp_serde cannot read it; pnpm had to write a transcoder for the same reason. The 6.96 MB row above is not a payload Rust can decode as-is.

So value deduplication has to happen in the protocol. Once styles are interned, the codec choice is worth 1.3x and nothing else.

Why styles are interned in Rust, not in the protocol

The obvious design is to make JS do it: send ["defineStyle", n, {…}] once and ["setStyleRef", id, n] afterwards. It was measured and rejected.

It is not free on the JS side, which is the first surprise. To emit a reference you must know the style's identity, and commitUpdate in host-config.ts resends newProps.style on every commit. A JSX style={{…}} literal is a fresh object every render, so a WeakMap misses on everything except hoisted constants. That leaves a content hash, and the cheapest content hash of a JS object is JSON.stringify. Measured with the interning inside the timed region:

path	JS time	wire bytes
`JSON.stringify(queue)`	26.31 ms	13.05 MB
intern, then stringify	23.35 ms	8.10 MB

1.13x, not the 1.5x an earlier version of this document claimed by timing a queue that arrived pre-interned.

The real objection is lifetime. examples/timeline.tsx writes left: clip.start * pxPerSecond inline, so a drag produces a distinct style every frame. A JS-owned table grows by one entry per frame and nothing releases it, and on that same frame the protocol sends a definition plus a reference where it sends one op today. It would make the interactive path slower and leaky to make the cold mount path faster.

Interning in Rust has neither problem. RetainedTree::intern_style hashes the raw payload, which is cheaper than building 80 Option fields, and sweep_styles drops any entry whose Arc::strong_count is 1 after each batch. A drag adds one entry per frame and releases it on the next.

The only thing given up is 4.95 MB of wire and 3 ms of JS.

Deliberately not doing

change	measured gain	why not
protocol interning in JS	1.13x JS, 1.6x wire	leaks on a drag and makes the update path send two ops where it sends one. Rust interning gets the memory win without either
MessagePack	13.4 ► 10.8 ms, 1.24x	a new dependency and a new codec on both sides, for a quarter
`CompactStyle`	none once styles are interned	80 property mappings and a rewrite of `apply_styles`, and it measured slower than `Box<StyleDesc>`
`CompactElement` dense tree	41.6 ► 9.2 MB, 4.5x	every reader of `RetainedElement` changes, and it needs JS id recycling before its numbers mean anything

What is left

Cheap, contained, unmeasured

Of the 592 bytes now left per element, none is the style:

element_type: String ► a u16 tag. Removes a heap block per element
events: HashSet<String> ► a u32 bitmask. HashSet is 48 B plus heap. Blocked on the custom-element trait signatures, which spell out HashSet<String>
HashMap<u64, RetainedElement> ► a dense Vec. JS already allocates ids as a counter, so the hash buys nothing
Buffer instead of String at the FFI. napi's String runs napi_get_value_string_utf8 twice, once to measure and once to fill, so a 13 MB batch is transcoded twice and allocated once more. BufferSlice is a pointer. This keeps JSON; it changes the container, not the format

Probably the real 1M-element blocker

Neither of these is about serialization, and neither is measured:

RetainedTree::mark_changed walks parent to root on every append, insert, style and text mutation. 72 009 appendChild ops at depth 10-15 is roughly a million map probes per mount
build_virtual_list runs every frame and touches every child, not just the visible ones: two map passes plus a child_ids.clone(). At 72k rows that is ~144k probes per frame; at 1M it is ~2M probes plus ~8 MB of Vec clone

Frame time

Everything measured here is mount and memory. At today's tree sizes a wheel event still costs Taffy on the visible rows and is unchanged. That stops being true at the 1M scale this work exists for, because build_virtual_list walks every child per frame, so a smaller element does start to matter.

Also, at 1M elements one applyBatch is roughly a 180 MB JSON string held twice. That is a separate wall, and no codec fixes it; a mount that large needs chunked batches.

Known gaps in the benchmark itself

Read these before quoting any phase-two number.

CompactStyle cannot deduplicate a style with a nested block. dedup_key stores the arena index of a hover or active, and every occurrence gets a fresh index. That is why the Rust side reports 100 unique styles where the JS side reports 90: six shapes carry a nested object across sixteen ops, and 90 − 6 + 16 = 100. chat.tsx barely uses hover; an app where every row has one would get zero style sharing from the prototype.
PropValue::visit_str runs the CSS colour parser on every string value before it knows the property, and quantizes to 8 bits per channel, which is lossy for the oklch theme. The type has to come from the key, not from sniffing.
verify() compares parents, child order, styled-ness and content presence. It does not compare element type, style content, text value, custom props or events, so a compact style that dropped half its properties would pass.
KEYS is one namespace for element types, event names and style property names, and the compact tree does 1 << (id % 32), so event bits collide.
The bench interners are thread-locals and are not reset between decode rows.
setCustomProp payloads still decode to serde_json::Value everywhere. They are 1.70 MB of the fixture and hold its largest strings. They should stay a Value, because build_element reads custom_props every frame and reparsing there would be a frame-time regression, but the clone is gone.
Escaped strings are rare here: exactly one of 312 198 value strings needed unescaping. MessagePack's guarantee that every string borrows is real and worth almost nothing on this payload.