SPEED_ANALYSIS.md

This is what the AI Claude answared to my question.

In this Rust code the deserializing is faster with rkyv due to zero copy buffer, but why is it also faster for serializing, copy is it linked to zero copy buffer ?

Great question! The faster serialization in Rkyv is not directly due to zero-copy deserialization, but rather due to fundamental differences in how the two libraries approach serialization. Let me explain:
Why Rkyv Serialization is Faster
1. Memory Layout Strategy

Rkyv: Creates a serialized format that mirrors the in-memory layout as closely as possible. This means:

    Minimal data transformation during serialization
    Direct memory copying for many data types
    Less CPU-intensive encoding

Prost (Protocol Buffers): Uses a more complex, compact wire format that requires:

    Encoding each field with type information and length prefixes
    Variable-length integer encoding (varints)
    More CPU cycles for transformation

2. Data Structure Differences

Looking at your data structure:
rust

Data {
    long_vector1: vec![1; 500_000],  // 500k identical integers
    long_vector2: digits,            // 10k varied integers
    name: "Test name to try",
    id: 12345,
    items: vec!["item1", "item2", "item3"],
}

Rkyv: Can serialize the Vec<i32> almost as a direct memory copy since it maintains native alignment and layout.

Prost: Must encode each integer individually using Protocol Buffer's varint encoding, which is more expensive even for simple repeated values.
3. Encoding Overhead

Protocol Buffers (Prost):

- Field tags and wire types for each field
- Length prefixes for strings and repeated fields
- Varint encoding for integers (even simple ones)
- ZigZag encoding for signed integers

Rkyv:

- Direct memory layout preservation
- Minimal metadata overhead
- Bulk copying of compatible data types

Zero-Copy Benefits (Deserialization Side)

The zero-copy advantage is primarily on the deserialization side:

    Rkyv: Can access data directly from the serialized buffer without copying
    Prost: Must decode and reconstruct all objects in memory

Code Evidence

In your benchmark, notice the serialization approaches:

Prost:
rust

data.encode_to_vec()  // Complex encoding process

Rkyv:
rust

to_bytes_with_alloc(&value, arena.acquire())  // More direct memory layout

Performance Implications

For your specific data (large vectors of integers), Rkyv's approach of preserving memory layout during serialization is much more efficient than Protocol Buffer's field-by-field encoding approach.

The ~6.6x serialization speedup (2.4ms vs 363µs) reflects this fundamental architectural difference, not zero-copy features. Zero-copy primarily benefits the read/access patterns, which is why both libraries perform similarly for simple data access in your benchmarks.