rust-performance

name	rust-performance
description	Guides Rust performance optimization. Use when profiling, benchmarking, reducing allocations, improving cache locality, choosing between rayon/async/threads, or applying SIMD/parallelism.

Performance Optimization

Layer 2: Design Choices

Core Question

What's the bottleneck, and is optimization worth it?

Before optimizing:

• Have you measured? (Don't guess)
• What's the acceptable performance target?
• Will optimization add significant complexity?

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Thinking Prompt

1. Have you measured?

   • Profile first → flamegraph, perf
   • Benchmark → criterion, cargo bench
   • Identify actual hotspots

2. What's the priority?

   • Algorithm (10x–1000x improvement)
   • Data structure (2x–10x)
   • Allocation reduction (2x–5x)
   • Cache optimization (1.5x–3x)

3. What's the trade-off?

   • Complexity vs speed
   • Memory vs CPU
   • Latency vs throughput

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Optimization Priority

1. Algorithm choice     (10x - 1000x)
2. Data structure       (2x - 10x)
3. Allocation reduction (2x - 5x)
4. Cache optimization   (1.5x - 3x)
5. SIMD/Parallelism     (2x - 8x)

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Common Techniques

Technique	When	How
Pre-allocation	Known size	Vec::with_capacity(n)
Avoid cloning	Hot paths	Use references or Cow<T>
Batch operations	Many small ops	Collect then process
SmallVec	Usually small	smallvec::SmallVec<[T; N]>
Inline buffers	Fixed-size data	Arrays over Vec

Tooling

Tool	Purpose
cargo bench	Micro-benchmarks
criterion	Statistical benchmarks
perf / flamegraph	CPU profiling
heaptrack	Allocation tracking
valgrind / cachegrind	Cache analysis

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Common Mistakes

Mistake	Why Wrong	Better
Optimize without profiling	Wrong target	Profile first
Benchmark in debug mode	Meaningless results	Always --release
Use LinkedList	Cache unfriendly	Vec or VecDeque
Hidden .clone() in loop	Unnecessary allocs	Use references
Premature optimization	Wasted effort	Make it correct first

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Anti-Patterns

Anti-Pattern	Why Bad	Better
Clone to avoid lifetimes	Performance cost	Proper ownership
Box everything	Indirection cost	Stack allocation when possible
HashMap for small sets	Overhead	Vec with linear search
String concat in loop	O(n²)	String::with_capacity or write!

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Related Skills

• ownership — avoid clones, use references in hot paths
• concurrency — rayon for data parallelism, tokio for I/O
• anti-patterns — performance anti-patterns