CRITICAL: Use for performance optimization. Triggers: performance, optimization, benchmark, profiling, flamegraph, criterion, slow, fast, allocation, cache, SIMD, make it faster, 性能优化, 基准测试

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? Will optimization add complexity? Performance Decision → Implementation Goal Design Choice Implementation Reduce allocations Pre allocate, reuse with capacity , object pools Improve cache Contiguous data Vec , SmallVec Parallelize Data parallelism rayon , threads Avoid copies Zero copy References, Cow<T Reduce indirection Inline data smallvec , arrays Thinking Prompt Before optimizing: 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 (2x 5x) Cache (1.5x 3x) 3. What's the trade off? Complexity vs speed Memory vs CPU Latency vs throughput Trace Up ↑ To domain constraints (Layer 3): Question Trace To Ask Latency requirements domain What's acceptable response time? Throughput needs domain How many requests per second? Memory constraints domain What's the memory budget? Trace Down ↓ To implementation (Layer 1): Quick Reference Tool Purpose cargo bench Micro benchmarks criterion Statistical benchmarks perf / flamegraph CPU profiling heaptrack Allocation tracking valgrind / cachegrind Cache analysis Optimization Priority 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 Common Mistakes Mistake Why Wrong Better Optimize without profiling Wrong target Profile first Benchmark in debug mode Meaningless Always release Use LinkedList Cache unfriendly Vec or VecDeque Hidden .clone() Unnecessary allocs Use references Premature optimization Wasted effort Make it work first Anti Patterns Anti Pattern Why Bad Better Clone to avoid lifetimes Performance cost Proper ownership Box everything Indirection cost Stack when possible HashMap for small sets Overhead Vec with linear search String concat in loop O(n^2) String::with capacity or format! Related Skills When See Reducing clones m01 ownership Concurrency options m07 concurrency Smart pointer choice m02 resource Domain requirements domain

m10-performance