/m10-performance

name: m10-performance
description: "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"
user-invocable: false

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):

"How fast does this need to be?"
    ↑ Ask: What's the performance SLA?
    ↑ Check: domain-* (latency requirements)
    ↑ Check: Business requirements (acceptable response time)

| 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):

"Need to reduce allocations"
    ↓ m01-ownership: Use references, avoid clone
    ↓ m02-resource: Pre-allocate with_capacity

"Need to parallelize"
    ↓ m07-concurrency: Choose rayon or threads
    ↓ m07-concurrency: Consider async for I/O-bound

"Need cache efficiency"
    ↓ Data layout: Prefer Vec over HashMap when possible
    ↓ Access patterns: Sequential over random access

---

Quick Reference

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

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)

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-* |