m10-performance

# 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