# Go Performance

> Use when optimizing Go code, investigating slow performance, or writing performance-critical sections. Also use when a user mentions slow Go code, string concatenation in loops, or asks about benchmarking, even if the user doesn't explicitly mention performance patterns. Does not cover concurrent performance patterns.

- Skill: `pngdeity/go-performance` (Agent Skill, multi-file: 4 files)
- Install (CLI): `npx skillmds@latest add pngdeity/go-performance`
- Raw SKILL.md: https://api.skillmd.com/api/skills/pngdeity/go-performance/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: pngdeity (https://skillmd.com/u/pngdeity)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/pngdeity/go-performance

---


# Go Performance Patterns

Performance-specific guidelines apply only to the **hot path**. Don't prematurely optimize—focus these patterns where they matter most.

---

## Available Scripts

- **`scripts/bench-compare.sh`** — Runs Go benchmarks N times with optional baseline comparison via benchstat. Supports saving results for future comparison. Run `bash scripts/bench-compare.sh --help` for options.

---

## Prefer strconv over fmt

When converting primitives to/from strings, `strconv` is faster than `fmt`:

```go
s := strconv.Itoa(rand.Int()) // ~2x faster than fmt.Sprint()
```

| Approach | Speed | Allocations |
|----------|-------|-------------|
| `fmt.Sprint` | 143 ns/op | 2 allocs/op |
| `strconv.Itoa` | 64.2 ns/op | 1 allocs/op |

> Read [references/STRING-OPTIMIZATION.md](references/STRING-OPTIMIZATION.md) when choosing between strconv and fmt for type conversions, or for the full conversion table.

---

## Avoid Repeated String-to-Byte Conversions

Convert a fixed string to `[]byte` once outside the loop:

```go
data := []byte("Hello world")
for i := 0; i < b.N; i++ {
    w.Write(data) // ~7x faster than []byte("...") each iteration
}
```

---

## Prefer Specifying Container Capacity

Specify container capacity where possible to allocate memory up front.

### Map Capacity Hints

```go
m := make(map[string]os.DirEntry, len(files))
```

### Slice Capacity

```go
data := make([]int, 0, size)
```

| Approach | Time (100M iterations) |
|----------|------------------------|
| No capacity | 2.48s |
| With capacity | 0.21s |

The capacity version is **~12x faster** due to zero reallocations during append.

---

## Pass Values

Don't pass pointers as function arguments just to save a few bytes. If a function refers to its argument `x` only as `*x` throughout, then the argument shouldn't be a pointer.

```go
func process(s string) { // not *string — strings are small fixed-size headers
    fmt.Println(s)
}
```

**Common pass-by-value types**: `string`, `io.Reader`, small structs.

**Exceptions**: Large structs where copying is expensive, or small structs that might grow in the future.

---

## String Concatenation

Choose the right strategy based on complexity:

| Method | Best For |
|--------|----------|
| `+` | Few strings, simple concat |
| `fmt.Sprintf` | Formatted output with mixed types |
| `strings.Builder` | Loop/piecemeal construction |
| `strings.Join` | Joining a slice |
| Backtick literal | Constant multi-line text |

> Read [references/STRING-OPTIMIZATION.md](references/STRING-OPTIMIZATION.md) when choosing a string concatenation strategy, using strings.Builder in loops, or deciding between fmt.Sprintf and manual concatenation.

---

## Benchmarking and Profiling

Always measure before and after optimizing. Use Go's built-in benchmark framework and profiling tools.

```bash
go test -bench=. -benchmem -count=10 ./...
```

> Read [references/BENCHMARKS.md](references/BENCHMARKS.md) when writing benchmarks, comparing results with benchstat, profiling with pprof, or interpreting benchmark output.

> **Validation**: After applying optimizations, run `bash scripts/bench-compare.sh` to measure the actual impact. Only keep optimizations with measurable improvement.

---

## Quick Reference

| Pattern | Bad | Good | Improvement |
|---------|-----|------|-------------|
| Int to string | `fmt.Sprint(n)` | `strconv.Itoa(n)` | ~2x faster |
| Repeated `[]byte` | `[]byte("str")` in loop | Convert once outside | ~7x faster |
| Map initialization | `make(map[K]V)` | `make(map[K]V, size)` | Fewer allocs |
| Slice initialization | `make([]T, 0)` | `make([]T, 0, cap)` | ~12x faster |
| Small fixed-size args | `*string`, `*io.Reader` | `string`, `io.Reader` | No indirection |
| Simple string join | `s1 + " " + s2` | (already good) | Use `+` for few strings |
| Loop string build | Repeated `+=` | `strings.Builder` | O(n) vs O(n²) |