GC pause for maps

maps.go

package main

import (
  "fmt"
  "os"
  "runtime"
  "time"
)

// Results of this program on my machine: (macos, go 1.14):
//
// for t in 1 2 3 4 5; do go run maps.go $t; done
//
// Higher parallelism does help, to some extent:
//
// for t in 1 2 3 4 5; do GOMAXPROCS=8 go run maps.go $t; done
//
// Output(go 1.14):
// With map[int32]*int32, GC took 456.159324ms
// With map[int32]int32, GC took 10.644116ms
// With map shards ([]map[int32]*int32), GC took 383.296446ms
// With map shards ([]map[int32]int32), GC took 1.023655ms
// With a plain slice ([]main.t), GC took 172.776µs
//
// Output(go 1.9):
// With map[int32]*int32, GC took 457.036664ms
// With map[int32]int32, GC took 22.160147ms
// With map shards ([]map[int32]*int32), GC took 432.721998ms
// With map shards ([]map[int32]int32), GC took 3.887943ms
// With a plain slice ([]main.t), GC took 80.547µs

func main() {
  const N = 5e7 // 5000w

  if len(os.Args) != 2 {
    fmt.Printf("usage: %s [1 2 3 4]\n(number selects the test)\n", os.Args[0])
    return
  }

  switch os.Args[1] {
  case "1":
    // Big map with a pointer in the value
    m := make(map[int32]*int32)
    for i := 0; i < N; i++ {
      n := int32(i)
      m[n] = &n
    }
    runtime.GC()
    fmt.Printf("With %T, GC took %s\n", m, timeGC())
    _ = m[0] // Preserve m until here, hopefully
  case "2":
    // Big map, no pointer in the value
    m := make(map[int32]int32)
    for i := 0; i < N; i++ {
      n := int32(i)
      m[n] = n
    }
    runtime.GC()
    fmt.Printf("With %T, GC took %s\n", m, timeGC())
    _ = m[0]
  case "3":
    // Split the map into 100 shards
    shards := make([]map[int32]*int32, 100)
    for i := range shards {
      shards[i] = make(map[int32]*int32)
    }
    for i := 0; i < N; i++ {
      n := int32(i)
      shards[i%100][n] = &n
    }
    runtime.GC()
    fmt.Printf("With map shards (%T), GC took %s\n", shards, timeGC())
    _ = shards[0][0]
  case "4":
    // Split the map into 100 shards
    shards := make([]map[int32]int32, 100)
    for i := range shards {
      shards[i] = make(map[int32]int32)
    }
    for i := 0; i < N; i++ {
      n := int32(i)
      shards[i%100][n] = n
    }
    runtime.GC()
    fmt.Printf("With map shards (%T), GC took %s\n", shards, timeGC())
    _ = shards[0][0]
  case "5":
    // A slice, just for comparison to show that
    // merely holding onto millions of int32s is fine
    // if they're in a slice.
    type t struct {
      p, q int32
    }
    var s []t
    for i := 0; i < N; i++ {
      n := int32(i)
      s = append(s, t{n, n})
    }
    runtime.GC()
    fmt.Printf("With a plain slice (%T), GC took %s\n", s, timeGC())
    _ = s[0]
  }
}

func timeGC() time.Duration {
  start := time.Now()
  runtime.GC()
  return time.Since(start)
}