Go: Understand the Design of Sync.Pool

Illustration created for “A Journey With Go”, made from the original Go Gopher, created by Renee French.

ℹ️ This article is based on Go 1.12 and 1.13 and explains the evolution of sync/pool.go between those two versions.

The sync package provides a powerful pool of instances that can be re-used in order to reduce the pressure on the garbage collector. Before using the package, it is really important to benchmark your application before and after the usage of the pool since it could decrease your performance if you do not understand well how it works internally.

Limitation of the pool

Let’s take a basic example to see how it works in a pretty simple context with 1k allocations:

type Small struct {
   a int
}

var pool = sync.Pool{
   New: func() interface{} { return new(Small) },
}

//go:noinline
func inc(s *Small) { s.a++ }

func BenchmarkWithoutPool(b *testing.B) {
   var s *Small
   for i := 0; i < b.N; i++ {
      for j := 0; j < 10000; j++ {
         s = &Small{ a: 1, }
         b.StopTimer(); inc(s); b.StartTimer()
      }
   }
}

func BenchmarkWithPool(b *testing.B) {
   var s *Small
   for i := 0; i < b.N; i++ {
      for j := 0; j < 10000; j++ {
         s = pool.Get().(*Small)
         s.a = 1
         b.StopTimer(); inc(s); b.StartTimer()
         pool.Put(s)
      }
   }
}

Here are two benchmarks, one without usage of sync.Pool and another one that takes…