May 14, 2014
Prelude
This post based on
real events
in docker repository.
When I revealed that my 20-percent-cooler refactoring made Pop function x4-x5
times slower, I did some research and concluded, that problem was in using
defer statement for unlocking everywhere.
In this post I’ll write simple program and benchmarks from which we will see,
that sometimes defer statement can slowdown your program a lot.
Let’s create simple queue with methods Put and Get. Next snippets shows such
queue and benchmarks for it. Also I wrote duplicate methods with defer and
without it.
Code
package defertest
import (
"sync"
)
type Queue struct {
sync.Mutex
arr []int
}
func New() *Queue {
return &Queue{}
}
func (q *Queue) Put(elem int) {
q.Lock()
q.arr = append(q.arr, elem)
q.Unlock()
}
func (q *Queue) PutDefer(elem int) {
q.Lock()
defer q.Unlock()
q.arr = append(q.arr, elem)
}
func (q *Queue) Get() int {
q.Lock()
if len(q.arr) == 0 {
q.Unlock()
return 0
}
res := q.arr[0]
q.arr = q.arr[1:]
q.Unlock()
return res
}
func (q *Queue) GetDefer() int {
q.Lock()
defer q.Unlock()
if len(q.arr) == 0 {
return 0
}
res := q.arr[0]
q.arr = q.arr[1:]
return res
}Benchmarks
package defertest
import (
"testing"
)
func BenchmarkPut(b *testing.B) {
q := New()
b.ResetTimer()
for i := 0; i < b.N; i++ {
for j := 0; j < 1000; j++ {
q.Put(j)
}
}
}
func BenchmarkPutDefer(b *testing.B) {
q := New()
b.ResetTimer()
for i := 0; i < b.N; i++ {
for j := 0; j < 1000; j++ {
q.PutDefer(j)
}
}
}
func BenchmarkGet(b *testing.B) {
q := New()
for i := 0; i < 1000; i++ {
q.Put(i)
}
b.ResetTimer()
for i := 0; i < b.N; i++ {
for j := 0; j < 2000; j++ {
q.Get()
}
}
}
func BenchmarkGetDefer(b *testing.B) {
q := New()
for i := 0; i < 1000; i++ {
q.Put(i)
}
b.ResetTimer()
for i := 0; i < b.N; i++ {
for j := 0; j < 2000; j++ {
q.GetDefer()
}
}
}Results
BenchmarkPut 50000 63002 ns/op
BenchmarkPutDefer 10000 143391 ns/op
BenchmarkGet 50000 72045 ns/op
BenchmarkGetDefer 10000 249029 ns/op
Conclusion
You don’t need defers in small functions with one-two exit points.
Update
Retested with go from tip as Cezar Sá Espinola suggested. So, here results:
BenchmarkPut 50000 54633 ns/op
BenchmarkPutDefer 10000 102971 ns/op
BenchmarkGet 50000 65148 ns/op
BenchmarkGetDefer 10000 180839 ns/op