-
support O(1) length method, like the issue decsribed here golang/go#20680
新增特性，O(1)复杂度的长度方法 -
reduce contention between Map operations with muti hash lock (doing), like the issue decsribed here golang/go#21035
目前sync map 对不同的key操作，对应的都是同一个锁，可以通过多锁的方式，减少竞争。（实现中）
In Go 1.9, sync.Map was introduced, but sync.Map dont support length method ,
when you need get the length of your map
获取map长度示例对比
import (
"sync"
)
length := 0
myMap.Range(func(_, _ interface{}) bool {
length++
return true
})it will lock your map, and take O(n) times
此操作会触发锁，复杂度O(n)
import (
"github.com/mojinfu/cmap"
)
length := myCMap.Length()cmap will not lock your cmap, and take O(1) times
此操作不会触发锁，复杂度O(1)
100 times Store(i, i) and Delete(i) in env goos: darwin ; goarch: amd64
| package | ns/op | B/op | allocs/op |
|---|---|---|---|
| sync.Map | 21230 ns/op | 5600 B/op | 499 allocs/op |
| cmap.Map | 24243 ns/op | 5600 B/op | 499 allocs/op |
- it means each Store or Delete action will take another 15ns
- cmap.Map中使用的原子计数器虽然线程安全，是通过底层硬件的支持作为保障的，这使得每次新增Key 删除Key，相对于sync.Map 都将有15ns的额外耗时
| map length | get length in sync.Map time-consuming | store in cmap.Map Extra time consuming |
|---|---|---|
| map长度 | 使用sync.Map通过range获取长度耗时 | 使用CMap存删造成的理论额外耗时 |
| 1 | 44.4 ns | 15 ns |
| 10 | 168 ns | 150 ns |
| 100 | 1527 ns | 1500 ns |
| 1000 | 22647 ns | 15000 ns |
| 10000 | 0.32 ms | 0.15 ms |
| 100000 | 8.01 ms | 1.5 ms |
-
it means when programme needs the length of the map(even for one time), use cmap package take the place of sync.map is better.
-
如果程序中需要获取Map长度，务必使用cmap来减少性能损耗。对长度的获取越频繁，使用cmap的必要性就越大。