cmap

support feather

1. 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
   

2. 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

sync.map usage

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)

cmap usage

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)

benchmark

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。