golang，一个map什么时候会发生gc？

Question

如果一个map只发生自身key对应的value的修改或删除，会造成很大的gc吗？
这个map很大

Answer 1

楼上研究很透彻，厉害，很多细节也是看了你的文章才知道，谢谢

Answer 2

其实这取决于这个map是什么样的map，以及你的go的版本，在go 1.4版本存在map gc的bug。具体细节可以参考我的这篇文章。
对于你的问题，在四种情况gc时间情况：

1. map[int32]*int32 即value为指针：

   With map[int32]*int32, GC took 1.66876088s

2. map[int32]int32 即value为对象引用：

   With map[int32]int32, GC took 14.169445ms

3. 将map分为多个小map(高性能map常用做法)

   With map shards ([]map[int32]int32), GC took 3.93556ms

4. 而使用slice代替map处理的话，即[]t

   With a plain slice ([]main.t), GC took 71.614µs
   

如果将GOMAXPROCS设置为4则结果如下：
runtime.GOMAXPROCS(4)

1. map[int32]*int32 即value为指针：

   With map[int32]*int32, GC took 1.5085196s

2. map[int32]int32 即value为对象引用：

   With map[int32]int32, GC took 15.583222ms

3. 将map分为多个小map(高性能map常用做法)

   With map shards ([]map[int32]int32), GC took 4.013019ms

4. 而使用slice代替map处理的话，即[]t

   With a plain slice ([]main.t), GC took 119.643µs
   

上述结果均在我的个人电脑(MacBook Pro (Retina, 13-inch, Early 2015))测试得出。

可以得出结论，gc时间受到map存储的类型和CPU核心数目影响，当然也和硬件机器的性能有关。就上面的数据而言，slice的gc时间是远远快于map的，而map存储指针则是最慢的，不推荐。

测试代码来自：http://play.golang.org/p/AeHS...
测试代码中的最初数据(作为参考)：

 With map[int32]*int32, GC took 2.831259536s
 With map[int32]int32, GC took 541.091193ms
 With map shards ([]map[int32]int32), GC took 325.312192ms
 With a plain slice ([]main.t), GC took 69.359µs

 Higher parallelism does help, to some extent:

 for t in 1 2 3 4; do GOMAXPROCS=4 go run maps.go $t; done
 With map[int32]*int32, GC took 2.347938525s
 With map[int32]int32, GC took 362.263322ms
 With map shards ([]map[int32]int32), GC took 89.884292ms
 With a plain slice ([]main.t), GC took 312.583µs