proposal: Go 2: variable length channel

[coder1966]

Proposal Details

The channel length can variable.

c := make(chan int , <MaxLen> [,<MinLen>] [,<TimeDuration>])

first:
cap(c) = <MinLen>

if full:
cap(c) = cap(c) * 2 // Max <MaxLen>

if len(c) < cap(c) /4 && timeout <TimeDuration>{
cap(c) = cap(c) / 2 // Min <MinLen>
}

[randall77]

What's the point of this exactly? Just saving memory when the channel is not being used much?

[coder1966]

What's the point of this exactly? Just saving memory when the channel is not being used much?

Yes, this is I want.

[Jorropo]

I'm unclear in what conditions the channel would grow.
Wouldn't cap(c) always be MaxLen because it would always grow on demand ? The actual ring being smaller would be an implementation detail.

---

That hell a lot of knobs and the performance impact of getting this wrong could be very bad compared to the gains.

---

It also seems to me you could implement this in a third party librairy, you would need a background go-routine per channel if you want select compatibility, but the timeout isn't free either.

[seankhliao]

Please fill out https://github.com/golang/proposal/blob/master/go2-language-changes.md when proposing language changes

[adonovan]

Channel buffers are rarely a significant fraction of total memory usage; most are of size zero, or one, or the number of worker goroutines. Making a language change for such a minor space optimization would be all cost and no significant benefit.

Therefore, this is a likely decline. Leaving open for four weeks for final comments.

[yangjinheng]

I have also encountered such a need before

[yangjinheng]

I have also encountered such a need before

probably like this, but this code may not be a good way

package main

import (
	"fmt"
	"time"
)

type User struct {
	Name string
	keys []string
}

func main() {
	users := []User{
		{Name: "foo", keys: []string{"a", "b", "c", "e"}},
		{Name: "bar", keys: []string{"f", "g", "h", "i"}},
	}

	// if there is a variable length channel
	// no need to calculate total
	total := 0
	for _, u := range users {
		for range u.keys {
			total += 1
		}
	}

	buffer := make(chan string, total)

	for _, u := range users {
		for _, k := range u.keys {
			go func(d string) {
				time.Sleep(time.Second)
				buffer <- d
				fmt.Println("Push", d)
			}(k)
		}
	}

	// close(buffer)
	// for range buffer {}

	for total > 0 {
		data, ok := <-buffer
		fmt.Println("Pop", data, ok)

		total--
	}

	close(buffer)
}

[qiulaidongfeng]

@yangjinheng The code you show seems to make chan use just the right amount of memory.
If a variable length chan is supported, the memory used by chan may not be just right.

[findleyr]

No change in consensus, so declined.
— rfindley for the language proposal review group