Go client program generates a lot a sockets in TIME_WAIT state

Question

I have a Go program that generates a lot of HTTP requests from multiple goroutines. after running for a while, the program spits out an error: connect: cannot assign requested address.

When checking with netstat, I get a high number (28229) of connections in TIME_WAIT.

The high number of TIME_WAIT sockets happens when I the number of goroutines is 3 and is severe enough to cause a crash when it is 5.

I run Ubuntu 14.4 under docker and go version 1.7

This is the Go program.

package main

import (
        "io/ioutil"
        "log"
        "net/http"
        "sync"
)
var wg sync.WaitGroup
var url="http://172.17.0.9:3000/";
const num_coroutines=5;
const num_request_per_coroutine=100000
func get_page(){
        response, err := http.Get(url)
        if err != nil {
                log.Fatal(err)
        } else {
                defer response.Body.Close()
                _, err =ioutil.ReadAll(response.Body)
                if err != nil {
                        log.Fatal(err)
                }
        }

}
func get_pages(){
        defer wg.Done()
        for i := 0; i < num_request_per_coroutine; i++{
                get_page();
        }
}

func main() {
        for i:=0;i<num_coroutines;i++{
                wg.Add(1)
                go get_pages()
        }
        wg.Wait()
}

This is the server program:

package main

import (
    "fmt"
    "net/http"
    "log"
)
var count int;
func sayhelloName(w http.ResponseWriter, r *http.Request) {
    count++;
    fmt.Fprintf(w,"Hello World, count is %d",count) // send data to client side
}

func main() {
    http.HandleFunc("/", sayhelloName) // set router
    err := http.ListenAndServe(":3000", nil) // set listen port
    if err != nil {
        log.Fatal("ListenAndServe: ", err)
    }
}

Answer 1

The default http.Transport is opening and closing connections too quickly. Since all connections are to the same host:port combination, you need to increase MaxIdleConnsPerHost to match your value for num_coroutines. Otherwise, the transport will frequently close the extra connections, only to have them reopened immediately.

You can set this globally on the default transport:

http.DefaultTransport.(*http.Transport).MaxIdleConnsPerHost = numCoroutines

Or when creating your own transport

t := &http.Transport{
    Proxy: http.ProxyFromEnvironment,
    DialContext: (&net.Dialer{
        Timeout:   30 * time.Second,
        KeepAlive: 30 * time.Second,
    }).DialContext,
    MaxIdleConnsPerHost:   numCoroutines,
    MaxIdleConns:          100,
    IdleConnTimeout:       90 * time.Second,
    TLSHandshakeTimeout:   10 * time.Second,
    ExpectContinueTimeout: 1 * time.Second,
}

Similar question: Go http.Get, concurrency, and "Connection reset by peer"