What version of Go are you using (go version)?

$ go version
go version go1.12.5 linux/amd64

Does this issue reproduce with the latest release?

no sure.
Only reproduce on my version

What operating system and processor architecture are you using (go env)?

$ go env
GOARCH="amd64"
GOBIN="/usr/local/go/bin"
GOCACHE="/home/yuan/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/data0/www/gopath"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build422863565=/tmp/go-build -gno-record-gcc-switches"

What did you do?

package httptest

import (
	"fmt"
	"io/ioutil"
	"net/http"
	"sync"
	"testing"
	"time"
)

var client *http.Client = &http.Client{
	Timeout: 500 * time.Millisecond,
	Transport: &http.Transport{
		DisableKeepAlives:   true,
		MaxIdleConnsPerHost: -1,
	},
}

func Benchmark_List(b *testing.B) {
	var (
		wg           sync.WaitGroup
		maxParallel  int       = 50
		parallelChan chan bool = make(chan bool, maxParallel)
	)
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		parallelChan <- true
		wg.Add(1)
		go func() {
			defer func() {
				wg.Done()
				<-parallelChan
			}()
			testHttp2()
		}()
	}
	wg.Wait()
}

func testHttp2() {
	req, _ := http.NewRequest("GET", "http://10.33.108.39:11222/index.php", nil)
	resp, err := client.Do(req)
	if err != nil {
		fmt.Println("error", err)
		return
	}
	_, err = ioutil.ReadAll(resp.Body)
	if err != nil {
		return
	}
	defer resp.Body.Close()
	fmt.Println("success")

}

What did you expect to see?

When I use ApacheBench to express the server 127.0.0.1:8080
ab -c 30 -n 10000000 -k http://127.0.0.1:8080
I think my code is ok。No bug.

What did you see instead?

But about every 3-4 minute, I saw the error in my log

net/http: request canceled while waiting for connection (Client.Timeout exceeded while awaiting headers)

my machine is ok.and this is my netstat.

LAST_ACK 2
CLOSE_WAIT 7
ESTABLISHED 108
SYN_SENT 3
TIME_WAIT 43